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Old 25th Dec 2022, 6:33 am   #1
dmowziz
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Default Simple Direct Conversion FM Receiver 88-108

Hii

Please, any insight, wisdom you can share will be great. Thanks


Firstly, please what is wrong with the schematic so far?
I'll add a limiter to the input. I want to study that this morning


From the mixer to the 3-section AF amplifier, will add a low pass but not sure what cutoff frequency yet.
After the AF amplifier then a discrete demodulator, to an audio jack to speaker

Please what antenna can I use?
I have a telescopic antenna from an FM board but no connector to it. is this good?
I remember making my first HF radio.. Trying to make it work (days, maybe up to weeks). After so many tries, realized the antenna was the problem
Then I thought, an antenna might be as important as the receiver


The first RF amplifier, no values to the components yet, I'll design that soon this morning so C17 and C18 will be combined.


I've done the oscillator (I'll redo the tuning part) and mixer separately. This mixer has a conversion loss of about 10 dB (even by simulation). I think there's something wrong with the topology. yeah? I can post the simulator output if it helps


Please for the mixer, the toroids. What inductance value should they be? How this value relates to frequency? I used a FT50-43. The toroid I have (which I think isn't good for VHF)

The AF amplifier (using 2N3904BU's), about -20 dB return loss from 92 to 1000 KHz. 50dB gain
Designed on simulator but no prototype




It seems the first wisdom I need is how to post the schematic without it being blurred

Last edited by dmowziz; 25th Dec 2022 at 6:56 am.
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Old 25th Dec 2022, 7:13 am   #2
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Default Re: Simple Direct Conversion FM Receiver 88-108

***I remember making my first and only below average HF radio..


Please here's a link to the schematic : https://bit.ly/3FNBVX8


Thanks thanks thanks thanks thanks thanks. Thank you very much
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Old 25th Dec 2022, 11:05 am   #3
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Default Re: Simple Direct Conversion FM Receiver 88-108

Quote:
Originally Posted by dmowziz View Post
I used a FT50-43. The toroid I have (which I think isn't good for VHF)
Fair-Rite number 43 ferrite material is no use for inductors at 100MHz and is getting fairly lossy as a transformer core. Above about 100MHz, it's best thought of as an RF absorber and a lot of it (and other brand equivalents) is used to absorb EMI.

Direct conversion receivers naturally demodulate double-sideband signals. They are usable for morse and single sideband signals, for example in the amateur band, but you also hear the frequency inverted image band.

If you try using one for FM, the receiver cannot tell the difference between frequency excursions above or below the tuned centre frequency. So your audio comes out full-wave rectified! with stupendous levels of distortion.

To differentiate between upper and lower sidebands, you need to have two receiver paths operating in quadrature, and then combine them either with a full-audio-band 90 degree phase shift network (no easy task for speech, worse for the bandwisth of music) or by DSP using a Hilbert-transformed filter.

The carrier leak at your mixers will create a strong interferer at 0Hz and this is usually combatted by AC coupling the two receiver paths, but this gives you a low frequency cut which can wind up cutting the low end of the audio bandwidth once you've taken care over rejecting the local oscillator phase noise.

So with the exclusion of IF DSP type radios (Which are superhets anyway) direct conversion FM reception is dramatically more difficult than AM SSB CW reception and it pushes you into compromises which greatly reduce the performance.

There are radios where conversion to effectively zero IF is used, and which perform well, but they have a number of fancy tricks and live almost entirely within the digital domain.

Have you been confused by the number of amateur radio direct conversion receivers there are, which can be useful, simple and cheap so long as you're wary of the image ambiguity? They do suit this quite well, but receiving/demodulating FM is a completely different matter. Direct conversion doesn't mean simple unless the mode you want to receive allows it.

There are simple SDR receivers with quadrature RF channels feeding a PC soundcard. They work, but their performance is limited by the accuracy of gain and phase matching of the two channels. Unwanted stuff is not filtered out in the normal sense, it is cancelled... any small error in the accuracy of cancellation creates a very large increase in the uncancelled residual.

David
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Old 25th Dec 2022, 4:32 pm   #4
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Default Re: Simple Direct Conversion FM Receiver 88-108

Could you perhaps use a pulse-counting discriminator with a near-zero IF? It wouldn't be hi-fi.

The other alternative I guess is the 'beer barrel discriminator' which used a really high-Q tuned circuit [typically a copper tube 'line' in something like a beer-barrel] and slope-detection.

http://electronbunker.ca/eb/FMCrystalSet.html

https://vacuumtubesinc.com/fmxtalsetpage
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Old 25th Dec 2022, 4:56 pm   #5
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Default Re: Simple Direct Conversion FM Receiver 88-108

[QUOTE=[I][I][I]G6Tanuki;1523828]Could you perhaps use a pulse-counting discriminator with a near-zero IF? It wouldn't be hi-fi /QUOTE]


Does anyone else remember this “Fidelity FM Tuner' article and circuit from PW in April 1965 (page 1136) ? –
https://worldradiohistory.com/UK/Pra...PW-1965-04.pdf
It features a pulse counter discriminator, virtually eliminates any alignment procedures, and (the author claims) offers a standard of quality not (then) available in any other way. It’s also a (comparatively) extremely simple circuit. This circuit has always intrigued me for its simplicity and cheapness. The author did develop a solid state version that was also published in PW (August 1967, p.250).
I did build it at the time, and it worked well (probably helped by our insanely good, line-of-sight reception from North Hessary Tor), but suffered from, I think, oscillator squegging. I was at the time a teenager with O’- levels looming on the horizon, and I didn’t get the chance to pursue it any further.
I’d be interested to know if anyone else has tried it.

I hope this isn't taking this thread off topic - it was the mention of a pulse-counter discriminator that prompted it.

Mike

Last edited by Boulevardier; 25th Dec 2022 at 5:18 pm.
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Old 25th Dec 2022, 5:58 pm   #6
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Default Re: Simple Direct Conversion FM Receiver 88-108

Sinclair did an FM tuner as a single phase-locked loop.

Standard FM demodulators can even be made at signal frequencies, but tuning them becomes an issue and other factors give poor performance unless you can have very high Q resonators in the demodulator. FM demodulators often need to hide behind a high-performance limiting amplifier.

david
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Old 29th Dec 2022, 11:29 pm   #7
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Default Re: Simple Direct Conversion FM Receiver 88-108

Thanks "guys" for the feedback

I did not read on FM but right now, just want to get a demodulated output without caring for much understanding and calculation from it until it works.

The schematic I am "following" (different mixer, amplifiers) is attached below. The demodulator is similar to what G6Tanuki posted?


Thanks Mike. I am sticking to a low part count demodulator (first image) just to get something working now.
Later on in the thread, I'll understand FM more and the demodulator part of the circuit you mentioned. PW August 1967. I attached it


@David. Please can you help in posting the schematic from the 2nd post? It gets blurred when I post it (Maybe file size). Thank you

Having issues with my first amplifier
https://www.vintage-radio.net/forum/...44#post1524544
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Old 31st Dec 2022, 9:44 pm   #8
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Default Re: Simple Direct Conversion FM Receiver 88-108

Philips TDA7000 - direct conversion FM receiver.

https://spectrum.ieee.org/chip-hall-...00-fm-receiver
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Old 1st Jan 2023, 9:18 am   #9
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Default Re: Simple Direct Conversion FM Receiver 88-108

The TDA7000 was not direct conversion. Rather it used a very low IF, namely 70 kHz, and by feedback to the VFO, reduced the deviation as seen by the demodulator to +/-15 kHz.


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Old 1st Jan 2023, 11:04 pm   #10
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Default Re: Simple Direct Conversion FM Receiver 88-108

Hi.
the OPs schematics in post 7 are both pulse counters, and are superhetrodyne. The incoming signals are converted to a low IF and amplified using untuned rc coupled amplification. The method of detection is simple, in that the signal from the mixer is amplified to limiting, giving effectively a pulse width modulated constant amplitude into a charge pump circuit. That in turn outputs a mean level dc approximately proportional to the duty cycle of the signal output from the IF stage. Providing the charge pump circuit output is kept to a lowish amplitude, the linearity and therefore fidelity of the resultant AF can easily equal that of the more common forms of demodulation.

There are some drawbacks of course, low IF means image rejection is non existent, they are touchy things to tune, and in their simple form can't be used to resolve stereo. But they do work. There have been constructional projects in the vintage hobby magazines for double conversion pulse counting receivers, however they are complex beasts.

The only much simpler system that can be used to resolve FM is the super regenerative circuit. Even with careful designing they tend to be noisy distortion ridden things. (Using slope detection for FM as they do), You can make them just about good enough for entertainment quality, but they are infuriating fiddly things, despite their simplicity of construction and low component count. Perversely, the self quenched super regenerative detector is as an incredibly difficult process to adequately explain as it is constructionally one of the most simple circuit to provide a working receiver.

Greg.
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Old 2nd Jan 2023, 12:02 am   #11
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Default Re: Simple Direct Conversion FM Receiver 88-108

There are some very reputable VHF/FM stereo tuners with pulse-counting discriminators, though not relying on down conversion to a very low IF and little other selectivity.

David
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Old 4th Jan 2023, 6:41 am   #12
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Default Re: Simple Direct Conversion FM Receiver 88-108

Quote:
Originally Posted by Radio Wrangler View Post
There are some very reputable VHF/FM stereo tuners with pulse-counting discriminators, though not relying on down conversion to a very low IF and little other selectivity.

David
After reading your comment I have been avoiding looking them up, trying to think of one for a memory exercise, (I have met some), the only name that came to mind is Fisher?

If it's the outfit I'm thinking of, the gear was built like a tank and sounded excellent. Company also did a lot of industrial measurement and control instrumentation ISTR.

I'm sure there's someone who can verify or correct that recollection.
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Old 4th Jan 2023, 12:29 pm   #13
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Default Re: Simple Direct Conversion FM Receiver 88-108

Kenwood AKA Trio did a great range of pulse count tuners really superb I have a KT601 in my collection
for the time state of the art
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Old 4th Jan 2023, 12:41 pm   #14
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Default Re: Simple Direct Conversion FM Receiver 88-108

Revox.

David
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Old 18th Jun 2023, 11:45 pm   #15
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Default Re: Simple Direct Conversion FM Receiver 88-108

Quote:
Originally Posted by G.Castle View Post
the OPs schematics in post 7 are both pulse counters
While you are correct about the latter receiver (PW Aug 1967), I believe you are incorrect in describing the other receiver (from S53MV) as a pulse-counting FM detector.

As Matjaz states in his website description, this receiver features a 200kHz IF with 3 limiter stages. The two diodes coupled to the last limiter by the 100pF and loaded by the 1k resistor is a stunningly cunning frequency-to-voltage converter. As he notes, since the impedance of the 100pF capacitor decreases as the frequency increases, the output voltage of the rectifiers will be directly proportional to the frequency. That demodulates the FM signal in a similar manner to conventional FM discriminators.

This works well with high deviation broadcast FM signals but I suspect it would not be as efficient for signals with, say, 5kHz peak modulation deviation such as is used on 2m amateur radio transmissions. That said, it'd be interesting to try.
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