Timeline for Could a vintage 9 transistor AM/FM implement a phase-locked loop of some form?
Current License: CC BY-SA 3.0
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| Feb 20, 2018 at 17:58 | comment | added | Dave Tweed | PLLs were used for FM demodulation long before they were used for frequency synthesis. There are at least two ways to do it: 1) make the loop bandwidth relatively wide, and take the audio output from the VCO control voltage, or 2) make the loop bandwidth relatively narrow and take the output directly from the phase detector. The latter only works with narrowband FM/PM (or digital FSK/PSK), in which the carrier phase can't slip by more than one cycle as a result of the modulation. | |
| Feb 20, 2018 at 17:44 | history | edited | Bimpelrekkie | CC BY-SA 3.0 |
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| Feb 20, 2018 at 17:42 | comment | added | Bimpelrekkie | @DaveTweed A PLL does not necessarily require a frequency divider. True, but I have never seen a design for an FM receiver with a PLL-based synthesizer without a frequency divider. The point of the frequency divider is that a reference frequency (for example from a crystal oscillator) is multiplied such that it can be the LO signal for the receiver. But I'll update my answer with "for a receiver". | |
| Feb 20, 2018 at 14:23 | comment | added | Dave Tweed | A PLL does not necessarily require a frequency divider. Besides which, all we're really talking about here is automatic frequency control, a simple form of frequency locked loop. | |
| Feb 20, 2018 at 12:19 | comment | added | uhoh | @glen_geek Indeed! Perhaps that is what I am partially mis-remembering, that AM/FM/FM-AFC three-position slide switch on some older transistor desktop radios of this era. Consider posting a supplemental answer, or a link? I can ask a new question if it helps (e.g. how did AFC work on early FM transistor radios?) edit: ...although I have just found this and this answer. | |
| Feb 20, 2018 at 12:07 | comment | added | Edwin van Mierlo | @DanMills fair-enough... I am here to learn as well. Thanks for the update | |
| Feb 20, 2018 at 12:05 | comment | added | Dan Mills | @EdwinvanMierlo I am afraid you do not understand FM modulation. What you say would be correct for an AM transmitter, but the spectrum occupancy of FM is way more complicated, being a set of sidebands of amplitude defined by the bessel function of the modulation index for simple tones and something much nastier for anything more interesting. The 200KHz channel spacing on FM predates stereo, and is a compromise even then.... | |
| Feb 20, 2018 at 12:02 | comment | added | glen_geek | A nine-transistor radio could have an AFC circuit (auto-frequency-control). Not quite a frequency-locked-loop, but it does tend to track the carrier. | |
| Feb 20, 2018 at 10:58 | comment | added | Bimpelrekkie | @EdwinvanMierlo Where did you get that 30 kHz from? The channel spacing of the FM band is 200 kHz (so 2.6 KHz is a little more than 1%) and the actual BW used depends on the modulation depth. To make an LC oscillator (as used in simple FM receivers) and have it vary less than 1% over temperature is impossible in my opinion. 10% over temperature would be doable perhaps. Better FM radios use AFC to compensate for this temperature drift. | |
| Feb 20, 2018 at 10:45 | comment | added | uhoh | I'm not interested in proving you right or wrong, but this has got me reading further about the electronic PLLs of the 1930's '40s and '50s which means my productivity for the rest of the day is now completely shot. (e.g. 1, 2) In the mean time, it does seem that I was thinking of FM detection in general, not any implementation of PLL. | |
| Feb 20, 2018 at 10:02 | comment | added | Bimpelrekkie | Your question is about PLLs in radios. How old the principle is and that one could make a mechanical PLL is irrelevant to your question. A mechanical PLL cannot receive radio signals nor does it mean that being an old principle less than 9 transistors are needed to build a PLL. But please prove me wrong and show me a 9 transistor (or less) PLL suitable for an FM receiver. | |
| Feb 20, 2018 at 10:00 | comment | added | uhoh | en.wikipedia.org/wiki/Phase-locked_loop#History I know current PLLs are sophisticated, but the principle and function of a phase-locked loop is fairly old, isn't it? | |
| Feb 20, 2018 at 9:58 | history | answered | Bimpelrekkie | CC BY-SA 3.0 |