I'm just an electronics hobbyist and your videos have taught me a lot. If you had been doing these videos 30 years ago I'm sure I would have changed my major to electrical engineering rather than finance. When you retire please consider teaching engineering or math at the college level. You would have a great impact on young minds.
I think he's having a much larger impact here, on youtube, teaching at the "hobbyist level", rather than teaching at the college level. Inspirational. Technical details (unfortunately?) are needed at some point to do advanced things, and those require a formal (university level) training, which can be more boring.
I wish I could get all the other RUclips "instructors" to watch your videos. You are so polished. You could make a living instructing the instructors and I wish you would.
I remember watching this video years ago, and I was really in the very early stages of my electronics hobby. I vaguely understood it, but I thought to myself that hey, nice video, but let's be honest, I'll never need to know how to do this... Well, I was wrong. Today I need to know how to measure output impedance, and I also could use a primer on transmission line termination. w2aew to the rescue! Your drawings and practical examples have taught me more than anything else. Thank you, genuinely!
Still valuable and excellent, and it will be even in two decades from now on. A really outstanding quality in terms of clarity, accuracy and style. Can't thank you enough for your work!
I am glad you still have this video posted. The equations used are easy to derive once you think about the relations, but when electronics is a hobby and not a field of study it sure speeds up the process to see it laid out so nicely as in this video.
Alan, when I watch your vids, it's like learning all over again. I must admit, I feel that same giggly sensation that I felt when I first learned these principles many years ago. I've found that a huge number of beginning electronics students, hobbyist and on occasion, even some tech's I've spoken to, have had a hard time understanding this. You sir, have accomplished the perfect explanation of the very concept of output impedance and how to measure it in a simple straight forward way.... I'm always amazed by your teaching skills which seem so easy but in fact, are difficult in practice. God indeed blessed you with an outstanding teaching ability and a keen mind as well. I'm so glad that you share this talent with others. It will unlock a new world of understanding for some they never knew existed. Again, Kudos!!! Job Well Done!
Just dropped in to see what was happening. LOL! Had just used the resistance substitution method (5:06) to get output impedance of an amplifier. Thanks again, Alan.
Hi Alan, great video and tutorial to measure output impedance. Most of us also wants to know how to measure an unknown network impedance from 100kHz to 1GHz or higher but this one can only be achieved using a network analyzer. In my experience in Electronics, EMC and RF, and before I started my ECErcuit RUclips channel, I been working in understanding how to measure unknown network impedance using spectrum analyser and signal generator. In somehow, I have managed and successfully measured unknown network impedance accurately from 100kHz to at least 100MHz. Measurement was achieved by deriving a formula through a series of derivation and calculations. I automate the system by writing and application to control the spectrum analyser and signal generator to process the measurement against the calibration reference using the formula which gives you the result of impedance plot and data. I'm thinking of sharing this work on my RUclips channel but I'm not sure if someone is interested to know. Cheers, ECErcuit.
For the beginers What Alan does here is : He devide's 2.5V bij 1000 ohms (1K) of R load. That give the current flows in R load which result 0.0025 Amps (2.5 milli amp's) that will also be current flows in Ro (since current is equal everywhere in a close circuit). We also know that when R load is connected and current start passing it creates 1.5 Voltage drop on Ro 4Volts - 2.5 (V drop on R load that is because the sommof all voltage drops must be 4Volts ). So 1.5V voltage drop on Ro devided withthe current flow's in it will give the value of R out which is 600 Ω (ohms).
Very nice presentation once again. A couple of observations: I did this on a breadboard with the output impedance of my awg set to 50 ohm. I used a small 100 ohm trim pot and adjusted the pot until Vpp on the oscilloscope measured the set voltage on the awg. I got 47 ohm. Not bad. It just seems to me that using a pot makes it much easier to fine tune the load resistance compared to a decade box. Secondly this method assumes that the circuit under test consists of linear circuit elements which can be modeled as a thevenin voltage and resistance. The failure of the first method (2:47) to work all the time may be an indication of this. Using a second load resistance measurement (6:54) may or may not satisfactorily adjust for this. Best way is to measure open circuit voltage (Vth), calculate voltage at several load resistance levels, calculate current V/R at each point and plot voltage vs current. (You can do a least squares approximation on the data if the line is not perfectly straight) Getting a straight line insures that you are actually dealing with linear circuit elements. As V (measured) = - Rth*I + Vth. The slope of this line is the negative of the output impedance (Rth).
Please note that the 50 ohm setting on your AWG is likely *NOT* setting the output impedance. The output impedance is *always* 50 ohms. This setting is telling the AWG what the *load impedance* is that you're connecting to. It uses this to properly set and display the voltage at the load.
@@w2aew Yes I know. When I say I set the output impedance I should have said the input impedance of the load. My Siglent SDX1032X has two options for load impedance 50 ohm and HiZ. When set at 50 ohm the open circuit voltage is twice the displayed voltage and when set at HiZ the open circuit voltage is equal to the displayed (set) voltage. The question I had is what is considered a high load impedance? If we desire less than a 1% discrepancy between the displayed and measured voltages (across the load) we have (at the Hiz setting) Vm (measured) = R*Vset/( R + 50 ). Where Vs is the set (displayed) voltage. If we require Vm to be >= 99% of Vset then R (load) must be greater than 4950 ohms. If, instead, can live with a 5% discrepancy then R must be greater than 950 ohms. In practice I actually tested this out and found that at a 700 ohm load the output voltage was within 5% of the displayed voltage and that at greater than 1750 ohms the output voltage was identical to the displayed voltage. So the question arises what if our actual load impedance is below 1750 ohm (for 1%) or less than 700 ohm (for 5%)? Well with the the awg set to HiZ we have Vm = Vset*R/( R + 50) we can turn this around (setting Vm to the desired voltage) and Vset = (1 + 50/R)*Vdesired. Eg lets say we have a 100 ohm load and desire 4 volts. We would then set the awg to Vset = (1 + 50/100)*4 = 6 volts. OTOH if the awg were set to expect a 50 ohm load and we had a load resistance R other than 50 ohm (but less than 700) then the formula would be 1/2 of this Vset = (0.5 + 25/R)*Vdesired.
Hello sir, can you make a video about how to measure the output impedance of RF amplifier circuit ? The methods in this video generally apply to relatively low frequency use cases, I want to know that is threre any approach to measure output impedance at RF frequencies , looking forward to your video, thanks a lot
Another EXCELLENT review!! If you are still considering suggestions for tutorials, Have you considered FET's and in particular their utility in audio. I have been amazed how well FET's sound in preamps and low power amps. Being a voltage device apparently is a plus at audio freq's. My experience has shown them to rival many tubes I have played with such as 45, 2A3, 6L6 tubes. I have played with MANY transistors and opamps at audio freqs but they sound flat and dull compared to FETs...Many of the people I know are OK with transistor theory but don't understand FET's....Another OUTSTANDING audio device is the nuvistor for low level work like preamps...Microphonic as heck but once tamed, they are VERY nice not to mention fast and reasonably cheap.. And a great big OSCILLATOR to you too...dit dit
Hey, great video. I was wondering if you'd be willing to post the derivation for the second formula. I can't seem to set up that equation from scratch. Much appreciated if you find the time.
Wow, this is great! Have you considered publishing all of your notepad explanations with links to videos (or a companion DVD...)? I think this would make a fantastic workbook for those of us without that elusive EE degree!
Dear Alan, I have RF magnitude and phase probe circuit with two toroids connected to 4 diode based peak detectors to give output voltage corresponding to magnitude and phase errors detected. I am somehow not sure how that voltage is mapped to magnitude and phase errors and in what ratio. This circuit is inside a 300W auto matching network which is used to do impedance matching. Load is a sputter source which is used to do sputtering. Generator is a 13.56 MHz, 300W unit. The circuit which I was talking about senses this magnitude and phase error and gives a feedback to a microcontroller which in turn rotates ganged series and shunt capacitor to match the impedance. I just want to understand the circuit better and make know how to make which motor move to get to 50 ohms. I am missing some bits and parts due to which my understanding about this thing is not complete. Please help me understand this thing. Let me know if you want any circuit diagram from or anything else. Thanks !
Lowering load Z (or R) also shifts the HPF created by output DC blocking capacitor (if any) upwards, so, f must be high enough compared to the cut-off freq of that.
Hi, Thank you for your great education videos! Would you please explain how to measure the input impedance of a receiver ( typically with input sensitivity of -110dBm). I tried to do the job using a nano VNA and attenuators, but it seems the measurement is not precise since the nano VNA cannot work with very low level signals. It would be much appreciated if you share your rich knowledge. Thank you!
The first formula is simply re-arranging the typical voltage divider equation: V1 = Vopen * (R1 / (R1+R2)), solving for R2. The second is starting with the ratio of two of these equations and solving for the source resistance.
The output impedance of the collector of a Darlington transistor is higher or lower compared to a bjt's collector output impedance? But I have heard is that a darlington's output impedance on The collector is much lower compared to a bjt's output and complaints on The collector but what is the advantage of the output impedance of a darlington's collector compared to a bjt's output impedance collector?
Thanks Alan for that video, it will definitely be usefull. Correct me if I'm wrong, but with some manipulations there would be a way to determine an input impedance as well, right ? I was thinking to perform the steps you described to evaluate the output impedance of an opamp circuit with 2 resistors, R1 and R2 as you did. Therefore once the opamp output impedance is known, measurements could be performed using the input of the circuit we want to know the input impedance in place of R2. Having access to the opamp output impedance value, along with the V1, R1 and the newly measured V2, we could deduct the value of R2, which would normally be equivalent to the input impedance of the circuit being tested. I understand that if either the opamp output or the tested circuit input input are significally inductive or capacitive, a close attention will need to be paid to the frequency, but generally speaking, I would be lead to believe that what I described could be achieved too. Let me know if that make sense. Keep up with the good work !
Perfect, thank you! This helps a diy guy a lot :) When measure a heaphone output that can have something between 1 and 30 Ohms, will it fit when using 50 and 100 Ohm resistors and do the third type of measurement explained in the video?
Hi Alan. I'm building a CW transmitter. I have a buffer amplifier after the crystal oscillator. I tried measuring the output impedance of that stage using the method you described (with two load resistors). I calculated the output impedance . I then tried another pair of resistors (with different values) to see if I come up with the same output impedance. I did not. It was quite different. Could the output impedance change based on the load?
Can I use the second method (about 6:42 on the video) to check the output impedance on my HF transceiver, see if it is truly 50 ohms? Perhaps use a 50 ohm dummy load and measure with my scope then drop in a 100 ohm dummy load and do the same measurement?
I’ve got most of the principles of ohms law figured out but I’m working on mastering impedance which some people blow-off and as just being resistance. My interest, as an amp tech, is to develop a checklist of measurements across every amp I fix. I was watching this video on measuring output impedance and am curious to know why, when (@3:14) you switched your scope to 1 m-ohm of input impedance, you identified that as an open circuit (and I know an open circuit causes a voltage spike .. assuming because of the pent up potential). As resistance goes, doesn’t the fact that 1 m-ohm exceeds the 600 ohms of output impedance mean that it applies more resistance? Isn’t 1-m ohm a bigger load? Or, is it “open” because the BNC-T was left unterminated?
I said that 1Mohm was effectively open circuit because it is so much higher than the 600 ohm output impedance of the source that the difference in loading of the 1Mohm load vs. a true open circuit would be negligible. I higher resistance load is a "lighter" load - easier to drive, because it doesn't draw much current. Low resistance loads are "heavier" loads, much harder to drive because they draw a lot of current.
OH GOD, finally got that equation. For a very long time, I am just tryin Rload backandforth to get 2Vloaded=Vunloaded, like dumb. Thank you so much w2aew!
I am aircraft technician and I need clarification on the following impedance subject related to ARINC 429 circuits. The transmitting source output impedance should be 75 W ± 5 W divided equally between Line A and Line B (line A and B are twisted pairs i.e 4 wires two for transmission and two for receive). This balanced output should closely match the impedance of the cable. The receiving sink must have an effective input impedance of 8k W minimums. …How is this physically measure?
Yet another excellent video. Can you use the same method to measure the input impedance of lets say a common emitter amplifier or any other type of electronic circuit?
Hi I have a question regarding conductance, can you measure conductance of a let say car battery ,this by a scope? I sort of measure the resistance and this in certain time peiods i think then in days or week or month so see how the resistance increases in time I think of sending a certain freq on one side collect it on the other side then calc the time this must increase when resistance increases or not?
I have a TC Helicon Harmonizer pedal. MIc in, through the effect then back out to mic in amp etc. It works as expected on many different amps, but does not work well with my Fender Acoustic Jr GO. The amp 100W and with the TC pedal, out put volume is lower than an plain acoustic guitar with no amplification. It is not my amp it is an incompatibility with this model of Fender, tried 4 amps. I am suspecting an impedance mis match. TC is 400 ohm out. I have no input data for the Fender but both channels accept XLR for mic or 1/4 unbalanced for a guitar. Anybody's thoughts on what might being going on, and also ways to test/measure or add something between the pedal and amp mic input greatly appreciated. Cheers
so ...how does a working circuit with many parts come out to be an of 50/6oo ohms? do they just place a 50/6oo ohms in parallel with entire device?.....taking into account the parallel total impedance...?.
When looking at the first stage/front end of an amplifier circuit, how can you tell if the first stage/front end is High impedance or low impedance? how can you tell how "sensitive' the front end will be to a low impedance input signal?
By examination of the circuit you should be able to approximate the input impedance. For the most part, if the amplifier isn't designed for RF use, the input impedance will most likely be relatively high.
@@w2aew If the audio amplifier doesn't have any resistor to ground its just only a decoupling cap connected to Q1 and transistor#2/Q2 feedback resistor is biasing the base of Q1 would this be considered an input that is low impedance or high impedance? Because mostly you put a 1meg resistor to ground on the input jack with the decoupling cap in series to the base of Q1 transistor. The 1meg resistor to ground is mostly what determines the High impedance. But if you don't have any resistor to ground the biasing resistors R1 and R2 set the bias voltage for the base of the Q1 transistor. I don't think R1 and R2 determine the input impedance they only set the biasing voltage of the base. But other amplifier configurations omit R1 and R2 and just use a feedback resistor from Q2/transistor#2 to set the biasing voltage for the base voltage of Q1. The input impedance gets really sensitive to low impedance.
im just a aermodellling hobbyist, right now i use the video transmitter with 5.8ghz freq , i need to know is this method can be used to measure the output impedance for my video trasmitter
Probably not, since most hobbyists don't have a way to measure the amplitude of a 5.8GHz signal. You can safely assume that is is likely 50-75 ohms as most RF outputs are.
No, you can't. The output impedance is determined by the active device design and can't be measured with a simple resistance measurements. Note that op amp output impedance is typically *very* low.
U have a great understanding of how circuits work and how to compare two or more method's of measuring things. Could u show us the sinad and signal to noise methods of measuring receiver sensitivity and any trick or tips u know of how and what to use to make these measurements with equipment on a budget. For example I can remember reading of a method of using an analogue multi tester to monitor carrier level. Thanks Alan
Allen, is there a way to calculate the feed point impedance of an Antenna? Like an EFHW uses a 49:1 impedance transformation. How do one come to a conclusion that for HF bands the feed point impedance of an EFHW is around 2500 Ohms and required a 49:1 impedance transformer? I couldn't find a satisfying reply anywhere.
Hi Allen I just watched your video on How to Measure Output Impedance. I wanted to measure the source impedance of a SI5351. When I use the 1/2 voltage method I get 49 ohms, very close. Next I tried the 2 resistor method and it only works out to be 1/2 of what I was expecting, 24 ohms. My R1 = 330 and R2= 814. V1=3.22 and V2=3.36. I'l double check my measurements and calculation. With this method should one of the resistors be close to 50 ohms? 73 Ken VA3ABN
Hi, Alan I guess my question is about the differences (if any) between… 'Output Impedance’ vs. 'Internal Resistance’, ... or ( 'Source Impedance' *). Are each of these terms completely synonymous? or is there enough of a difference between them where they are all needed in order to properly describe slightly different things at work. --- In the class I’m taking we did an experiment to determine the INTERNAL RESISTANCE (R_int) of a voltage supply. The 'SPECIAL CASE’ section of your video here basically covers the same process and formula we used… with the minor difference that… V-open / 2 = R_int (in our experiment). In the experiment we did I used a potentiometer as the R_load,… and adjusted it until V_load = V_open/2. After which, the POT was measured to determine it’s resistor value… and we approximated that to being (R_int). [ ... we ultimately did this for a variety of R_load values… and calculated the Power Differences as R_load values shifted away from matching what R_int is; … and more calculation were done to determine dB gains and losses-relative to the values of the matched pair ( R_int = R_load ). ] I guess In hindsight… we were doing a little more than just finding out what R_int was… it was also about maximizing power transfer, …and maybe starting to flirt with impedance matching too? though, I’m still not 100% solid on understanding everything that impedance is. I have been reading about it (a lot)… but still need more real experience with it, and intuitive understanding… in terms of circuit building, and more hands on testing. Thanks for the Great Channel, Alan… I really LOVE IT. and it’s helping me a lot in my attempts to learn about electronics. Take Care, Jim * My mention of the 'Source Impedance' comes from watching your impedance matching videos, which I'm just now starting to look at. .
Do you have a video about what impedance has to do with measuring tube radio circuits, like why you use a VTVM meter instead of a multi meter, when checking components in the circuits..
I don't have a video, but it is a relatively simple topic to explain. Many tube circuits and nets are very high impedance. This is because the plate output impedance is quite high, and the grid is a very high (near open circuit) impedance. If you use a multimeter to measure these circuits, the input impedance of the multimeter will often be much lower than the circuit node impedance, and thus will alter the voltage when you connect it. A common multimeter impedance is 20,000 ohms per volt, which means that if you have the meter set to a 50V scale, its input impedance is 50*20000, or 1Mohm. At lower voltage ranges, the input impedance is much lower. A typical VTVM or modern DMM has an input impedance of 10Mohms, thus will load (and alter) the circuit to a much lower extent.
@@w2aew So a mulitmeter will drop the readings, or change the circuit more than a VTVM, which will do what when it drops? I guess I dont understand what impedance is or does..
@@kennynvake4hve584 Impedance is just a fancy word for resistance. Actually, it refers to the resistance + reactance - more completely called complex impedance. But for the purposes of this discussion, let's just say that impedance = resistance. Each circuit node can be "modeled" as a voltage source with a series resistor (Kirchoff's Law). Thus, the voltage at each node can be changed by drawing current from it (with a load of some sort). Some circuit nodes have a very low impedance (resistance), meaning that the voltage stays fairly constant even when a load is applied. A power supply node or supply rail in circuit is an example of a low-impedance node. The voltage remains constant even when drawing current from it. Other nodes in a circuit can have much higher impedance (resistance), which means that the voltage can change if you draw some current from it while measuring it. The circuitry connected to the Grid of a vacuum tube will often be high impedance (resistance). So, if you connect a multimeter or VOM to it, it may change the voltage at that node. Consider the following example: A circuit node that is operating at 3V has a 100Kohm impedance - meaning that it "looks like" a 3V voltage source with a 100Kohm resistor in series. If you use a multimeter with a 20,000 ohms/volt rating and set it to a 5V scale, the meter will have a input resistance of 5*20,000, or 100Kohms. If you connect this meter between our test node and ground, the circuit's resistance and the meter resistance will form a voltage divider, and the voltage on that node will drop to 2.5V. It is the same as putting a 100Kohm resistor between that node and ground. You can see from this example that a VTVM with a 10Mohm input resistance will have much less of an effect: 5V * (1M/(1M+100K)) = 4.55V
A wonderful collection of videos. I could listen to your voice all day... but can you help me with a problem on home made swr meters? I have constructed many types using single or double pcb's.... The forward measurement is spot on but the reflected is always higher. I'm thinking, if the impedance of the stripline is out, that's my basic error to be added to the real error on the test load.... SO. how do I measure the impedance of any swr meter?
Hi sometimes our signal is very high frequency like ghz and we can't measure amplitude by an oscilloscope or even an spectrum analyzer how we can measure output impedance in this case?
A four port VNA is typically used to measure the small-signal output impedance in this case. If you are talking about large signal or high power outputs, then often a load-pull analyzer ($$$) is used.
ok, so now I can measure output impedance of the local oscillator and than match it with the diode mixer🙂 If I understand you correctly we can measure input impedance with the same way?
This shouldn't be necessary because the LO input of a diode mixer is non-linear (commutating diodes), and thus is not impedance matched (or realistically matchable).
How do you measure the output impedance of high frequency devices, for example a crystal oscillator (if it makes sense) or an amplifier? As usual, thanks for your videos!
This video shows one method to measure complex input impedance: ruclips.net/video/eYN7dhdt1Dw/видео.html ...or you can use an inexpensive analyzer like the NanoVNA: ruclips.net/video/Sb3q8f0NBZc/видео.html ruclips.net/video/xa6dqx9udcg/видео.html
is this a good response of a signal gen? cause ideally i think it should not be affected when loaded. the feedback circuit should take care of maintaining the amplitude
High frequency signal generators typically have a 50 ohm output impedance. If you use feedback to set the amplitude regardless of load, then the effective output impedance is then lower than 50 ohms, thus giving the generator a poorer output return loss.
When converting high impedance to lower impedance the signal drops down 20dB or more. This means that high impedance audio signals has more current drive, power, energy compared to low impedance audio signals? because what is the difference between an audio signal that is high impedance audio signal compared to a low impedance audio signal? it must have to do with the signals energy and power?
@@w2aew But when concerting from High impedance audio signal to a low impedance audio signal its a 20dB loss from input to output, any reasons why? The high impedance audio signal is 20dB lower when converting it to a low impedance audio signal. If low impedance has more current and more power it should be +20dB not -20dB
@@waynegram8907 Makes perfect sense. It's a simple resistive voltage divider. A high impedance source effectively has a high resistance in series with the output. A low impedance input looks like a low value resistor. So, you have a resistive voltage divider which makes a lower voltage appear at across the low impedance load.
Than you for thus video. So, in order to measure the output impedance of my sound card must I use the 1 Kohm resistor and the formula you sent as? The measure woudn t be correct if I only use my multi-meter Omhs parameter? Thank yuo so much.
You can't use an ohmmeter to measure output impedance because it depends on the active properties of the circuit. The ohmmeter *only* works on unpowered devices.
Just out of curiosity - What if the R_load that I am interested in is in the order of a few megaohms? You used 1 megaohm for V_open but what if R_load is higher than that?
@@w2aew I am currently looking at driving p-n diodes in reverse bias using a sine wave generator at 200MHz. The diodes have input impedance of several megaohms (2-4 as far as I know). Given the frequency I am looking at and the method you showed, would it be safe to use the calculated output impedance of the sine wave generator, as explained in the video, to find the actual voltage across the diodes? Or does frequency play a role here?
@@Terrar-fr1bk The reverse biased impedance of the PIN diodes is so much higher than the generator, it can be considered an open circuit. However, you may need to consider the PIN diode's capacitance under reverse bias and what it's capacitive reactance is at 200MHz.
Ok, great explanation! But, I would like to find out the output impedance of my signal gerenator, and not the resistance. Because, the open-circuit output voltage of it changes with frequency. Please, how would I do this?
It is *very* likely that the change in output amplitude is *not* due to the presence of reactance in your output impedance - it likely has more to do with the signal generator circuit itself or the measurement method.
Ok, thanks a lot w2aew! I was thinking that I could get some kind of a model of the signal generator's output. Maybe, with an impedance like in a transmission line. But, I don't know if that is possible. Anyway, thank you so much for your attention!
not really. As mentioned early in the video, we use a low frequency to avoid transmission line, termination, etc. issues that occur at higher frequencies.
w2aew , how do you measure output impedance in an RF circuit? For a small signal I could use a test load and calculate what the value is, but what about a high power amplifier like for ham radio?
Did you see my video on how to measure inductance with a scope and some basic parts? Measure Capacitors and Inductors with an Oscilloscope and some basic parts
nice video :) will this method work with square wave generator say from 555 timer or we can only measure the impedance of sine wave generator using this method
It should work well with a square wave output too. Just keep in mind that many logic circuits have a different output impedance for the high state and low state.
Question, on Leader LFG 1300S generator requires a 50 ohm cable. can you explain this to me? Does it mean that different generators require different cables for testing? Thanks
The vast majority of coaxial cables used with test and measurement equipment are 50 ohm cables. This means that the coaxial cable has a 50 ohm characteristic impedance. For low frequency signals, this is unimportant. But for high frequency signals (above about 1MHz or so), or for signals with fast rising/falling edges (faster than 40-50ns), the 50 ohm cables and proper termination begins to be important. You may want to review this video on transmission lines and terminations: ruclips.net/video/g_jxh0Qe_FY/видео.html
Good quality RG-58/U such as Belden 8262 would be a fine choice. Maximum length depends on your frequency of operation, it all depends on the amount of loss you're willing to tolerate. Hundreds of feet are often used at frequencies below 100MHz.
Hello Alan, my other question is when you have multiple test equipment such as an oscilloscope, frequency counter, signal generator, and they all have different impedance output such as 50 ohms, 75 ohms, 600 ohms or 1 meg ohms. What coax cable is appropriate when connecting the test equipment ???
There should only be one termination on the line. Other devices should be in high impedance mode. The cable should match the source and terminating resistance. So signal generator is 50 ohm source, freq counter is high impedance connected to a tee, o scope at other end terminated in 50 ohms, using 50 ohm coax between each.
Some active devices instead of running in a resistive mode, run in a constant current mode. For example in a high negative feedback amplifier, the output voltage changes very little with a changing load, so the impedance appears to be very low and the inverse of the impedance called damping factor is used. A common high performance amp with a damping factor of 30 would have an apparent impedance of 1/30th of an ohm or 33 miliohms. However protection first limits current to about 10A(varies) and then the output protection relay or other shutdown trips. Now at 60 volts at 10 Amps, the limit is about 600Watts peak into 6 ohms. Protection kicks in before the output can be dropped to 1/2 voltage.
If I recall correctly, it started off as a kit based on the EXAR XR2206CP monolythic function generator IC - followed up by an op amp based output buffer that i designed to give more flexibility in output amplitude, polarity and offset.
@@w2aew Alright, thanks for a response! Suprisingly, It looks like XR2206CP is still being made. I'm asking because, i'm thinking of making a generator myself, although i have to say that i find my smartphone to be quite good at it for testing the average audio stuff.
Many times you may be wanting to measure the output impedance of a generator, amplifier, circuit, etc. that is AC coupled, and thus a DC signal won't work.
One example is when an unloaded output causes the output to saturate or hit a limiting condition prior to reaching the theoretical open circuit voltage.
A simple example would be a current source output. If open circuited, the output will saturate before it reaches the Thevenin equivalent open circuit voltage.
It depends a lot on the circuit. As the frequency goes up, the output impedance can start to become reactive vs purely resistive, and this simple method doesn't work anymore.
This was a home-made function generator that i designed and built 25 years ago. Unfortunately, I have lost the documentation of the design, so I'd have to do some reverse engineering to recover the design.
***** Don't sweat it. Just thought I noticed the same kind of distortion on the peaks of the sine wave as I have on my function generator I built with an XR2206. Might be just seeing things though. In any case, great work on your videos. And if you get a minute, I'd love to hear your input (read: see a video) about good grounding techniques (esp. in regards to audio electronics.) Thanks!
Let me check it up...am still trying to find out how to calculate input and output impedance for audio systems and then how to design circuitry for impedance matching
I'm just an electronics hobbyist and your videos have taught me a lot. If you had been doing these videos 30 years ago I'm sure I would have changed my major to electrical engineering rather than finance. When you retire please consider teaching engineering or math at the college level. You would have a great impact on young minds.
I totally agree
I think he's having a much larger impact here, on youtube, teaching at the "hobbyist level", rather than teaching at the college level. Inspirational. Technical details (unfortunately?) are needed at some point to do advanced things, and those require a formal (university level) training, which can be more boring.
I wish I could get all the other RUclips "instructors" to watch your videos. You are so polished. You could make a living instructing the instructors and I wish you would.
Alan, I often find myself going back to your videos to brush up on calculations I don’t use every day. It’s a great resource.
I remember watching this video years ago, and I was really in the very early stages of my electronics hobby. I vaguely understood it, but I thought to myself that hey, nice video, but let's be honest, I'll never need to know how to do this...
Well, I was wrong. Today I need to know how to measure output impedance, and I also could use a primer on transmission line termination. w2aew to the rescue!
Your drawings and practical examples have taught me more than anything else. Thank you, genuinely!
Glad to hear it! I hope you found the videos on transmission lines and terminations!
I like your videos as they teach the fundamentals with real world, hands-on, models. If you don't already, you would make a great teacher!
Maybe when I retire...
Still valuable and excellent, and it will be even in two decades from now on. A really outstanding quality in terms of clarity, accuracy and style. Can't thank you enough for your work!
I am glad you still have this video posted. The equations used are easy to derive once you think about the relations, but when electronics is a hobby and not a field of study it sure speeds up the process to see it laid out so nicely as in this video.
Love that step resistor box. I need to make or buy one. You explained this process much MUCH better than my "engineer" instructor. Thanks Allan
Alan, when I watch your vids, it's like learning all over again. I must admit, I feel that same giggly sensation that I felt when I first learned these principles many years ago. I've found that a huge number of beginning electronics students, hobbyist and on occasion, even some tech's I've spoken to, have had a hard time understanding this. You sir, have accomplished the perfect explanation of the very concept of output impedance and how to measure it in a simple straight forward way.... I'm always amazed by your teaching skills which seem so easy but in fact, are difficult in practice. God indeed blessed you with an outstanding teaching ability and a keen mind as well. I'm so glad that you share this talent with others. It will unlock a new world of understanding for some they never knew existed. Again, Kudos!!! Job Well Done!
Just dropped in to see what was happening. LOL! Had just used the resistance substitution method (5:06) to get output impedance of an amplifier. Thanks again, Alan.
Hi Alan, great video and tutorial to measure output impedance. Most of us also wants to know how to measure an unknown network impedance from 100kHz to 1GHz or higher but this one can only be achieved using a network analyzer. In my experience in Electronics, EMC and RF, and before I started my ECErcuit RUclips channel, I been working in understanding how to measure unknown network impedance using spectrum analyser and signal generator. In somehow, I have managed and successfully measured unknown network impedance accurately from 100kHz to at least 100MHz. Measurement was achieved by deriving a formula through a series of derivation and calculations. I automate the system by writing and application to control the spectrum analyser and signal generator to process the measurement against the calibration reference using the formula which gives you the result of impedance plot and data. I'm thinking of sharing this work on my RUclips channel but I'm not sure if someone is interested to know. Cheers, ECErcuit.
For the beginers What Alan does here is : He devide's 2.5V bij 1000 ohms (1K) of R load. That give the current flows in R load which result 0.0025 Amps (2.5 milli amp's) that will also be current flows in Ro (since current is equal everywhere in a close circuit). We also know that when R load is connected and current start passing it creates 1.5 Voltage drop on Ro 4Volts - 2.5 (V drop on R load that is because the sommof all voltage drops must be 4Volts ). So 1.5V voltage drop on Ro devided withthe current flow's in it will give the value of R out which is 600 Ω (ohms).
I watch these videos many times. Each tim I do I find something more. Thank you.
Exactly the tutorial I was looking for.
Explained very well.
Very nice presentation once again. A couple of observations: I did this on a breadboard with the output impedance of my awg set to 50 ohm. I used a small 100 ohm trim pot and adjusted the pot until Vpp on the oscilloscope measured the set voltage on the awg. I got 47 ohm. Not bad. It just seems to me that using a pot makes it much easier to fine tune the load resistance compared to a decade box. Secondly this method assumes that the circuit under test consists of linear circuit elements which can be modeled as a thevenin voltage and resistance. The failure of the first method (2:47) to work all the time may be an indication of this. Using a second load resistance measurement (6:54) may or may not satisfactorily adjust for this. Best way is to measure open circuit voltage (Vth), calculate voltage at several load resistance levels, calculate current V/R at each point and plot voltage vs current. (You can do a least squares approximation on the data if the line is not perfectly straight) Getting a straight line insures that you are actually dealing with linear circuit elements. As V (measured) = - Rth*I + Vth. The slope of this line is the negative of the output impedance (Rth).
Please note that the 50 ohm setting on your AWG is likely *NOT* setting the output impedance. The output impedance is *always* 50 ohms. This setting is telling the AWG what the *load impedance* is that you're connecting to. It uses this to properly set and display the voltage at the load.
@@w2aew Yes I know. When I say I set the output impedance I should have said the input impedance of the load. My Siglent SDX1032X has two options for load impedance 50 ohm and HiZ. When set at 50 ohm the open circuit voltage is twice the displayed voltage and when set at HiZ the open circuit voltage is equal to the displayed (set) voltage. The question I had is what is considered a high load impedance? If we desire less than a 1% discrepancy between the displayed and measured voltages (across the load) we have (at the Hiz setting) Vm (measured) = R*Vset/( R + 50 ). Where Vs is the set (displayed) voltage. If we require Vm to be >= 99% of Vset then R (load) must be greater than 4950 ohms. If, instead, can live with a 5% discrepancy then R must be greater than 950 ohms. In practice I actually tested this out and found that at a 700 ohm load the output voltage was within 5% of the displayed voltage and that at greater than 1750 ohms the output voltage was identical to the displayed voltage. So the question arises what if our actual load impedance is below 1750 ohm (for 1%) or less than 700 ohm (for 5%)? Well with the the awg set to HiZ we have Vm = Vset*R/( R + 50) we can turn this around (setting Vm to the desired voltage) and Vset = (1 + 50/R)*Vdesired. Eg lets say we have a 100 ohm load and desire 4 volts. We would then set the awg to
Vset = (1 + 50/100)*4 = 6 volts. OTOH if the awg were set to expect a 50 ohm load and we had a load resistance R other than 50 ohm (but less than 700) then the formula would be 1/2 of this Vset = (0.5 + 25/R)*Vdesired.
ok I have to say im hooked on your videos. You explain this stuff very clearly. Thank you for your time
Thank goodness for your videos! They help me understand my Measurements and Troubleshooting laboratories.
Great video! Very well put together, perfectly explained, and super useful information. Thanks for your work!!
very straightforward! You're a good teacher. Thanks! :D
Hello sir, can you make a video about how to measure the output impedance of RF amplifier circuit ? The methods in this video generally apply to relatively low frequency use cases, I want to know that is threre any approach to measure output impedance at RF frequencies , looking forward to your video, thanks a lot
Fantastic tutorial! I just discovered your channel.... now I gotta watch all your videos. Thanks....
Thanks. This is helpful. Best wishes from New Zealand.
Another EXCELLENT review!!
If you are still considering suggestions for tutorials, Have you considered FET's and in particular their utility in audio. I have been amazed how well FET's sound in preamps and low power amps. Being a voltage device apparently is a plus at audio freq's. My experience has shown them to rival many tubes I have played with such as 45, 2A3, 6L6 tubes. I have played with MANY transistors and opamps at audio freqs but they sound flat and dull compared to FETs...Many of the people I know are OK with transistor theory but don't understand FET's....Another OUTSTANDING audio device is the nuvistor for low level work like preamps...Microphonic as heck but once tamed, they are VERY nice not to mention fast and reasonably cheap..
And a great big OSCILLATOR to you too...dit dit
Occ-il-Later, you be a funny man. It's the imaginary component that trips me up when trying to figure out Impedance for RF
Hey, great video. I was wondering if you'd be willing to post the derivation for the second formula. I can't seem to set up that equation from scratch. Much appreciated if you find the time.
I always learn something from your videos. Great work. Thanks!
Man you rock! Great at explaining things. Very good teacher. Thanks!
Wow, this is great! Have you considered publishing all of your notepad explanations with links to videos (or a companion DVD...)? I think this would make a fantastic workbook for those of us without that elusive EE degree!
Do you have a PDF of all of your notes ? This would be great as either an e-book. Great videos.
Thank you for making this video.I just used it to measure the impedance of my flea market function generator.
Dear Alan,
I have RF magnitude and phase probe circuit with two toroids connected to 4 diode based peak detectors to give output voltage corresponding to magnitude and phase errors detected. I am somehow not sure how that voltage is mapped to magnitude and phase errors and in what ratio. This circuit is inside a 300W auto matching network which is used to do impedance matching. Load is a sputter source which is used to do sputtering. Generator is a 13.56 MHz, 300W unit. The circuit which I was talking about senses this magnitude and phase error and gives a feedback to a microcontroller which in turn rotates ganged series and shunt capacitor to match the impedance. I just want to understand the circuit better and make know how to make which motor move to get to 50 ohms.
I am missing some bits and parts due to which my understanding about this thing is not complete. Please help me understand this thing. Let me know if you want any circuit diagram from or anything else. Thanks !
Lowering load Z (or R) also shifts the HPF created by output DC blocking capacitor (if any) upwards, so, f must be high enough compared to the cut-off freq of that.
Just found your channel. Very interesting and informative. Also, I REALLY like that scope :)
what connector to input banana lead from resistor box which has T shape ? thank s
Hi, Thank you for your great education videos! Would you please explain how to measure the input impedance of a receiver ( typically with input sensitivity of -110dBm). I tried to do the job using a nano VNA and attenuators, but it seems the measurement is not precise since the nano VNA cannot work with very low level signals. It would be much appreciated if you share your rich knowledge. Thank you!
How about testing the output impedance of a ham radio in the HF range from 160 meters to 10 meters?
Very well explained. Is there a derivation for the first and last formula?
The first formula is simply re-arranging the typical voltage divider equation:
V1 = Vopen * (R1 / (R1+R2)), solving for R2. The second is starting with the ratio of two of these equations and solving for the source resistance.
***** That's perfect. Thank you!
The output impedance of the collector of a Darlington transistor is higher or lower compared to a bjt's collector output impedance? But I have heard is that a darlington's output impedance on The collector is much lower compared to a bjt's output and complaints on The collector but what is the advantage of the output impedance of a darlington's collector compared to a bjt's output impedance collector?
your videos are indispensable. thank you.
Thanks Alan for that video, it will definitely be usefull.
Correct me if I'm wrong, but with some manipulations there would be a way to determine an input impedance as well, right ?
I was thinking to perform the steps you described to evaluate the output impedance of an opamp circuit with 2 resistors, R1 and R2 as you did. Therefore once the opamp output impedance is known, measurements could be performed using the input of the circuit we want to know the input impedance in place of R2.
Having access to the opamp output impedance value, along with the V1, R1 and the newly measured V2, we could deduct the value of R2, which would normally be equivalent to the input impedance of the circuit being tested.
I understand that if either the opamp output or the tested circuit input input are significally inductive or capacitive, a close attention will need to be paid to the frequency, but generally speaking, I would be lead to believe that what I described could be achieved too.
Let me know if that make sense.
Keep up with the good work !
Perfect, thank you! This helps a diy guy a lot :)
When measure a heaphone output that can have something between 1 and 30 Ohms, will it fit when using 50 and 100 Ohm resistors and do the third type of measurement explained in the video?
Hi Alan. I'm building a CW transmitter. I have a buffer amplifier after the crystal oscillator. I tried measuring the output impedance of that stage using the method you described (with two load resistors). I calculated the output impedance . I then tried another pair of resistors (with different values) to see if I come up with the same output impedance. I did not. It was quite different. Could the output impedance change based on the load?
Can I use the second method (about 6:42 on the video) to check the output impedance on my HF transceiver, see if it is truly 50 ohms? Perhaps use a 50 ohm dummy load and measure with my scope then drop in a 100 ohm dummy load and do the same measurement?
Yes, that would work, provided your scope doesn't alter the impedance.
Gotta say it. You have brilliant Forest Mims style graf paper skillz.
I’ve got most of the principles of ohms law figured out but I’m working on mastering impedance which some people blow-off and as just being resistance. My interest, as an amp tech, is to develop a checklist of measurements across every amp I fix. I was watching this video on measuring output impedance and am curious to know why, when (@3:14) you switched your scope to 1 m-ohm of input impedance, you identified that as an open circuit (and I know an open circuit causes a voltage spike .. assuming because of the pent up potential). As resistance goes, doesn’t the fact that 1 m-ohm exceeds the 600 ohms of output impedance mean that it applies more resistance? Isn’t 1-m ohm a bigger load? Or, is it “open” because the BNC-T was left unterminated?
I said that 1Mohm was effectively open circuit because it is so much higher than the 600 ohm output impedance of the source that the difference in loading of the 1Mohm load vs. a true open circuit would be negligible. I higher resistance load is a "lighter" load - easier to drive, because it doesn't draw much current. Low resistance loads are "heavier" loads, much harder to drive because they draw a lot of current.
@@w2aew Oh, doh. Not even sure why I asked. So obvious now. Thank you.
OH GOD, finally got that equation. For a very long time, I am just tryin Rload backandforth to get 2Vloaded=Vunloaded, like dumb. Thank you so much w2aew!
I am aircraft technician and I need clarification on the following impedance subject related to ARINC 429 circuits. The transmitting source output impedance should be 75 W ± 5 W divided equally between Line A and Line B (line A and B are twisted pairs i.e 4 wires two for transmission and two for receive). This balanced output should closely match the impedance of the cable. The receiving sink must have an effective input impedance of 8k W minimums. …How is this physically measure?
None of those measurements are impedances.
Hello Alan, thank you for your great explanations. Is it possible to put a link with the schematics of your generator ?
I built that homemade generator over 25 years ago, and the schematic is lost unfortunately.
Great video. Love the HP-15C!
really awesome tutorial video for me. I have been wondered this issue for long time. Thanks.
Yet another excellent video. Can you use the same method to measure the input impedance of lets say a common emitter amplifier or any other type of electronic circuit?
Nice and straightforward.
Mind doing a video on building V/I Curve Tracer?
Hi
I have a question regarding conductance, can you measure conductance of a let say car battery ,this by a scope?
I sort of measure the resistance and this in certain time peiods i think then in days or week or month so see how the resistance increases in time
I think of sending a certain freq on one side collect it on the other side then calc the time this must increase when resistance increases or not?
I have a TC Helicon Harmonizer pedal. MIc in, through the effect then back out to mic in amp etc.
It works as expected on many different amps, but does not work well with my Fender Acoustic Jr GO.
The amp 100W and with the TC pedal, out put volume is lower than an plain acoustic guitar with no amplification. It is not my amp it is an incompatibility with this model of Fender, tried 4 amps.
I am suspecting an impedance mis match. TC is 400 ohm out. I have no input data for the Fender but both channels accept XLR for mic or 1/4 unbalanced for a guitar.
Anybody's thoughts on what might being going on, and also ways to test/measure or add something between the pedal and amp mic input greatly appreciated. Cheers
Great video, sir. But how to measure the output impedance of an RF signal generator with a Directional Coupler and a Spectrum Analyzer?
Increase the load until you see a 6 dB power drop. That's the output impedance.
Thanks for all the videos!
so ...how does a working circuit with many parts come out to be an of 50/6oo ohms? do they just place a 50/6oo ohms in parallel with entire device?.....taking into account the parallel total impedance...?.
Usually the output driver is low impedance, and a resistor is added in series to get to 50 ohms.
When looking at the first stage/front end of an amplifier circuit, how can you tell if the first stage/front end is High impedance or low impedance? how can you tell how "sensitive' the front end will be to a low impedance input signal?
By examination of the circuit you should be able to approximate the input impedance. For the most part, if the amplifier isn't designed for RF use, the input impedance will most likely be relatively high.
@@w2aew If the audio amplifier doesn't have any resistor to ground its just only a decoupling cap connected to Q1 and transistor#2/Q2 feedback resistor is biasing the base of Q1 would this be considered an input that is low impedance or high impedance? Because mostly you put a 1meg resistor to ground on the input jack with the decoupling cap in series to the base of Q1 transistor. The 1meg resistor to ground is mostly what determines the High impedance. But if you don't have any resistor to ground the biasing resistors R1 and R2 set the bias voltage for the base of the Q1 transistor. I don't think R1 and R2 determine the input impedance they only set the biasing voltage of the base. But other amplifier configurations omit R1 and R2 and just use a feedback resistor from Q2/transistor#2 to set the biasing voltage for the base voltage of Q1. The input impedance gets really sensitive to low impedance.
im just a aermodellling hobbyist, right now i use the video transmitter with 5.8ghz freq , i need to know is this method can be used to measure the output impedance for my video trasmitter
Probably not, since most hobbyists don't have a way to measure the amplitude of a 5.8GHz signal. You can safely assume that is is likely 50-75 ohms as most RF outputs are.
Can you use an ohmmeter to measure the output resistance of an opamp straight away? Are the probes across vin1 and vout or vin2 and vout of an opamp?
No, you can't. The output impedance is determined by the active device design and can't be measured with a simple resistance measurements. Note that op amp output impedance is typically *very* low.
U have a great understanding of how circuits work and how to compare two or more method's of measuring things. Could u show us the sinad and signal to noise methods of measuring receiver sensitivity and any trick or tips u know of how and what to use to make these measurements with equipment on a budget. For example I can remember reading of a method of using an analogue multi tester to monitor carrier level. Thanks Alan
Allen, is there a way to calculate the feed point impedance of an Antenna? Like an EFHW uses a 49:1 impedance transformation. How do one come to a conclusion that for HF bands the feed point impedance of an EFHW is around 2500 Ohms and required a 49:1 impedance transformer? I couldn't find a satisfying reply anywhere.
That's exactly what an antenna analyzer does.
nice video can we use true RMS DMM without peak measure capability instead of oscilloscope
Hi Allen
I just watched your video on How to Measure Output Impedance.
I wanted to measure the source impedance of a SI5351. When I use the 1/2 voltage method I get 49 ohms, very close.
Next I tried the 2 resistor method and it only works out to be 1/2 of what I was expecting, 24 ohms. My R1 = 330 and R2= 814.
V1=3.22 and V2=3.36. I'l double check my measurements and calculation.
With this method should one of the resistors be close to 50 ohms?
73 Ken VA3ABN
Hi, Alan
I guess my question is about the differences (if any) between… 'Output Impedance’ vs. 'Internal Resistance’, ... or ( 'Source Impedance' *).
Are each of these terms completely synonymous? or is there enough of a difference between them where they are all needed in order to properly describe slightly different things at work.
---
In the class I’m taking we did an experiment to determine the INTERNAL RESISTANCE (R_int) of a voltage supply.
The 'SPECIAL CASE’ section of your video here basically covers the same process and formula we used… with the minor difference that… V-open / 2 = R_int (in our experiment).
In the experiment we did I used a potentiometer as the R_load,… and adjusted it until V_load = V_open/2.
After which, the POT was measured to determine it’s resistor value… and we approximated that to being (R_int).
[ ... we ultimately did this for a variety of R_load values… and calculated the Power Differences as R_load values shifted away from matching what R_int is; … and more calculation were done to determine dB gains and losses-relative to the values of the matched pair ( R_int = R_load ). ]
I guess In hindsight… we were doing a little more than just finding out what R_int was… it was also about maximizing power transfer, …and maybe starting to flirt with impedance matching too? though, I’m still not 100% solid on understanding everything that impedance is. I have been reading about it (a lot)… but still need more real experience with it, and intuitive understanding… in terms of circuit building, and more hands on testing.
Thanks for the Great Channel, Alan… I really LOVE IT. and it’s helping me a lot in my attempts to learn about electronics.
Take Care,
Jim
* My mention of the 'Source Impedance' comes from watching your impedance matching videos, which I'm just now starting to look at.
.
In general, the "internal resistance" and "output impedance" in this context are basically the same thing.
Do you have a video about what impedance has to do with measuring tube radio circuits, like why you use a VTVM meter instead of a multi meter, when checking components in the circuits..
I don't have a video, but it is a relatively simple topic to explain. Many tube circuits and nets are very high impedance. This is because the plate output impedance is quite high, and the grid is a very high (near open circuit) impedance. If you use a multimeter to measure these circuits, the input impedance of the multimeter will often be much lower than the circuit node impedance, and thus will alter the voltage when you connect it. A common multimeter impedance is 20,000 ohms per volt, which means that if you have the meter set to a 50V scale, its input impedance is 50*20000, or 1Mohm. At lower voltage ranges, the input impedance is much lower. A typical VTVM or modern DMM has an input impedance of 10Mohms, thus will load (and alter) the circuit to a much lower extent.
@@w2aew So a mulitmeter will drop the readings, or change the circuit more than a VTVM, which will do what when it drops? I guess I dont understand what impedance is or does..
@@kennynvake4hve584 Impedance is just a fancy word for resistance. Actually, it refers to the resistance + reactance - more completely called complex impedance. But for the purposes of this discussion, let's just say that impedance = resistance. Each circuit node can be "modeled" as a voltage source with a series resistor (Kirchoff's Law). Thus, the voltage at each node can be changed by drawing current from it (with a load of some sort). Some circuit nodes have a very low impedance (resistance), meaning that the voltage stays fairly constant even when a load is applied. A power supply node or supply rail in circuit is an example of a low-impedance node. The voltage remains constant even when drawing current from it. Other nodes in a circuit can have much higher impedance (resistance), which means that the voltage can change if you draw some current from it while measuring it. The circuitry connected to the Grid of a vacuum tube will often be high impedance (resistance). So, if you connect a multimeter or VOM to it, it may change the voltage at that node. Consider the following example: A circuit node that is operating at 3V has a 100Kohm impedance - meaning that it "looks like" a 3V voltage source with a 100Kohm resistor in series. If you use a multimeter with a 20,000 ohms/volt rating and set it to a 5V scale, the meter will have a input resistance of 5*20,000, or 100Kohms. If you connect this meter between our test node and ground, the circuit's resistance and the meter resistance will form a voltage divider, and the voltage on that node will drop to 2.5V. It is the same as putting a 100Kohm resistor between that node and ground. You can see from this example that a VTVM with a 10Mohm input resistance will have much less of an effect:
5V * (1M/(1M+100K)) = 4.55V
A wonderful collection of videos.
I could listen to your voice all day...
but can you help me with a problem on home made swr meters?
I have constructed many types using single or double pcb's....
The forward measurement is spot on but the reflected is always higher.
I'm thinking, if the impedance of the stripline is out, that's my basic error
to be added to the real error on the test load....
SO. how do I measure the impedance of any swr meter?
Hi sometimes our signal is very high frequency like ghz and we can't measure amplitude by an oscilloscope or even an spectrum analyzer how we can measure output impedance in this case?
A four port VNA is typically used to measure the small-signal output impedance in this case. If you are talking about large signal or high power outputs, then often a load-pull analyzer ($$$) is used.
@@w2aew everything can be done with $$$$ 😁
ok, so now I can measure output impedance of the local oscillator and than match it with the diode mixer🙂 If I understand you correctly we can measure input impedance with the same way?
This shouldn't be necessary because the LO input of a diode mixer is non-linear (commutating diodes), and thus is not impedance matched (or realistically matchable).
@@w2aew thank you! sorry for the stupid question, because I am just a beginner)
@@Avrora707 No worries - it is not a stupid question. A lot of people don't realize that the LO of a diode ring mixer isn't a nice 50 ohm load.
How do you measure the output impedance of high frequency devices, for example a crystal oscillator (if it makes sense) or an amplifier?
As usual, thanks for your videos!
Hi very nice video! But is there a similar way to mesure INPUT impedance of receiver?
This video shows one method to measure complex input impedance:
ruclips.net/video/eYN7dhdt1Dw/видео.html
...or you can use an inexpensive analyzer like the NanoVNA:
ruclips.net/video/Sb3q8f0NBZc/видео.html
ruclips.net/video/xa6dqx9udcg/видео.html
is this a good response of a signal gen? cause ideally i think it should not be affected when loaded. the feedback circuit should take care of maintaining the amplitude
unless i could design my output impedance to be relatively small so that it would be negligible in the voltage divider
High frequency signal generators typically have a 50 ohm output impedance. If you use feedback to set the amplitude regardless of load, then the effective output impedance is then lower than 50 ohms, thus giving the generator a poorer output return loss.
Great video. Thanks for sharing your knowledge.
When converting high impedance to lower impedance the signal drops down 20dB or more. This means that high impedance audio signals has more current drive, power, energy compared to low impedance audio signals? because what is the difference between an audio signal that is high impedance audio signal compared to a low impedance audio signal? it must have to do with the signals energy and power?
Just the opposite! High impedance sources can't deliver as much current or power as low impedance sources.
@@w2aew But when concerting from High impedance audio signal to a low impedance audio signal its a 20dB loss from input to output, any reasons why? The high impedance audio signal is 20dB lower when converting it to a low impedance audio signal. If low impedance has more current and more power it should be +20dB not -20dB
@@waynegram8907 Makes perfect sense. It's a simple resistive voltage divider. A high impedance source effectively has a high resistance in series with the output. A low impedance input looks like a low value resistor. So, you have a resistive voltage divider which makes a lower voltage appear at across the low impedance load.
Than you for thus video. So, in order to measure the output impedance of my sound card must I use the 1 Kohm resistor and the formula you sent as? The measure woudn t be correct if
I only use my multi-meter Omhs parameter? Thank yuo so much.
You can't use an ohmmeter to measure output impedance because it depends on the active properties of the circuit. The ohmmeter *only* works on unpowered devices.
Nicely done, as always.
Just out of curiosity - What if the R_load that I am interested in is in the order of a few megaohms? You used 1 megaohm for V_open but what if R_load is higher than that?
That gets tricky to measure since most voltage measuring equipment will have 1Mohm (scope), or 10Mohm (10x scope probe or DMM) input impedance.
@@w2aew I am currently looking at driving p-n diodes in reverse bias using a sine wave generator at 200MHz. The diodes have input impedance of several megaohms (2-4 as far as I know). Given the frequency I am looking at and the method you showed, would it be safe to use the calculated output impedance of the sine wave generator, as explained in the video, to find the actual voltage across the diodes? Or does frequency play a role here?
@@Terrar-fr1bk The reverse biased impedance of the PIN diodes is so much higher than the generator, it can be considered an open circuit. However, you may need to consider the PIN diode's capacitance under reverse bias and what it's capacitive reactance is at 200MHz.
Excellent! Thanks for posting this !
Ok, great explanation! But, I would like to find out the output impedance of my signal gerenator, and not the resistance. Because, the open-circuit output voltage of it changes with frequency. Please, how would I do this?
It is *very* likely that the change in output amplitude is *not* due to the presence of reactance in your output impedance - it likely has more to do with the signal generator circuit itself or the measurement method.
Ok, thanks a lot w2aew! I was thinking that I could get some kind of a model of the signal generator's output. Maybe, with an impedance like in a transmission line. But, I don't know if that is possible. Anyway, thank you so much for your attention!
Hi, If I want measure output impedance in RF circuit example in my "colpitts oscillator". I can measure how you explained?
not really. As mentioned early in the video, we use a low frequency to avoid transmission line, termination, etc. issues that occur at higher frequencies.
w2aew , how do you measure output impedance in an RF circuit? For a small signal I could use a test load and calculate what the value is, but what about a high power amplifier like for ham radio?
Awesome!
Please tell us how to measure inductance at high freq. When LCR meter only measures up to 1khz?
Did you see my video on how to measure inductance with a scope and some basic parts?
Measure Capacitors and Inductors with an Oscilloscope and some basic parts
Another very good video! Thanks very much Alan!
nice video :) will this method work with square wave generator say from 555 timer or we can only measure the impedance of sine wave generator using this method
It should work well with a square wave output too. Just keep in mind that many logic circuits have a different output impedance for the high state and low state.
Question, on Leader LFG 1300S generator requires a 50 ohm cable. can you explain this to me? Does it mean that different generators require different cables for testing?
Thanks
The vast majority of coaxial cables used with test and measurement equipment are 50 ohm cables. This means that the coaxial cable has a 50 ohm characteristic impedance. For low frequency signals, this is unimportant. But for high frequency signals (above about 1MHz or so), or for signals with fast rising/falling edges (faster than 40-50ns), the 50 ohm cables and proper termination begins to be important. You may want to review this video on transmission lines and terminations:
ruclips.net/video/g_jxh0Qe_FY/видео.html
I would like to make my own bnc cables for testing. What cable ? brand and number? and what is the maximum length ?
Thanks
Good quality RG-58/U such as Belden 8262 would be a fine choice. Maximum length depends on your frequency of operation, it all depends on the amount of loss you're willing to tolerate. Hundreds of feet are often used at frequencies below 100MHz.
Hello Alan, my other question is when you have multiple test equipment such as an oscilloscope, frequency counter, signal generator, and they all have different impedance output such as 50 ohms, 75 ohms, 600 ohms or 1 meg ohms. What coax cable is appropriate when connecting the test equipment ???
There should only be one termination on the line. Other devices should be in high impedance mode. The cable should match the source and terminating resistance. So signal generator is 50 ohm source, freq counter is high impedance connected to a tee, o scope at other end terminated in 50 ohms, using 50 ohm coax between each.
Some active devices instead of running in a resistive mode, run in a constant current mode. For example in a high negative feedback amplifier, the output voltage changes very little with a changing load, so the impedance appears to be very low and the inverse of the impedance called damping factor is used. A common high performance amp with a damping factor of 30 would have an apparent impedance of 1/30th of an ohm or 33 miliohms. However protection first limits current to about 10A(varies) and then the output protection relay or other shutdown trips. Now at 60 volts at 10 Amps, the limit is about 600Watts peak into 6 ohms. Protection kicks in before the output can be dropped to 1/2 voltage.
Just out of curiosity, what is this signal generator built upon? 20-30 years ago, i'm guesing it's opamps? or is it discrete?
If I recall correctly, it started off as a kit based on the EXAR XR2206CP monolythic function generator IC - followed up by an op amp based output buffer that i designed to give more flexibility in output amplitude, polarity and offset.
@@w2aew Alright, thanks for a response! Suprisingly, It looks like XR2206CP is still being made. I'm asking because, i'm thinking of making a generator myself, although i have to say that i find my smartphone to be quite good at it for testing the average audio stuff.
can you do a video on impedance matching??
+GOWTHAM KRISH (Gowtham Shanmugaraj) Big topic - it is on my list - hopefully I'll get to it later this year.
***** I'm waiting
can i use the same process for measuring input impedances making my test circuit as the load?
that's one way to do it.
So why did you use a sine wave? Cannot we just use a DC output?
Many times you may be wanting to measure the output impedance of a generator, amplifier, circuit, etc. that is AC coupled, and thus a DC signal won't work.
Are able to provide the schematic for that wave generator?
I built that generator over 30 yrs ago, the schematic is long lost...
@@w2aew thank you for the reply. Most importantly thank you for sharing all your knowledge. Hope to shake your hand some day. Have a good one.
will it work with the input of a spectrum analyzer?
This technique is used for output resistance measurements, not input impedance.
@@w2aew DUH, never mind. my bad
Thank you. The length was for just bench work mostly.
What is the brand and model of your oscilloscope in this video? Thanks
It is a Tektronix 2465
Hi Alan, for the second case, please give an example of when not working with no load.
One example is when an unloaded output causes the output to saturate or hit a limiting condition prior to reaching the theoretical open circuit voltage.
***** Could you give an unequivocal example of what you say is very abstract
A simple example would be a current source output. If open circuited, the output will saturate before it reaches the Thevenin equivalent open circuit voltage.
***** Got it thanks !!!
***** If I understand you had in mind such a scheme?
screenshot.su/show.php?img=0e6dc6caf1938140fb7836ffb0b6b617.jpg
Yet please tell me to what maximum frequency does this method work?
It depends a lot on the circuit. As the frequency goes up, the output impedance can start to become reactive vs purely resistive, and this simple method doesn't work anymore.
***** Got it thanks !!!
Great video. Subscribed. Just wondering what you used for your funct generator.
This was a home-made function generator that i designed and built 25 years ago. Unfortunately, I have lost the documentation of the design, so I'd have to do some reverse engineering to recover the design.
***** Don't sweat it. Just thought I noticed the same kind of distortion on the peaks of the sine wave as I have on my function generator I built with an XR2206. Might be just seeing things though. In any case, great work on your videos. And if you get a minute, I'd love to hear your input (read: see a video) about good grounding techniques (esp. in regards to audio electronics.) Thanks!
Where do I learn the concepts like reflection
This video might help: ruclips.net/video/g_jxh0Qe_FY/видео.html
Let me check it up...am still trying to find out how to calculate input and output impedance for audio systems and then how to design circuitry for impedance matching