HTML-код
- Опубликовано: 6 окт 2024
- This video describes a simple RF demodulator / detector probe that you can use with your DMM or oscilloscope to measure the relative amplitude of an RF signal in your circuit. It is very useful for tuning or adjusting IF stages during alignment, filter responses, peaking RF circuits, and examining amplitude modulation characteristics. The video describes the circuit and its operation, and demonstrates how to use it and the typical results you'll achieve. It uses a pair of 1N34A Germanium diodes, but could also be built with other low capacitance, low bias diodes. Notes from the video can be found here:
www.qsl.net/w/w...
When I was a boy, I saw a magazine article about modifying virtually any common portable 9V transistor radio into a signal tracer. It involved cutting off the tuned part of the board and feeding a signal directly to the high gain amplification section. The design included a switch to select between a simple capacitor input and a diode detector for demodulating a signal. I used a cheap walkie talkie instead of an AM radio. I had more fun with that device than just about anything else at the time. I put a little stereo jack on it so I could easily switch between a ground clip and probe or a pair of alligator clips. The amount of gain was incredible. I assume it was on the order of 80 or 90dB of gain. I didn't have scope, so this and homemade logic proves were about as advanced as it got for me. I did have a VOM. Those tools, magazines and a mid 70s ARRL handbook got me started. The RF and AF sections are nicely sperated on most of those old radios because of the way they had to build the board to allow the speaker voicecoil to poke through it. There's usually three traces on a narrow section of the board that wraps around the speaker. Power, ground and a signal line.
I have built several with a single 1N34, but this one works much better. It's better than the old Eico probe I was using. Thanks
Amazing!! I'm attempting learn electronics and I am at the stage now I need this probe. Looks like m first build project. Thanks.
i really like to learn from you rather than any other person in this RUclips world , so please learn me more about the RF world.
thank you so much
Nice circuit and awesome construction!
I just built this RF Probe and it works really well, thank you.
Great video. For a second I thought you were gonna show standing waves on the microstrip.
Awesome, will be making one of these, many thanks.
Just saw this video. I think I will build one for the shack. Having a problem with a Yaesu FT-747GX and need to measure the pre-driver output RF voltage. Found an RF Voltmeter schematic online and built that but it sucks LOL. At least I know you tested your design. Thanks for posting this. 73s. Kb8NIV.
The roll-off would be better defined and the sensitivity preserved by NOT putting a 1nF cap to ground but inserting a 47K resistor between diode and coax cable to scope or meter. The 1 megohm input resistance (DC coupled) of the 'scope is your DC path to ground. When using a meter it should have a 1 megohm resistor across its input. The coax cable capacitance and 'scope input time constant capacitance is sufficient to remove RF cycles from the recovered envelope - plus you have best video response for sweep aligning.
That's a cool idea - using the 100pF or so of cable+scope input capacitance as the storage cap - I'll have to give that a whirl...
Thanks for your answers to my questions and the efforts you put into the videos. Very educational. The 1.8V RMS you measured corresponds to about 5Vpp which as you stated is because the circuit sees the charging of the cap from you peak to peak detector. My first attempt at building the circuit worked but 2 things: 1.) As I changed frequency I saw a much large attenuation of the signal. My connection lengths were much larger compared to yours. You kept all leads very tight and short. Pretty sure my caps were 1nF. 2.) After working for a while, it stopped working. Both 1N34A diodes were found open. Maybe too much current? Voltage spec is very large.
Question: If the 50 ohm transmission line is exactly 50 ohm then the scope output should be exactly equal to the RF source value correct. If not exactly 50 ohms, the scope signal observed will differ from the RF source correct?
nicely done!
Excellent video thanks.
Nice simple probe. Going to make one.
This is so cool! Definitely going to make one.
Just made this - not as pretty as yours, but works perfectly. Thanks :)
Great circuit and give sign of power if some calculation made
Not sure if anyone has ever said this, but your handwriting and circuit drawing style is almost identical to Forrest Mims! I had to look up your callsign to make sure you weren't him :)
Great video
Very handy design,clever idea this rf probe.
Thanks...
I'm planning on making an UHF bug mic... And I don't have good gear... (I only have an 10Mhz analog scope circa 1970 or older; no spectrum analyzer, no func gen (I use a soundcard for this); no lcr meter and and 2 mid-class DMMs)... So I would love to make myself this tool but only if it's usefull up to ~450Mhz... so... what should be improved to have this working up to those freqs ? I imagine that the answer could be "a really fancy fast schotky diode for the attack... and an accumulator cap small enough to avoid the RF passing through it and being dumped to ground... something that I think should work fine if I increase the discharge resistor because I'm not interested in the AM demodulation feature... only in the detect/Vpp meassurement feature ... I'm not an EE; just a young amateur so feel free to let me know if I'm talking nonsense
yes, fast schottky would work well, and caps in the tens or hundreds of pF would be a good place to start. Include a series resistor between the last cap and the DMM/scope to isolate the capacitance from the demod probe.
Fantastic videos! I am learning a lot through your channel :) A quick (and maybe silly) question, at 7:00 in this video, what is the reason for using the RF detector probe you built (blue trace), when you can see the RF envelope on the Oscilloscope (yellow trace) anyway? Is the probe more useful because is it ONLY providing the envelope shape, and not all of the complex wave components that make up that envelope shape in the yellow trace? Is that correct?
There can be a number of reasons. In addition to what you've suggested, it is also possible that your scope, meter, etc. does not have the bandwidth to see the RF signal directly.
@@w2aew thanks for clarifying that. Makes perfect sense 👍 73
So many old transistors and diodes have been replaced by newer better items but the 1N34A is an exception
I built something like this, its much more accurate with a short ground, just use two pins next to each other, your sample probe pin and a ground pin just as short and touch then to the ckt at the same time will give good response up to mid hundeds of Mhz.
Good project. I have a few 1N34As around the shack, but need to find a source for some more I guess.
Here's one place you can find some:
www.danssmallpartsandkits.net/
Fantastic and educational vdo tutorial.I need to troubleshoot a vhf 144mhz transmiter, will this probe be sufficient to check for a faulty final stage? There is a 4 pin PA mosfet module for a drive stage . I'd like to to check that, then the final transistor's base and the collector to see if maybe this transistor needs a replacement. Either part is damaged ,so I am looking for a tool to locate where the rf generation stops, and point me to a faulty part. Thank you in advance.
It should give you a good relative indication.
For a more accurate probe you can use a high speed OoAmp and dual-diode Schottky in a "ideal diode" circuit config. Another advantage is much higher input impedance and thus reduced loading on the signal source.
Yes, of course you'll need a very fast op amp to be able to measure VHF, UHF and above, and the probe will require power.
Even with a fast op amp such as an LT1222, these are very difficult to get good linearity in the super diode configuration. From personal building experience, and not Spice simulations, what works far better for low levels is using the op amp as a DC log amp with a diode or BJT junction in the feedback, thus keeping the amplification on the more manageable DC side of the probe circuit. I have not regarded this worth the trouble since just a few miilivolts RMS can be read on a DMM without amplification, and a correction chart or table can be made for the non linear portion of the response curve. I have found this to be more than sufficiently sensitive and accurate.
I have also built single detector diode RMS output probes with a forward biased diode linearization network on the DC side, with trimpots for calibration (similar to the Fluke 85RF probe design), and achieved excellent linearity from about 200mVrms to 20Vrms and an essentially flat frequency response from 100kHz to 30MHz. (Not tested beyond those parameters.)
ZlayaCo6aka I
How exactly did you design and make the 50-ohm microstrip? Might be a cool thing to show in a video.
There are several online calculators that you can use to compute the dimensions for the conductor and dielectric thickness to get 50 ohms. In this case, I have a bag of pre-etched strips that are the correct width for 50 ohm microstrip when glued down to a ground plane.
Yes would be interesting to do some transmission line stuff and how you built the strips
@@w2aew I'm sure this is a silly question but here I go... The 50 ohm strip..... with nothing hooked to it could you measure the resistance with a ohm meter and would you have to measure from the farthest points on the strip? Thank you.
@@MoHntr You can't measure the impedance of a transmission line with an ohmmeter - it will just look like a near zero ohm conductor.
Thanks....A definite item to build for my shack.
How would one go about testing the 1st RF signal in a 2meter transceiver ? Got one with a very weak front end. I think the 1st RF is the problem area.
Thanks again for the probe details.
Very good info as ever , I do need to build one i have some BAT85 diodes that might work , I am setting up a TS-430s and it asks for a RF VTVM so this might get around that ,
If all you need to do is "peak" an RF signal, then this will work fine. If you need to make a specific amplitude measurement, then you'll need to use some test signals to try to establish some calibration curve of the detected amplitude vs. the actual RF amplitude. In most cases, all you may need is to see the modulation shape, or peak the response of a signal. Any of these "relative" measurements will work fine with this detector probe.
Thanks for sharing
Thank you!
Is this probe a contact style ? I built your RF sampler for an O-scope instead of buying attenuators , cheaper that way , I put it in a plastic project box 4" x 3" x 1-5/8 looks good , I used 2 PL-259's for the main in and out , and 2 BNC for the outputs , all amphenol of course but I haven't tried it yet , kinda waiting on my new bench I'm gonna build real soon.... I have room for my scope on my current bench , but I can't keep it there all the time ......
Your schematic shows 5 arrows indicating "ground" but you stated you "split" the ground on the circuit board in two places.
So my question is do all 5 of these "arrows" go to a common ground point?
All of the grounds in the schematic go to a common ground point. The "splits" in the copper are to create the other isolated nodes (input pin and diode connections).
Thank You Alan for your quick reply, I appreciate it!
Good information .
Thanks for the Video.
Pretty neat thanks
I have a dumb question. If the germanium diodes have a 0.2-0.4v minimum "conducting" voltage, then how do they detect/rectify such low volt signals you refer to. That made me start thinking about how crystal radios work when the germ is able to detect such low signal levels. I'm missing something important here obviously.
The "turn-on" voltage isn't a binary thing. The I-V curve is exponential, there is conductions below 0.2V, it's just much lower. The "hard limit" of "off" and "on" is a simplification to make circuit analysis easier.
Ahh- that makes sense. My computer background is clouding my reasoning- I was in fact thinking of it as the 0/1 thing! Thanks. Great videos. Keep them coming. My brain needs them.
Could you use something like this to measure rf on the outside of a coax feeder to determine how effective a choke balun was ? If not can you give any direction to what could be used.
Many thanks Paul
+Paul Fleming Take a look at near field probes....
Love all your videos - qualified and instructive! Exactly how can one use a diode detector circuit similar to yours with a DMM to get the most accurate rf power verification of a 50 ohm signal generator? Something of a poor man's calibration reference. I saw a comment on the TinySA group that one can do this with a schottky diode and a few resistors. Then calculate from resulting DC voltage with an accurate DMM. 1.414 times rms voltage minus 0.4v forward drop across diode. Wondering the setup and limitations of this concept.
The response from a simple diode detector like this will be somewhat non-linear with power, and will vary with frequency. So, you'd have to develop a chart or graph to use with it - which would involve the use of some calibrated RF measurements. At RF frequencies, various parasitics will keep it from responding with the ideal response.
@@w2aew OK, makes sense. I have plenty of typical non-calibrated equipment and signal sources. In this scenario how would you reach best measurement only one signal point - 30MHz, level of -25dBm? (This just happens to be the calibration output of the TinySA device, with plenty of users looking for an answer to this!)
@@donb3967 Really not possible to achieve a decent calibration over frequency and power level with a single point calibration. The response will be non-linear vs. power. Also, take caution with the TinySA "calibration" output, I would bet dollars-to-donuts that the output is a square wave, so the *total* output power is going to be different than the "calibration power" at the fundamental, which would adversely affect your (broadband) detected power result.
Thank you Sir for your splendid explanation. I had a doubt to ask if you wouldn't mind. I wanted to know whether there is a particular relationship between the range of frequencies and the Resistance and Capacitance values you have made use of in the circuit. Eagerly waiting for your response.
Not really super critical. As I go higher in frequency, I would likely go a little lower in capacitance.
@@w2aew Ok, got it. Thanks.
I love your videos. For someone like me who passed the technician, & general in one night, and missed the extra class by 2 questions, & went back & got it the next testing session. I need all the help I can get! I need to go take the VE exam. That club is getting really geriatric, and they need some young(er) blood. I have the thing that looks kind of like a defibrulator, only it has giant scope probes, instead of paddles and a 100,000:1 transformer in it, so it works with my linear amps. It's a nice diff trans probe set. I have a 100:1 probes that will do 600 V P-P. Those have been in my kit forever. I do guitar, bass guitar, keyboard, and harmonica amps, as well as pedal steel, electric fiddle, etc. The latter keep you thinking. 73 KI7AQJ
Hi w2aew, it looks like you used RG178 (50 ohm) to connect the probe to your 'scope. If you were to build an active RF probe requiring power, could you use a balanced shielded audio cable to connect the probe to a DMM or oscilloscope so that shield was negative for the power source and probe end, red was the positive voltage and white was the signal? Or, would the balanced audio cable have the wrong impedance, i.e. 50 ohms, so therefore it would be advisable to use RG178 plus an additional wire for the positive side of the power supply to power the active probe? The reason I ask is I have seen some active probes that demodulate RF signals from the DIT and play them through audio amplifiers. I presume that in this case impedance is of no importance since the audio from the probed signal is the only part of the signal "filtered" off to an audio amp, not a 'scope where impedance matching is important?
For the low frequency output of your probe, the audio cable should work.
When you made your first DMM measurements you had a RF input of 1V and you measured 1.8V. Can you explain the reasons behind the differences. Sounds like you measurement was in RMS. What units are your RF source in (PPV, RMS)? If everything was ideal how would an ideal measurements differ from 1.8V? I did understand as you went up in frequency this value went down, but given your circuit this is still a usable tool. I think your point was there is some attenuation but given the simplicity of the circuit still a decent response from the probe.
The answer is given earlier in the video where I describe how the circuit works. I was measuring a 1Vrms signal, which is about 2.828Vpp. The circuit behaves somewhat like a peak-peak detector, with some losses due to the diode voltage drops, so it gives about 1.8V response.
Hi Alan thanks for sharing. Could I use 500pf and 1500pf caps instead of the 1nf you use? I guess that changes the frequency response, Am I right?.
@@tonyfremont Thanks tony!
Thanks for the video. Will it work with FM also?
It will not demodulate FM.
I know this video is from a while ago, but maybe you can answer a question I have. I was given a LUNAR Industries inc. PAG 818 and it looks like a power detector. I have looked
everywhere on the web for information on this unit, but came up empty. I was wondering if you've every heard of this company or product? I would love to know the specs on it, so I can put it to work. Thanks and thank you for all the great videos. 73 N9JOD
Sorry - never heard of that company so I have no further info to share.
Another great video clip and circuit.
I guess you used clamper circuit, to raise the voltage level followed by a diode detector. Great design, in my opinion.
I copied the design and posted it in my circuit book.
I always had this question that if we mix a frequency with itself, say using ring diode mixer, or Gilbert cell, or any other non linear device, can we detect its level at DC? F1-F1=0 , which is DC.
I know in the direct conversion receivers, or regenerative receivers they do something like that, but they do not want to detect signal level rather the modulation signal.
What is these same frequency signals are out of phase, do get any information about the phase difference?
Also, I am thinking of designing high frequency high impedance differential probe for low voltages 1uV to 1V , if you have any reference let me know.
This called homodyne - and the DC output will vary with the phase. So, unless the phases are locked, you can't detect power that way.
@@w2aew Thanks,
So then if we feed RF and LO of a mixer from the same source, then the phase will be locked since the only difference is the path, so we should be able to detect the power.
I guess the only way to find out is to built the circuit and try it. I will use MC1496 to investigate this concept.
Shouldn't it be in a shielded enclosure?
Yes, ideally you'd house it in a shielded enclosure. I built it "open" so I could show the details of the circuit construction.
I really like the probe you designed and made. My question is, can it act as a Signal Tracer if instead of going to the scope I connect it to an audio amplifier and if so would I be able to sig. trace hearing the radio station all the way to the front end converter tube of my AM radio.
Yes, see my other post on this thread about modifying an old 9v transistor radio into an awesome signal tracer.
Alan the 1N34A is rated at a PIV of 65 Volts. The 1N270 & 1N4148 are rated at a PIV of 100 volts. What would you set up for higher voltage applications, say more than 50 volts and less than 500 volts ? Would you string diodes together in series or set up something like a capacitive voltage divider ?
I think the 1N270 would be better than the 4148. However, as you suggest, I'd probably attenuate the signal with a resistive or capacitive divider as my first choice for higher power operation.
Very well done, sir!! Just one question: Can I use the 1N60 instead the 1N34A?
Yes, you can
I built this circuit a feed it with a 1 mhz unmodulated signal. The output is zero. I am using 1n4148.
Hi, i built the probe using two OA90 germanium diodes. With a sine input 0 to 4V centered on 2V i found that voltage value is (Vpeak - 2 Vd) / sqrt(2) where Vd is 0.3.
There is a thing i don't understand.. connecting the multimeter negative to ground and positive to diode as shown in your schematic i found a negative value. The absolute value is correct but why the sign is negative ??
The only explanation that I can come up with is that the diodes were installed backwards.
***** Hi, i saw at 3:26 of video that black sign of upper diode is connected to groud clip. Since the black sign on diode is Katode it seems that you have Kathod to ground. while in the schematic presented it should be Anode to ground.
With Kathod to ground configuration the voltage is positive, while using the Anod to ground as in the schematic the voltage is negative.
Fabio Capozzi I look better to the video and found that upper diode is connected to center pin. Was only an assembly problem. Tnx !!
Thank you, what if we have to go and measure a signal at a higher frequency (2 to 4Ghz)Any advice ?
Probing 2-4GHz is very difficult, as lead lengths become big parasitic elements. Keep EVERYTHING as short as possible and you can have success.
Can I use it to test a VHF 2M mobile radio, only for "sniffing" RF on transistor collector, MOSFET drain and PIN diodes in a TX line?
Very nice video. I have build an Signal Tracer out of a radio and I wanted it to detect RF frequency and listen the modulation as audio. The problem however is that it doesn't come out as audio, in fact it you can hear almost nothing, after testing from 17Khz to 3Mhz. The demodulator consists of: a germanium diode, followed by a 330pF and a 470K which resistor parallel to it, one end goes to ground and the other end to the audio amplifier. Please help, thank you very much :)
+ciprianwiner Do you have an input capacitor? That is, in order, probe tip-input capacitor (I use a .001uF, 500V ceramic disc) - diode from the probe side of the input capacitor to ground - then an isolation resistor (100K is a good value) from that junction to the probe's output to the meter or scope. Another filter cap of 220pF to ground from the output side of that resistor is optional to smooth out the final output. A diode load resistor across the diode itself is also optional (470K is a good value for most applications). The probe will not woirk without the input capacitor, since this is what charges to the peak level of the RF when the diode conducts.
+Rob T Yes there is a input capacitor but it's 10uF because I didn't think in mattered... the thing is that the Signal Tracer has to work with audio about 99% of the time (I use it as an amplifier) so 1n capacitor is just too small for audio frequency. I will built a separate probe for it for work on radio frequency. Thanks for pointing out what I have done wrong. Cheers :)
I appreciate the lucid explanation of the rf probe. Is this similar to the detectors used in MFJ antenna analyzers?. Can you make a video on how to choose the component values depending on the frequency of interest ( say HF, VHF, UHF)? Thanks for the excellent videos. - VU3KWK
I've never looked at the detector circuit used in the MFJ analyzer, so I don't know if this is similar.
it uses a shunt detector to read the differential and ground referenced voltages of the RF impedance bridge
nice circuit but it is very hard to find germanium diodes! please suggest where are the places to find germanium diodes in junk parts :)
Could this be modified to monitor a 100V carrier output from an RF PA? Maybe just add a 100K resistor at the input? Or would it be better to place a voltage divider before the diodes (move the 10K and the additional 100k to the input) ? I'm thinking to make a monitor for my transmitter using an Arduino which has a max input of 5v. So I'll need to map the attenuated 0-5V voltage reading to reflect the true 0-100V voltage output.
I'd probably start with a RF sampler or directional coupler in order to get a more controlled and safe sampling level - you could follow that with a simple envelope detector like this...
How the 990mV RMS when measured with probe becomes 1.8V. There must be some relations. I am trying to understand this. Is it simply just double of the RMS voltage?
It's a combination of the rectification of the diodes, the diode drop, the diode capacitance and the storage caps. Since there isn't an easy/fixed mathematical relationship, thats why I made the plot shown at 2:31
Is there a method by which we can regulate the range in which the RF demodulator fuctions?
Thanks, this is really helpful! Can I ask what sort of lead you used to connect to the detector and when you connect it to your scope is it a standard high input impedance port?
Yes, just a standard high impedance input, connected via simple coax. Remember, the output of the probe is just the envelope of the signal and won't have much useful high frequency content.
w2aew ok brilliant, thank you very much, will be making one of these :)
How would Ge diode compare versus Si Schottky diode in this case. Frequency response ?
About the same.
Hi Alan. Would you object if I translated some of the info into French and shared it with club members in West Quebec? 73, Chris VE2MW
That would be OK with me!
Thank you. I will include the link to your video. The majority are bilingual but a few are not. 73 Chris
hello, i have a few question about the design,; first, could it work with digital modulation, for exmaple ask?; what frequency range it works? thanks
Can this be used to measure/confirm the Percentage of Modulation if using a Precision E-200-C signal generator? I want to confirm that the number on the control matches the actual percentage of modulation within the signal. Thank You!
Probably not linear enough as you approach 100%. Better off making measurement on the RF envelope itself.
@@w2aew Much Appreciated, Thank You! Would you ever consider a video on how to check a signal generator's modulated RF output for accuracy to 400kHz, and to track the percentage number on the dial to actual percentage of modulation?
@@Pwaak Take a look at my latest video
ruclips.net/video/-D-hkFkCH7I/видео.htmlsi=Bj0PbPn79amD6qMb
@@w2aewThank you for your descriptive presentation! Going the analog route :2465B. (Replaced C1016,18,20,51,52 Rifa/Wima and R1018) 73 Tony N2TSO
Very nice design. I want to build a similar RF detector to detect WIFI signals...What kind of input/antenna would you recommend for this?
The antenna is the *easy* part. The biggest problem is that even if you successfully build a band pass filter for the 2.4G ISM band, you are still left with the fact that there are a lot of signals that exist in that band. Besides WLAN, you've got Bluetooth devices, 802.15.4 / Zigbee devices, microwave ovens, and a host of other license free wireless devices. It would be virtually impossible to pick out WLAN transmissions with just a simple power detector like this.
***** Ah I see...Well what if I want to build a high pass filter, where all signals above, say, 40 MHz are picked up...Would a simple RC circuit at the output of the diode accomplish this?
Johnny Que A filter at the output of the diode won't do it, because the low frequencies would already be detected. You'd have to add the filter before the diode.
***** Ah thanks a lot! I tried it in simulation and it seems to be working...now time to try it on the real system :) I put a high-pass filter at the antenna input, and I put an op-amp at the output to boost the voltage (since the voltage detected at the input may be quite low with the antenna).
I was not clear on how this might be handy if you have an oscilloscope with the same frequency range ? If we wanted a general detector to measure higher frequencies say mobile phone frequencies ghz how might this need to be modified ?
Sometimes it is handy to be able to view the amplitude envelope of an RF signal. To make one for higher frequencies, you'd have to use diodes rated for much higher frequency.
Can you give the details on how you built the 50 ohm transmission line micro stripe. I would like to build one and not sure of the dimensions and material? I web site I found gives the W and T dimensions but still could use some guidance. Assuming the Length does not matter.
Length does not matter. The width of the microstrip trace is a function of the thickness of the insulating material between the trace and the ground plane, and the dielectric constant of the material. Several calculators help you get the dimensions:
www.eeweb.com/tools/microstrip-impedance
www.pasternack.com/t-calculator-microstrip.aspx
chemandy.com/calculators/microstrip-transmission-line-calculator-hartley27.htm
Still can't understand why we need first (vertical on scheme) diod. Seems it just would short negative half period voltage from capacitor directly on the ground. I know this is basic scheme from some recivers (like detector reciver or AM demodulator part from more complex recivers) and so shame to get stuck in it with understending.
This first diode allows the negative half cycle of the RF input to charge-up the input capacitor, so when the positive half cycle comes along, it transfers this charge to the grounded capacitor via the 2nd diode. Thus, to boosts the detected voltage a bit.
It seems I understood it from this point of view. After negative half cycle we have source in serial connection with cap: "+" of source (on positive half way) to "-" of charged capacitor (who got charge through first diod on negative half cycle) and then them voltages stack up. Thanks for answer as well!
So you can build this and connect it directly to your scope..then touch it to plate voltages, or grid voltages?? Can you hook it directly to align the IF stages in a TUBE radio...???
No, absolutely not! Don't go anywhere near plate voltages with a low-voltage probe like this!
how do I see people aligning there radios with scopes..is it a special high voltage scope..or are the probes different...?? mine says 600volts p-p 300V(DC + AC peak)
Many tube radios use plate voltages less than 300V, so your probes will be OK for that. Have you watched my video on oscilloscope basics, and looking at signals inside an All American Five tube radio?
ruclips.net/video/niBGkqlh2XY/видео.html
👍
How nano volt signal power can be detected?
You would typically need some kind of narrowband detection, such as with a spectrum analyzer or lock-in amplifier.
Do you have any references about measuring the reflections of an RF signal. Suppose I inject a RF signal into IC input. Any ideas on how to measure how that signal is reflected?
This is most often (and most properly) performed using a VNA or a TDR.
So how does one get a job at tektronix?
Like most electronics/tech companies - keep an eye out for openings in your area of expertise on the company careers page - and apply!
careers.fortive.com/tektronix
@@w2aew Wow, thank you so much for the reply.
im not a rf guy (yet) but is there anything that uses RF under about 200mV? Because you could bias the diodes with a coin or button cell to remover that drop at the low milivolts but would it be worth it?
+Laharl Krichevskoy Not worth it in my opinion, at least not for a sensitive germanium diode. Adding DC bias does not add much improvement. A germanium detector probe hooked up to a 10M ohm digital meter, for example, will still read something like 0.4mVDC when the RF input to the probe is only about 10mVrms.
Ok, I'm thoroughly confused here. The whole time you are measuring with the DMM, it is set to DC Volts. What's up with that?
Yes. The RF Demodulator/Detector produces a DC voltage that is proportional to the RF signal amplitude. If the RF signal has some amplitude modulation, then the output of the detector will follow that (which is shown on the scope)
@@w2aew Ah, yes, I wasn't thinking about the circuitry in the probe, it is being rectified. Yep. Good deal.
I was looking at the AC signal on the o-scope and looking at the DMM and it was driving me nuts. Wasn't even thinking about the probe you were using. Makes total sense now. Thanks!!!
i know my question will be completely off topic, this video made me curious ; so if i wanted to detect the satellite RF that we get using a dish or LNB specifically as input with a similar circuit, would i get a current that i may use a speaker or led as an indicator in the output
The signal from the LNB will be too weak to drive a speaker or LED, and maybe even too weak to get a decent response from a passive RF probe like this.
w2aew ok thank you for the informations, i was excited to make an amazing gadget that you plug to LNB then align your dish and get responds even though of using the smartphone as a monitor by getting the output through audio jack just and acts like headset mic
Hello again, Can i use nte112 diodes in this circuit ? Thanks
Yeah, they'd probably work well.
@@w2aew Thanks !
Can you give me some trigger levels etc that you use to get 2 tone looking hollow on your scope
It's not a trigger setting - it is possible only if the scope supports intensity graded waveform display - so it really depends on your scope waveform display features...
@@w2aew it’s a rigol hdo1104 latest model I’m hoping I can figure it out
@@plentygood422 see if there is a waveform or display persistence control
Kind sir. The probe input does not need to be grounded? If I use SMA connector as RF input, the SMA shouldn't be grounded, correct? Also how did you determine the capacitor and resistor value for the detector? I have a carrier frequency of 115 MHz and message frequency of 1 MHz-5 MHz. Are there any formulas I can use to calculate the values? Thanks so much.
It is grounded (note the black clip lead). If using an SMA as input, it should be grounded also. I'd leave the cap values, and just adjust the bleed resistor (10k) as needed to follow your amplitude variations. It will likely have to be lower for your 5MHz modulation signal. Note that this will also increase the ripple from the carrier.
w2aew I appreciate the response. Learning a lot from your videos. I noticed this circuit configuration is similar with voltage multiplier(doubler). Is it the same concept?
Hi Alan
Please can you explain: I Used the probe with DMM and measured voltage 33 V output from AM transmit into a 50 ohm dummy load. Radio set o 5W. Converting P to P to RMS (33 x.7) 23 V. Using online RF power calculator it is 10 watts. This is double the power shown on the Bird watt meter.
Regards Stan
What frequency was the AM transmitter running at? And, what DMM where you using?
@@w2aew Frequency was 14Mhz and DMM was Fluke 77
DMM was set to VDC
Measurement taken fro a BNC T piece. Could this be an issue?
@@stanholmes4293 This detector probe is not going to be calibrated - it is mainly useful to get relative readings, as mentioned right at the beginning of the video.
@@w2aew Yes I did take note of that, but as the power was double I thought I was doing something stupid. The one you made have you tested it for accuracy? Understand it is great to adjust for max readings.
Thanks for your prompt responce.
I built one to probe a fm bug, unfortunately I don't know if my detector isn't working or my transmitter is not oscillating. #fail
If you have any kind of signal generator - doesn't have to be RF - you can use that to check it out.
Hmm, Ge still available?? If not, I guess I have a goldmine in my parts box ;-)
You can still get them, just not dirt cheap anymore. Of course, hot carrier diodes like the 1N5711 would work well too.
Very useful and helpful project!...Thank's!
Could a BNC connector be used instead of the SMA?
Yes, of course.
*****
Thank's for the reply! I wondered if there might be some subtle influences added to the probe's circuit due to the different style connector.
Can you calculate power from this circuit?
The response is somewhat non-linear, so you'd have to make a lookup table from a known power level.
wonderfull its a poormans vtvm
This looks like the sort of demodulator probe used on those audio output signal tracers. How much does the input impedance of the signal tracer amplifier load the probe. Recently just built a vellemen signal tracer kit and I’m using a bought demodulation probe but I’m suspecting the 50kohm input impedance of the amplifier will load the circuit under test. I note a lot of the classic signal tracer like the HealthKit were valve based and high impedance. regards Chris. PS if you get a minute I’m trying to help one of your countrymen and fellow radio dabbler, here ruclips.net/video/c_ysLO0ouFU/видео.html
50k load shouldn't bother the RF detector at all. I posted a reply on the video, suggesting that he start with checking DC bias points throughout.
w2aew Thanks he is a nice guy, fly’s planes how great is that? Thanks for the heads up on the impedance. Best regards Chris
Any chance you could share the schematic? Thanks!
I don't have it in an electronic format - it was hand drawn. Only 5 parts. Simply pause the video around the 1:00 mark and copy it down.
Thanks
I am looking to build this but it seems 1N34A diodes are not available anymore. What would you suggest as the best substtitue?
While they are a little harder to find than they used to be, there are definitely still available.
w2aew thanks Alan
SMA connector!!
well
"the probe is quite simple...". Yeah, no. It's quite simple to EEs maybe. It's not simple to people with some basic electronics under their belt. But playing devil's advocate... why would anyone with only basic electronics knowledge be watching your videos. Oh well, it was way over my head but I'm sure I learned something.
Maybe watching some of my videos on the basic properties of diodes and capacitors, as well as my videos on peak detectors, will give you some foundation for what is happening in this video.
Thanks Alan. I'm just being a negative Nelly... don't mind me. I'll get there eventually.@@w2aew
@@Wil_Bloodworth No worries - there's a lot to learn, and hopefully some of my videos will help you along the journey.