DIY FM radio

[Martinn]

Hi all,

when I was a kid, I once tried building a FM radio (was from one Heinz Richter book IIRC), which was a total fiasco as I lacked measurement equipment, parts and of course know-how.

Now, 100 MHz is within easy reach of most of my instruments (with the notable exception of a still missing RF signal generator) and of course I have learnt a lot since then. I'm not a RF developer though, but a FM superhet seems easy enough.
Some years ago I found a FM radio project in the Elektor magazine. I was excited, but it turned out to be a breakout board for some integrated receiver chip with the project being wiring its SPI interface to an Arduino board. Quite disappointing.

So, while there's still FM radio broadcast, I'm looking for suggestions building a discrete (transistor or tube) FM superhet radio. There are kits of unknown quality on Aliexpress, but I'd rather build my own. My guess is that getting hold of specialty components like IF band filters (455 kHz), tunable inductors or variable dual capacitors (for tuning) might be difficult. Maybe there are specialist vendors for that still stocking stuff made decades ago, who knows.
Any ideas? Projects from last millenium magazines?

Thanks! Martin

[dobsonr741]

Once I built an FM receiver with CA3189, TDA1062, uA758. It was discrete enough, with ICs covering the main functional areas, 10.7Mhz ceramic filter, while still had a few LC filters to fiddle with. This is what I would still recommend building. You can have modernities like varicap tuning, AFC, etc.

[BrokenYugo]

FM broadcast usually uses a 10.7MHz IF.

Consider starting with a super regen, those have a much lower parts count, no IF cans or other carefully aligned filters, etc. Then move on to a superhet design.

Also be thinking of how you want to do demodulation, that can get complicated with FM and discrete parts.

Parts can be had new/NOS, doesn't take a ton of digging. Or scrap some dumpster boomboxes, those IF cans are fairly standard and typically color coded.

[Terry Bites]

For utimate simplicity have a look at the Si4730/31/34/35-D60.
TDA7088s etc are also simple but just as unsatisfying.

You could cook one from scratch- see attached. That would be something to be proud of.

BrokenYugo is right, 10.7MHz is vey much the norm. 10.7MHz IF transformers and filters are still widely available.

[Martinn]

You could cook one from scratch- see attached. That would be something to be proud of.

The Elenco kit looks great and is still available! Although it'll be > $100 with shipping and import, saving the hassle of finding the right components is probably worth it. Thanks!

[jbb]

I saw this (University course support material?) video series online which appears to be a great resource.

https://youtu.be/r_p7AHsSOdw?si=H-lUyYiEI5xgNl9d

[Martinn]

As it turns out, it is near impossible to get hold of the required (mainly filter) components for a FM radio build.
In the meantime, I purchased this kit from Ali and it was quite fun to build (and works quite well)!
https://www.aliexpress.com/item/1005003202947613.html

[Jwillis]

As it turns out, it is near impossible to get hold of the required (mainly filter) components for a FM radio build.
In the meantime, I purchased this kit from Ali and it was quite fun to build (and works quite well)!
https://www.aliexpress.com/item/1005003202947613.html

I just finished this one https://www.aliexpress.com/item/1005003506101273.html?spm=a2g0o.order_list.order_list_main.38.71191802toHcw3. Figuring out how some of the connections are made was somewhat of a challenge because markings on board are all in Chinese.
Yours looks like it had better translation.

[Martinn]

Yours looks like it had better translation.

Not really.
But the description was quite well made, particularly the alignment procedure description (once I ran it through Google Translate)!

BTW: I thought I could add a RDS (or RBDS depending on where you live) decoder board. But it seems these went firmly extinct, with even the decoder chips hard to come by.

[Conrad Hoffman]

One would only do this for the educational and entertainment value! I'd start by looking at schematics for things like the Sansui 2000x and similar era receivers. They were mostly discrete, though they used IC amplifiers in the IF chain. You can do discrete. Also look at the old Dynaco tuners. They were well known for not needing special test equipment to align them. You can still find dual capacitors and probably ceramic filters. You can wind your own transformers. Some of the MPX boards were discrete, so well within your capabilities. Go stage by stage, testing until each stage does what it's supposed to. Hook 'em together and you should have a radio!

[Benta]

Back in the '70s/80s, one of the tuner brands to go to was Larsholt.
I attach a schematic (and Danish text) on the Larsholt 7251 tuner module plus associated modules/circuits.
Might inspire you?
https://www.brugtgrej.dk/attachedfiles/20200223113028_5e525444a3856.pdf

[Jwillis]

BTW: I thought I could add a RDS (or RBDS depending on where you live) decoder board. But it seems these went firmly extinct, with even the decoder chips hard to come by.

A little over my head but I did find this, if it's the kind of thing your looking for https://www.skyworksinc.com/-/media/Skyworks/SL/documents/public/data-sheets/Si4706-D50.pdf

Seems a lot of once common semiconductors have gone the way of the Dodo in the past 3 or 4 years. Stupid lock down.

[newbrain]

A similar schematics, from the Italian "Nuova Elettronica" ("New Electronics") magazine.
Based on SO42P and TDA1200, varicap tuning, pptional stereo decoder.
SO42P is just a double balanced mixer, so I think that any equivalent would work.

The article gives indication on howto build the needed coils.

https://win.adrirobot.it/nuova_elettronica/pdf/nuova-elettronica-048.pdf

It works, I built it with a home made PCB many years ago.
Sensitivity was not exceptional, in the following issue they added a dual gate MOSFET based pre amp, for mono reception, though, it was quite OK.

[cncjerry]

You can build one with a mixer to form a downconverter, couple matching components, filter, and one of the SDR programs pretty much in an afternoon these days.  You can use a signal generator for the VFO. Not descrete of course, but it would also receive about everything.  Man, times of changed.

Jerry

[chilternview]

There was a Maplin design back in the late 80's for a FM radio based on the TDA7000 chip which only needs 2 simple coils and no extra filters:

https://www.worldradiohistory.com/UK/Mapelin/Maplin-Project-Book-09.pdf

I made one back then and still have it, it works surprisingly well in a good signal area.

[Mike244]

Hi,
just a tip: The FM radio is a fairly complex device and building it without having experience with HF is probably not the best idea...
I would advise you to buy a simple tuner(Kenwood/Sony/Sansui) from the early 70s and try to restore it to factory condition. I hope you have the necessary equipment: FM generator, sweep generator 9-12 MHz, tube or FET HF voltmeter?

BR
Mike

[robert.rozee]

Hi all, when I was a kid, I once tried building a FM radio [...]

you might like to have a look at something similar to this:
https://www.mikroe.com/ebooks/radio-receivers-from-crystal-set-to-stereo/fm-receiver-with-one-transistor-and-audio-amplifier

typing single transistor FM radio receiver into google (hint:look under images) gives a number of hits, many involving a very simple front end followed by an LM386 or similar audio amplifier driving a speaker. while these designs may not provide anything near stellar performance, they will give the learner the fun experience of hearing the sound of the 'local radio station' out of their creation. they get to wind a simple coil, and do a bit of soldering. there is even no harm  in using a pre-built LM386 module for the audio side.

what might be neat would be if someone modified such a simple front end to use a varicap diode for tuning. i read on one website about using a couple of red LEDs or 1N4001 diodes as varicaps:
http://www.hanssummers.com/varicap.html


cheers,
rob   :-)

[coromonadalix]

https://ca.robotshop.com/products/elenco-am-fm-radio-kit-combo-ic-transistor

[David Hess]

Ceramic filters for the 10.7 MHz and 455 kHz IFs are still available, and if you do not want to deal with transistor IF amplifiers, operational amplifiers can do it.  For FM demodulation I would use a PLL, or implement a Costas Loop for FM and AM demodulation.

Maybe have a little fun and use a higher first IF, or upconvert first.

There are plenty of inexpensive integrated PLLs and DDSs so I would skip analog oscillators except for crystal ones for the fixed LOs.

[Martinn]

Thanks for the many ideas!

I already built the elenco kit. Very nice, but strange combination - on the one hand, they explain how to read resistor color rings, on the other hand I have serious doubts that without understanding of how a FM superhet works and a decent set of instruments (RF source with modulation, scope or spectrum analyser) you get a single tone out of the FM part. Far from beginner level. With proper instrumentation adjustment is more or less straightforward, although pointless in the AM case - after I had finished the kit, I found out there are no more AM transmitters left. Shortwave radio is a different story though, but this kit does not cover SW.

My initial idea for designing a FM radio was to practise impedance matching, VNA work and maybe some smith chart magic. As it seems all discrete FM tuners use these square "filter cans" in various colors, which cover both inter-stage impedance matching and usually also filtering. These completely vanished from the face of the earth - including data sheets or design data. So one could design those filter/transformers from scratch - which I feel is effort going out of hand.

One interesting problem remaining would be RDS (RBDS) decoding. Have a look at the spectrum after the FM demodulator (from chinese kit):
Below the 19 kHz pilot tone there is the L+R signal, then left and right of the suppressed 38 kHz carrier the L-R signal for stereo (here you can see the signal is actually stereo). Above (second cursor) at 57 kHz there's the RDS data stream, 1187.5 Hz 2-PSK modulated.
https://en.wikipedia.org/wiki/Radio_Data_System
I wonder how one could demodulate that (not using a RDS decoder chip like TDA7330). Maybe a phase locked 38 kHz pilot tone is already available from the stereo decoder, if that would help.

SI4706: Amazing - just connect antenna and SPI interface - but where's the fun in that?

Maybe have a little fun and use a higher first IF, or upconvert first.

Interesting idea. Could you explain how you would do that?

TinySA also does an upconversion https://tinysa.org/wiki/pmwiki.php?n=Main.TechnicalDescription
to 433 MHz, then to second IF 870 kHz. I guess those values come from what filters are available.

[David Hess]

Maybe have a little fun and use a higher first IF, or upconvert first.

Interesting idea. Could you explain how you would do that?

Usually all stages convert to a lower frequency, but that does not have to be the case.  A wider tuning range becomes possible if the first mixer stage converts to a higher frequency.  The disadvantage is less gain because the IF frequency is higher, and the first IF filter is less selective, but microwave amplifiers and filters are readily available now and inexpensive, and a higher first IF makes preselection easier.

[MarkT]

Indeed there's a wide range of RF modules/amps/filters/mixers from companies like Minicircuits although perhaps a bit more pricey that fully discrete design.  Though it might be fun to make a Gilbert Cell active mixer (recreate the obsolete SA602 in discrete parts, basically), there are still useful looking chips - I found the AD608 which is active mixer plus IF strip, for instance, which can simplify things (if you keep it modular you can rework parts of the design in isolation which is handy).

These days you'd probably take the 38kHz wide signal and digitize directly with an audio ADC at 96 or 192kSPS and demodulate stereo using DSP.

[David Hess]

These days you'd probably take the 38kHz wide signal and digitize directly with an audio ADC at 96 or 192kSPS and demodulate stereo using DSP.

Just make sure that your ADC has a full power bandwidth to support the IF frequency if you are going to subsample it.  I think Analog Devices makes some delta-sigma ADCs for IF applications?  But sampling ADCs are commonly used for this.

Just to be clear, this means for example sampling the 455 kHz IF at say 192 kSamples/second which is way below the Nyquist frequency, but more than twice the IF bandwidth.  The sampling ADC becomes a free mixer with a local oscillator equal to the sampling frequency.  Samplers and mixers are essentially the same thing.