I was able to set the deviation on an FM transceiver using this method using a mobile phone, an SDR receiver, and an oscilloscope. It took a couple attempts because the makeshift signal generator did not keep a constant audio level as the frequency changed. Once I realized the problem, I changed my method a bit. Take the specified deviation in the service manual and divide by 2.405. Use a tone generator to produce tone at this frequency. Set the tone level to what is specified in the service manual by checking with an oscilloscope. Play tone into radio input while transmitting. Watch the waterfall on the SDR transceiver, and adjust the radio's deviation until the carrier frequency disappears. When I was finished, the resulting adjustment was within about 1.5% of what the radio arrived with.
Tnx Peter and congratulate you on doing this all in English so that you can reach a large audience.I'm wondering if you do this as a hobby or professionally. It does not matter, you are doing a great job!
I used to give analog and digital modulation lectures long time ago. I wish I had got an SA to show my students the modulations spectrum in the frequency domain. Thanks.
Hi Peter, an informative presentation. I used my OWON XSZ1015P Spectrum Analyser and my Marconi 2031 Sig Gen to test it out at 28MHz with 5KHz deviation at -10dbm. I found 2 lower frequency nulls #1 at 581.4Hz, #2 at 909.4Hz and #3 at 2.083KHz all nulling down the fundamental to about -90dbm. The Spectrum Analyser in ''modulation mode'' showed a Peak deviation of 5.05KHz, 5.04KHz and 5.01KHz. All quite close results. Based on the modulation Index of 2.4048 for 1st Bessel null this calculates to an audio freq of 2.076KHz for 5.00 KHz deviation. So I am confused by the 1st 2 nulls as they should not be there. I think the best way is to to use the formula to calculate the audio freq based on Audio (KHz) = Deviation/2.4048 and trim it for a null on the SP. Cheers Mike
Great job Peter, I loved the way you tied things back to the mathematical fundamentals, it gives me a warm and fuzzy feeling ;o) Keep up the great work and thanks for sharing.
Very cool, Peter! Thanks for the thorough description. I've heard of the Bessel Null function in FM broadcasting, but it's neat to see what it means and how it's derived. You should do more theory discussions like this! :-)
It worked! I was able to get an estimate of the deviation variation of an old signal generator using my basic Siglent 100MHz oscilloscope. But not in real time (the scope only takes 1024 points, not enough frequency resolution) but saving the signal values (about 20000 points) and doing DFT via software and using Carlson bandwidth formula.
Great video for introduction to FM and FM deviation. However the carrier null shown does not seem to be the first null (2.4). Audio frequency at start was very low which means very high m. As the fm is increased m decreases and the null shown is a higher order null. Deviation by calculation should be much higher than 5 KHz.
Great and many thx ! I didn't have a clue that the Bessel functions have something to do with the spectrum of a FM modulation, now you just proved it. I vote to have now some more expalantions (of the math side), could you give us a site or something to read more about this ? When I study for the HAREC license, these little formulas sound silly, I would be happy and remind easyier if I can unterstand the math under the formulas.
Ein sehr guter Vortrag. Ich würde es für begrüßen, wenn Sie diesen Votrag für alle, die nicht englisch verstehen, eine deutsche Version zu veröffenlichen.
8:56 but this diegramm is a little bit confusing , I mean for FM because I usuelly can't see the modulation like that on my oecilloecope unless there is a really slow signal like 10 Hz or something so then I can see sinewave expanding on the screen...
FM deviation it's not easy to understand, but we appreciate a lot what you have done for your followers. It's not so clear 4 me, how it's possible see deviation without carrier signal. :(
Hi Peter, I am trying to fix my HP8590L spectrum analyzer and would like to seek your advice on some of the issues. 1. I tried to measure the signal path from the RF to the IF with a 500Mhz scope which I borrowed but I cannot see any sensible signal between the output and the input of the different stages. Of course at 2.4 Ghz I would expect issues with the scope shorting the signal due to capacitance but at 300 Mhz I expected it to work. When I disconnect the wires from the SA I see on the screen that the signals disappear hence there must be a signal inside the cable its just that my scope doesn’t seem to pick it up. Do you have a recommendation how I could check the signals, I do not have another spectrum analyser. I tried a small hand made coil (contact less) but didn’t help either. Is it impossible to measure in your opinion ? 2. I see a nice signal on the SA at a SPAN of 100khz and above. The moment I tur the nob below 100Khz the signal becomes useless. Can you recommend where to search ? www.eevblog.com/forum/repair/hp8590l-spectrum-analyzer-calibration-signal-issues/new/?topicseen#new Highly appreciated Bjoern
Good evening, thanks for the interesting video. I have a question: @minute 37'30" you said "...we have seen that our Bessel zero is @ 2.4". I searched backward but I didn't understand how you can say that. Please can you help me to understand? Thanks again. Andrea
I think the bessel 0 point you have achieved here is not the first bessel 0 point (i.e modulating index =2.41) because if it was so then while you were increasing the frequency of the audio signal amplitude of the central tone should go from negative to zero as shown in the bessel graph. But in your case amplitude of central tone was coming from positive to zero while you were increasing the frequency so maybe it was any one of the next bessel 0 points.
squelch is used in receivers not transmitters. The squelch basically mutes the receiver when the signal strength falls below a certain level. Once the signal strength is above a certain level the squelch "opens" and the receiver un-mutes. The squelch level threshold is usually controlled by the user depending if they want to hear strong signals or weak signals.
Dear Sir , thanks a lot , please I have question : If I set the carrier ( from waveform generator ) to say 10 KHz , set the deviation ( FM modulation from the waveform generator ) to 1 KHz , so is it possible to view the analog FM signal on the Oscilloscope ? Thanks and best regards