Measuring FM Deviation using the Bessel NULL Method (030a)

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Electronics for the Inquisitive Experimenter

Жыл бұрын

In this video I will be showing you how to measure FM Deviation using the Bessel Null Method.
This can be done with either a CW Receiver or a Spectrum Analyzer.
HERE IS THE PROMISED LINK TO BESSEL PLAY SPREADSHEET:
docs.google.com/spreadsheets/...
Time Markers for Your Convenience
----------------------------
00:05 Introductory Comments
01:11 Background of the Bessel Null Method
03:12 The numbers YOU need to know
04:07 The Setup
05:12 What you need: See the Carrier ONLY
06:56 A Tour of My Setup
06:59 The Hardware
07:56 Settings
07:57 Modulation Frequency
08:52 Spectrum Analyzer
09:15 The Radio
10:27 Measuring FM Deviation
12:15 Calculating the FM Deviation
13:14 Setting an FM Deviation
14:49 Setting CTCSS Deviation
16:56 Final Comments and Toodle-oots
-----------------------------

Пікірлер: 23

@WECB640 Жыл бұрын

Bravo Ralph! Thank you for sharing your knowledge and uncovering what (sadly) many amateurs do not know. 73 OM

@eie_for_you Жыл бұрын

Thanks, man! Many do not have the need to know, so they do not seek the knowledge. 😞

@patrick4228 9 ай бұрын

And yet another video that has given me a welcomed insight. Finally my measurements are matching with what I expected and the proof that my deviation meter was right the whole time. Plus I'm starting to understand what deviation is all about. I had to watch the video at least three times before all the pieces fell into place but that makes the "AHA!" moment all the greater. I am lucky to own a 'real' spectrum analyser, but it seems I can get the same results with my much cheaper SDR receiver. Here, I can also see the carrier and its amplitude going up and down. Perhaps handy for those on a budget. Many thanks! Again!

@eie_for_you 8 ай бұрын

WOW! I'm so glad that this helped the light go on for you. 🙂 I have an SDR that I've played with a bit (nothing serious...yet). Either I haven't found the right software to use with it or just haven't stumbled across the methodology. Not real impressed with its performance as an HF receiver. 😕

@patrick4228 8 ай бұрын

Thank you for the reply!I have the SDRPlay and use their SDRUno radio software or the RSP Spectrum Analyser, both are free to download. I am very satisfied with this SDR + software, but they are certainly nothing special or expensive. If it might interest you, I would be happy to send you some screenshots of both programs with "a vanished carrier" on them. Maybe the settings will tell you something. @@eie_for_you

@hugocoolens Жыл бұрын

Great video. Very well done.

@eie_for_you Жыл бұрын

Thank you! We use this method all the time to set the deviation on our repeater system ... both the CTCSS tone and the "program audio."

@scotttodd3506 Жыл бұрын

I need to check the calibration of a broadcast FM monitor but the book I have with the various recommended tone frequencies is in storage and not accessible until later this spring. Thanks to your video I was able to calculate a couple of frequencies I could use, not only to set the meter at 100%, but also to check tracking in the middle at 50%

@eie_for_you Жыл бұрын

Woohoo! Great news! I am so glad this helped! 🙂

@visitslovenija 8 ай бұрын

Thanks Ralph. Heck, I think your idea to use a receiver set for narrow band cw is a better way to check it than using a spec analyser. :)

@eie_for_you 8 ай бұрын

You are welcome! I have to agree with you! The pip on the S.A. is below visibility and I can still hear the tone. Using a narrowband CW receiver it does seem to be easier to judge when the carrier is completely extinguished.

@SpinStar1956 Жыл бұрын

SUBSCRIBED!!! Just ran across you channel and realy like this video. I got given a broken repeater and have been trying to both get is up and to understand enough t correctly operate it. I have 2 old 8640B/Option 323 but they are not table enough nor give me the resolution of signal or modulation to be able to use them. I ran into the same problem trying to obtain a 'Service 'Monitor' and am unwilling to (what seems like) throw money away on a crap-shoot! I was a little taken aback when I saw the cost of your RIGOL RF generator. Would you of one with comparable specs but maybe up to 500MHZ? Anyway, pretty tough going here but am pushing ahead... Thanks & 73...

@eie_for_you Жыл бұрын

And your comment just popped up in my list of comments! WOW! The Rigol RF generator I have (DSG815) goes to 1.5 GHz. I am very happy with it. Unfortunately, I have no recommendations for you except what I have found so useful, my own DSG815. Price point-wise, you are not going to get better for the $$ (my opinion)

@stargazer30721 Жыл бұрын

Great video! I am trying to help take manage a VHF packet system. How could I use a similar setup for setting deviation on an FM transmitter for 9600 baud packet? I have a TNC that I can adjust the signal level going into the transmitter. The manual for the TNC says that FM deviation should be 2.4kHz. Since 9600 baud is FSK and not AFSK, will your method still work? I know just enough to be dangerous! Please help!

@eie_for_you Жыл бұрын

I am glad you found this video helpful! How would I do it? I would use this method exactly as described with a signal generator and all. Replace the TNC with an audio signal generator connecting its output to the port where the TNC would be connected, select the appropriate audio frequency for your deviation (2.4 KHz/2.4048 = 998.0 Hz) and then adjust the output amplitude of the signal generator until you get your null. Now you know the amplitude you need as an audio input to your transmitter to produce the desired deviation - for your digital signal, I'd be concerned with peak-peak voltage. Replace the signal generator with your TNC at the transmitter's input and adjust the amplitude of the TNC audio output to be the same peak-to-peak output amplitude that you had with the signal generator. You could use the MAX/MIN method of measuring deviation that I describe in the next video (kzfaq.info/get/bejne/l7qmeqZ8s9SrgKs.html) to verify the final deviation. I hope this helps. :-)

@hienphamminh1405 Жыл бұрын

Please explain to me where the audio signal generator connects to?

@eie_for_you Жыл бұрын

Well, this depends on what device you are adjusting. If you are working on a radio transmitter, then you need to find out what the amplitude of the normal audio source is and where it is applied. Then you set up your audio signal generator to mimic the amplitude of the normal source and connect it to that input. The object is to choose a place where the audio we use to make the measurement "looks" like it is coming from the source it normally would be coming from, but is, in reality, our signal generator where we have control. If it were, for instance, an FM broadcast station, I'd connect my signal generator to one of the inputs to the control board and set the audio level to 0vu to run the test on the FM transmitter's deviation. In the case of the demonstration I performed in the video, I connected it to the external audio input of the RF signal generator. Hope this helps

@craigdupree1687 Жыл бұрын

What was the book? (I like math)

@eie_for_you 11 ай бұрын

"The Handbook of Electrical Engineering", Research & Education Association, c.1996; Section D, page 70. Good luck in finding a copy. 🙂

@Ibelieve218 Жыл бұрын

So If I wanted to see my modulation at say 146.520 fm. I would set my analyzer at 300 hz resolution band width and my span 1 mhz modulation (source would be speech using microphone).

@eie_for_you Жыл бұрын

Well, Neal, if you are going to be checking FM deviation using a microphone and speech, then I would suggest the "MIN/MAX" method instead. This is great for checking a dynamic. live signal like speech. My video on that can be found here: kzfaq.info/get/bejne/l7qmeqZ8s9SrgKs.html The Bessel-Null method only works with a single, specific tone given a desired deviation. CTCSS tone has to be shut off on the monitored RF. Hope this helps.

@Ibelieve218 Жыл бұрын

@@eie_for_you will thank you.i own the hp esa1500a and just got the rigol dsa815

@eie_for_you Жыл бұрын

@@Ibelieve218 It's a great spectrum analyzer for the money! I got the one with the tracking generator so I can use it for "response-type" measurements on filters and the like.