Making a narrowband FM disciminator  - Page 1
Forum section: electronics / rf-microwave

author: met12
author_rank: Newbie
timestamp: January 30, 2024, 11:22:04 am
content: Hi,I am trying to make an FM demodulator circuit which can be used on narrowband FM.I had been following the tutorial from a MrVideoTronix youtube video, on the foster-steeley discriminator.https://www.youtube.com/watch?v=bhd4S0-AlZU.. His schematic used can be found at this link.https://www.patreon.com/posts/58257211.I also used a transformer binocular core with 6 windings for each coil. However, I have not yet included his amplifier part.When testing the basic circuit, I only get a difference of about 2mV every 100kHz. I know that the amplifier will help this, but I am looking to demodulate a narrowband FM signal, with 10.7MHz centre frequency and 2.5kHz deviation, and such a small frequecny change has little effect in the circuit.I would be interested in hearing if anyone else has managed to built a narrowband FM demodulator, and if they any advice on adjusting this circuit.

author: Karel
author_rank: Super Contributor
timestamp: January 30, 2024, 01:08:40 pm
content: Quote from: met12 on January 30, 2024, 11:22:04 amHow narrow-band is narrow-band? Narrow-band as in e.g. 2 mtr FM amateur radio or very-narrow-band as in FM CB-radio?In the latter, the best solution is to add a second mixer to create a 455 KHz intermediate frequency.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 30, 2024, 01:28:05 pm
content: Quote from: met12 on January 30, 2024, 11:22:04 amWelcome to the EEVblog forum, but I'm afraid there is no attachement.You can edit your first post and attach the schematic, otherwise not many will spend the time to watch the video.

author: met12
author_rank: Newbie
timestamp: January 30, 2024, 01:47:58 pm
content: Quote from: RoGeorge on January 30, 2024, 01:28:05 pmThanks for pointing that out, hopefully it is fixed now!

author: met12
author_rank: Newbie
timestamp: January 30, 2024, 01:56:19 pm
content: Quote from: Karel on January 30, 2024, 01:08:40 pmI am looking to create a demodulation circuit capable for narrowband mode with 2.7kHz max bandwidth. My issue right now is that at 10Mhz IF, I can just about get the deviation down to about 100kHz (only 2mV difference on the output), but even downconverting to 455kHz I am not sure if I'll be able to get a suitable output difference to show single kHzs?

author: Karel
author_rank: Super Contributor
timestamp: January 30, 2024, 02:12:40 pm
content: The Foster-Seeley detector is used at 10.7 MHz only in case of wideband FM (usually broadcast FM radio's).For narrow-band FM (communication only) almost all radio's use a second IF-stage at 455 KHz.This is done for two reasons:1. easier IF filtering in order to increase selectivity and suppress neighbour channels2. higher output of the FM detector (because the output is related to delta-Freq. / Freq.)

author: RoGeorge
author_rank: Super Contributor
timestamp: January 30, 2024, 03:11:05 pm
content: Getting a higher Q for the transformer should sharpen the selectivity, and thus the slope of the discriminator will appear steeper, producing a higher voltage in the detector (for the same frequency deviation).Another try to increase the selectivity of T1 might be to lower the coupling capacitor, Cc=10nF.Two coupled LC tuned on slightly different frequency will influence each other. There is a tendency they shift each other's resonant frequency (the more coupling factor K they have, the more they influence each other). Made once a short video to see the effect for myself:Maybe tuning both the primary and secondary of the T1 on the same frequency, and loosening the coupling factor will increase the selectivity enough to get a significantly higher demodulated voltage, but I have very little experience with RF, so no idea how correct these arguments are, or how effective would any of these be in practice.

author: Karel
author_rank: Super Contributor
timestamp: January 30, 2024, 03:22:25 pm
content: Quote from: RoGeorge on January 30, 2024, 03:11:05 pmWhile true, this is difficult to achieve in practise, mainly for stability reasons.Resonance circuits with very high Q suffer from temperature & mechanic instability.Ofcourse this can be (at least partly) mitigated by using special construction techniquesbut it's usually cheaper & simpler to add a second IF-stage.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 30, 2024, 04:02:14 pm
content: Have you considered using a PLL?

author: met12
author_rank: Newbie
timestamp: January 30, 2024, 04:20:13 pm
content: Quote from: wasedadoc on January 30, 2024, 04:02:14 pmI have the CD74HCT4046A PLL which I could use. I had moved onto radio specific circuits after I was having a similar issue using this. I could only get a stable output when the resistors are adjusted so that the output changes over a span of 500kHz for 10MHz, which again means the output has no real response for 2.5kHz deviation . If you have any advice for getting this PLL working for my specifications that would also be really helpful.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 30, 2024, 05:05:33 pm
content: Quote from: met12 on January 30, 2024, 04:20:13 pmWould suggest converting again to a lower frequency, maybe a few tens of kHz and then PLL.

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2024, 05:54:34 pm
content: The classic way to do this 30-35 years ago would be to use a Philips SA605/615 mixer/limiter/quad detector chip and a suitable LC quadrature detector network using a couple of caps and a tunable inductor.You could mix from 10.7MHz down to 455kHz using a 10.24MHz crystal (The SA605 has a built in oscillator that supports a crystal connection) and then use the hard limiter section at 455kHz and then fit a quadrature detector at the limiter output. At the mid point of the limiter, it's usually best to fit a bandwidth limiting filter if you want the detector to work down to really tiny signals. Otherwise, wideband noise will limit the dynamic range.Google for Philips SA605 to see typical circuits. I've made this circuit quite a few times over the years using just the SA604A limiter and detector chip at 455kHz. It will also limit and detect at 10.7MHz although the limiting isn't quite as good.Google for Philips SA604A to see the limiter/detector chip. This will easily work at 10.7MHz.However, all of this is old school. Today this stuff would usually be done in the digital domain.The other option is to steal parts from an old FM CB radio although CBs seem to be attracting large prices on ebay. Probably better to sell the CB intact!

author: iMo
author_rank: Super Contributor
timestamp: January 30, 2024, 05:56:27 pm
content: In case you target an VHF/UHF FM narrowband receiver, you may try with the single chip radios, like the RDA1846 (perhaps obsolete today) or the newer variants (the popular chip from the Baofeng radios). People played with it a lot, so perhaps you could download the software for it from github.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: February 05, 2024, 07:40:18 pm
content: Quote from: G0HZU on January 30, 2024, 05:54:34 pmOr get an old CB (or cheap Baofeng) and just build a simple converter to convert the signal up or down to a range covered by the receiver. Using the latter as a tunable IF.No internal modifications required so you retain the radio's resale value when you're finished with it.An example where I convert 1.8 MHz to 27 MHz (for reception of AM signals).

author: FredWDaniel
author_rank: Newbie
timestamp: February 05, 2024, 10:47:09 pm
content: You might consider looking at a Motorola Syntor-X receiver schematic as they use a 10.7 MHz low IF and demodulate using a quadrature detector & a crystal. I am not sure what IC they are using, but asking the Motorola forum athttps://batboard.batlabs.com/might yield the common industry part number, rather than the replacement part number in the manual. There are several other Motorola receivers of similar designs dating back at least 25 years.The schematic is athttps://www.repeater-builder.com/motorola/syntor/syntor-x/syntor-x-uhf-rx.pdf

author: LaserSteve
author_rank: Super Contributor
timestamp: February 06, 2024, 08:17:18 pm
content: Narrowband FM IF chips exist...Start with MC3357P and its current family MC3372 and ML13150They are now made by 3rd party suppliers such as Lansdale and New Japan Radio..search string is "narrowband FM integrated circuit Motorola RCA ".. The numbers I gave you are obsolete, but I assure you some are still available and in production...OK, NJR has merged:https://www.nisshinbo-microdevices.co.jp/en/pdf/datasheet/NJM2592_E.pdfMouser has a few types, select your deviation carefully...https://www.mouser.com/ProductDetail/Nisshinbo/NJM2593V-TE1?qs=Gh61b4mt6ULoEQRCfHjYdw%3D%3DSteve