Video: FM demodulation using a Pulse Counting FM Discriminator - Page 1
Forum section: electronics / rf-microwave

author: w2aew
author_rank: Super Contributor
timestamp: August 23, 2019, 08:30:05 pm
content: This video presents a little known FM demodulation technique that is quite simple and works very well - called a Pulse Counting FM Discriminator:

author: GerryR
author_rank: Frequent Contributor
timestamp: September 02, 2019, 12:23:59 am
content: That is a neat technique. I haven't absorbed it all yet and thought it worthwhile to bump it up to make sure others do not miss this. (The more I learn, the dumber I feel!) Will Rogers said, "We're all ignorant, just on different subjects." I guess this is one of my areas.

author: 0culus
author_rank: Super Contributor
timestamp: September 02, 2019, 02:34:50 am
content: I built one on perfboard, and it indeed works as advertised. Very cool little circuit.

author: Circlotron
author_rank: Super Contributor
timestamp: September 02, 2019, 06:11:54 am
content: Probably one improvement would be to make sure the pulse width stayed absolutely constant. The one in the video seemed to vary a its width little with frequency and that would affect linearity.

author: Kalvin
author_rank: Super Contributor
timestamp: September 02, 2019, 12:32:01 pm
content: The main point was to demonstrate the concept, not to create optimal solution.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2019, 02:28:34 am
content: Quote from: Circlotron on September 02, 2019, 06:11:54 amPulse width implies a measurement of the average of a current * time product but this circuit actually transfers fixed charge through the capacitor on every cycle so pulse width is irrelevant within wide bounds. I assume when w2aew says "pulse width", he is referring to the variation in pulse width as the output frequency changes and not the time allocated to discharging the capacitor.Jim Williams discusses the difference in these two approaches, current balancing and charge pumping, in the appendix of Linear Technology application note 14. The charge pump method used here is one of the oldest methods of doing frequency-to-voltage conversion and is found in all kinds of circuits including precision ADCs, tachometers, and step generators. Getting equal accuracy out of the current balancing approach is much more difficult.Analog Devices wrote an excellent application note which discusses the techniques for improving the accuracy of the current balancing methods. They include thing like quartz stabilizing a one-shot timer.

author: Circlotron
author_rank: Super Contributor
timestamp: September 03, 2019, 02:34:26 am
content: Okay. I was thinking of doing it using discrete logic. Fixed width rail to rail CMOS pulse and then low pass filter it. To be sure, sometimes a handful of bipolar transistors has more going on than meets the eye.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 03, 2019, 03:56:38 am
content: I think Marconi may have used this technique in one version of their FM Deviation Monitor.At a country TV site, we wanted to use "Bessel zeros" to check the "Dev Mon" accuracy, to which end, we had a Drake DR7 sent up from the Depot.Alas for our enthusiasm, the Drake cut off sharp at 30MHz, & the transmitter sound drive unit monitor output, which we had hoped to use to look for our "zeroes" was at the standard IF output at 33.4MHz.------Bummer!But wait! --- the DR7, unusually for comms receivers, tuned down to 10kHz, & looking in the Marconi manual revealed an IF of 100kHzWe managed to dangle a pickup wire near that IF, & picked up enough signal to "do our thing".We wearily packed up our gear & never thought any more about why the IF was at 100 kHz.(At least, I didn't--- the other bloke was a bit of a "Electronics geek", so he might have!)I'm unlikely to ever find out if it was a Pulse Counting FM Discriminator, as all that old gear is lost in the mists of time.The RSGB Manual used to have a section describing this method, as did "73" magazine, back in the 1970s..

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2019, 04:23:05 am
content: Quote from: Circlotron on September 03, 2019, 02:34:26 amWith CMOS, a big improvement could be made by implementing a simple "flying capacitor" type of charge pump frequency-to-voltage converter. This can be done with one analog switch type of IC or an LTC1043. No fixed pulse width is required.Bob Pease did a lot of work with precision charge pump voltage-to-frequency converters using bipolar technology but they are more complicated. Reading his articles on these gives a lot of insight into precision analog design.Nobody goes to that much effort for FM audio demodulation because other error sources are more significant.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 03, 2019, 04:33:14 am
content: Has anyone built a working FM demodulator using discrete digital logic?

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2019, 05:30:32 am
content: Quote from: NiHaoMike on September 03, 2019, 04:33:14 amI has been done many times as part of a phase locked loop.The pulse counting FM discriminator could be done using a digital one-shot. Use the zero-crossing to trigger a count down timer clocked at a higher frequency to produce a constant width output pulse. Average the output to get demodulated FM. Except for the RC output filter, that could be done completely with digital logic.There is a voltage-to-frequency converter ADC based on the principle but it suffers from some weird non-linearities.

author: w2aew
author_rank: Super Contributor
timestamp: September 13, 2019, 06:53:02 pm
content: Quote from: Circlotron on September 02, 2019, 06:11:54 amAbsolutely true. If you make a better limiter, that would help keep the PW constant.

author: aurelZ
author_rank: Contributor
timestamp: September 16, 2019, 05:50:31 pm
content: I build last year pulse counting fm receiver.It work on just 1,5V battery ,sound is good for such a simple design.