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- Опубликовано: 28 июн 2024
- This video shows how to measure the modulation index/percentage of an AM (amplitude modulated) RF carrier. The basics of the calculations are shown, as well as techniques for setting up the scope to get a high quality waveform for the measurement. Notes for the video can be found here:
www.qsl.net/w2aew/youtube/AMme...
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I'm not a radio ham, but love your videos and often learn something new - you're a great teacher!
That is fine. Ham Radio does give you the ability to really get into radio electronics. As a matter of fact, I got into ham radio to learn more about radio electronics - and electronics in general - but also to build my own little radios. I think videos from w2aew are really good.
Thanks for giving those of us with more modest scopes a way to get similar results.
Amateur radio is all about learning new things from others, We never stop learning. Thanks Alan for another great tutorial. 73
Detailed explanation along with pictorial view of whats going on and how to achieve results! Very much appreciated, Thank You!
Thanks for the video. The info on the sampling options on the scope I didn't know. Thanks for teaching me.
Thank you so much. I've learned a great deal from you watching all your videos. Incredibly grateful you are willing to share your expert knowledge.
Great video, as usual. Trigger hold off might be another option to get stable picture, and it helped me to grasp thaf concept using it on AM signal.
Interesting video as usual. 👍 Seeing you using the HP 15C inspired me to get mine out of the drawer and put some batteries in it. Still working after more than 40 years. Wow.
Another great video, thanks! For triggering on am AM signal on either an analog or digital scope, I'd recommend using "Hold-off Time". just set the hold-off time to the modulated frequency's period (the reciprocal, e.g. 1/Frequency) divided by 2. e.g. 1 KHz = 1/1000/2 = 0.0005 Seconds or 500uS. Works great for me.
As I recall from my old days of AM radio engineering, we used a trapezoid pattern to measure modulation. An RF sample would be sent to the vertical scope input, and the modulating audio would be used for the horizontal sweep. This produces a trapezoid that can be measured in the same way as the method you used. The peak and trough measurements are used in the same formula to determine the modulation index. With a high voltage RF sample, we would connect it directly to the vertical deflection plates via screw terminals on the side of the scope provided for that purpose (don't try this at home!). I don't know how this would work with a digital scope (don't have one, and back in the day digital scopes using vacuum tube were scarce!).
Another great tutorial Alan, took copious notes for my "W2AEW Electronics Course"! ;-) 73 - Dino KLØS
Well, that was a nifty demonstration of what you can do with a lab grade instruments, an arbitrary function generator and digital oscilloscope. Now run through a similar demonstration showing how to come up with the modulation percentage of a hobby grade RF signal generator using an analog oscilloscope.
Sure thing - here's a follow-up showing the measurement with an older analog scope and a few different inexpensive RF signal generators: ruclips.net/video/zfYK1vU3JMY/видео.html
Alan your videos are always informative and entertaining! Keep up the good work. 73
Great job Alan. Thank you.
Another nice video Alan. Thanks.
As always, a great video. Learned something again.
Thank you Alan.🤓
No worries, OM! Your videos are extremely enjoyable.
Thank you, Alan. :-)
Very good video and the RPN demo was a bonus!
Thank you very much indeed Sir....learnt something new....
Wow! I learned several new tricks from this one! Thanks!
Excellent video thank you!
Fact my communication engineering exam is today The topic is AM modulation
Great video, as always. Love your calculator!
OM, that calc has a 288 pages manual! You read them then you go for Advanced functions handbook and read 186 pages more... 🥵
Thank you.
You sure have some nice toys! BTW RPN is the way to go!
Thanks, I learned something new.
If you have offset dc voltage, it might be better to measure Vp_to-Vp and Vt_to-Vt and divide them by 2 to get Vp and Vt.
In fact, you could just use the peak to peak and trough to trough measurements directly. No need to divide them by two.
@@w2aew Yes you are right, because you can factor 2 out in the fractions.
BRILLIANT !! .
Interesting. I need to update my scope. I'll always keep my analog unit for old stereo work,but at 100 mhz, and low memory depth; well lol
"Arbitrarily picked by me" for the phone region of the 20 meter ham band :)
Although you’d probably get scolded for operating AM in that portion of the band ;-)
Amazing brother🌹🌹😎😎🌹
You could also half your measurement errors by measuring peak-to-peak values. Once again, the factor of 2 will cancel. Oscilloscopes are designed for our 1% ability to observe trace positions on their screens... assuming the measurement occupies at least half of the screen. Note that 1% of 50 is 0.5 and 1% of 80 is 0.8.
Thank you so much for your videos. I do learn something each time I watch them. Can you tell me if you have a video that would describe the modulated signal from a CB radio? I am trying to simply understand what a waveform is saying about a modulated am signal from a CB on my scope. I would like to look at the waveform and be able to quickly tell by viewing the envelope what the signal is telling me by the shape of the envelope?.
In the US, CB radios use AM or SSB signals. This video talks about this:
ruclips.net/video/i29pI7okLe4/видео.html
Thank you!
Exelent as allways. Could you make a video about chirp/lora modulations and explane how it works and pro/cons ? 73 de LA4XUA
Thanks, Alan! How about FM modulation?
I have a few videos on measuring FM deviation already.
@@w2aew Ok, thanks. i'll watch
Can't you use the hold off time set to 90% of your modulation period together with the high threshold level to have a stable trigger?
That can help too.
Thanks! So how does a voice (in AM Radio) get added to this carrier? What would the waveform look like? Perhaps a future video?
Take a look at the videos in the description. Your voice waveform would replace the sinusoid shape of the envelope.
@@w2aew Sorry, missed that part! Love your stuff.
Hello Alan, I'm going to try to get my HAM license this year and while reading technical literature, I often bump on the terms "Standing Wave" in relation to transmitters. Now, from my courses in physics I remember what a standing wave is supposed to be, but in relation to HF I must admit that I'm a bit mystfied about the principle of "matching" an antenna to a transmitter for full power transmission.
I understand it has something to do, mainly, with matching the impedance of an antenna (+ cable between tx and antenna) to your tx but the subtleties elude me a bit. Could you give us a practical example of how and when this "matching" is done please?
These videos might help:
ruclips.net/video/g_jxh0Qe_FY/видео.htmlsi=oOCkMVm51RG4zDyP
ruclips.net/video/M1PgCOTDjvI/видео.htmlsi=jHprvpzIDf4HxqIF
ruclips.net/video/DovunOxlY1k/видео.html
What would the triggering look like on a analog scope like and 475 or 485? Would you just have to accept the jitter, or could you use HF reject to get stable waveform?
Careful level adjust and tweaking of hold off should work well.
@w2aew Would any of your suggestions work for a stable measurement of a VFO output or oscillator around 2 MHz, 5 MHz, or 7 MHz? Not just the waveform but also the numerical voltage and frequency measurements. When I try it on a digital oscilloscope, they seem to bounce around. Maybe it could be a suggestion for a new video?
Depends a lot on the scope and the techniques it uses for acquisition and measurement. If the signal is unmodulated, then averaging can help stabilize things a lot.
@@w2aew Yes, unmodulated CW. But really any AC measurement whether audio frequencies, unmodulated RF, SSB, AM, FM, PSK, FSK, etc. I have tried to do the averaging and include multiple cycles in the waveform but the results "seem" to be off when I do that. No EE here. Just an amateur. So, only a guess. To me, this is the hardest thing about using test equipment is knowing how to properly do a measurement for a given situation or signal. I have a new Siglent SDS814X HD 100 MHz 4-channel digital scope. As for frequency, I can hook up to my HP frequency counter to give me a decent measurement. But then I am left wondering about the voltage which seems to bounce around 50 kHz. I always keep my scope and probes on 10X and keep coupling on AC. I use the included 100 MHz probes for the 100 MHz scope. I did read others suggestions on other forums to use a 3 turn coil and get it near the oscillator to get a reading. The reading I am doing is after the buffer after the VFO signal is generated but before the transmit drive stage at a test point on a low-power (< 5W) CW transceiver.
@@danielmorgan7702 Measuring any kind of modulated RF signal (SSB, AM, etc.) is very tricky with a digital scope - mainly because you often want the horizontal scale set to something like 200us/div or more so that you can see the shape of the audio modulation. But, at this horizontal scale, the sample rate of the acquired waveform is often way too slow to properly sample the RF carrier. For example, consider the following scenario - an AM signal at 14.3MHz. If you set the horizontal scale to 200uS/div, that means that the entire waveform is 2mS long. If you use 10,000 points of waveform memory, you'll have a waveform sample every 2ms/10,000 or 200nS, which implies a sample rate of 5MS/s, which is severely undersampling your 14.3MHz signal. Ideally you'd want the sample rate to be >>2.5x the RF carrier frequency, much better if it is more like 10-20x the RF frequency. Your scope would need to be setup with very deep memory, and thus would be quite sluggish...
👍Thank you sir.
How would you measure the modulation index using an analog scope.
The only major difference would be an analog only scope would not have cursors to aid in the measurement. One would have to count divisions on the grid on the screen.
@@BryanTorok Some analog scopes do have cursors! (the 2465 for instance) Also, some aspects of the measurement setup are easier on the analog scope -- the sampling part which likely is the major hurdle for the digital scope can be disregarded. Further, the ch2 trigger trick can of course be re-used, if the source can supply such a signal. Many function generators have a sync out that is excellent as a trigger source, letting the main out drive the modulation on the R/F generator. If using the built-in modulation of a RF generator, there often is an AF output that can be used as trigger source. But, yes, counting divisions is what one would have to do if nothing else is available.
Thank you@@BryanTorok
@@mansnilsson4382 It has been decades since I used a hybrid analog scope with digital measurement. They were pretty slick for the time and actually under some circumstances provided a better image than modern all digital scopes.
Here's a followup showing how to use an analog scope for the measurement:
ruclips.net/video/zfYK1vU3JMY/видео.html
73' 88'
Do you know about, how to read the program inside a microchip? I heard the firms can read secret probrams of Pics. What are the keywords of thise topic ?
Don’t know, outside of my wheelhouse.
Modulations with a modulation level >100% are interesting.
I am surprice that you not read the value after you enable envelop funtion. In my profetional job I am making a lot of pulse power mesurement on RF switches using Keysight oscilloscop. One of the biggest issue is to get stable envelop. What I can say the Hiber traanssform funtion what they are using for that is useless. Look like the people do not undestand that making static calkulation in mathab do not much the dynamic responce in real life dynamic signal. The only thing that really works is max hold with carries in free run. With this approch you can use relatively low carrer frequency to generate interpolation for fast switching signal. Unfortunatly max hold is very slow.
I didn’t use the envelope function for two reasons. It’s wasn’t needed because my sample rate fast fast enough to accurately sample the RF, and I didn’t want to include the peaks of the random noise on the signal.
Very nice.
Excellent video. We used the same o-scopes in our lab. We didn’t work with AM modulated signals. Great tips! Still using HP-10 C calculator?
HP-15C actually.