Thread: FM "deviation"
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Old 2nd Apr 2010, 2:49 am   #34
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
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Default Re: FM "deviation"

Quote:
Originally Posted by G8HQP Dave View Post
There is a world of difference between maintaining accurate P and P+90 over 300-3kHz for communications purposes, and the requirements of a decent stereo receiver. Probably the best method for communications is polyphase, but that would be useless for stereo as P is very frequency dependent. Getting accurate phase and amplitude tracking of three signals at F, 38-F and 38+F is hard! Nowadays I suppose you could do it with a digital filter.
Agreed that the examples I have quoted, basis familiarity, would not necessarily scale up to wide band audio.

Still, the PLL, phase-shifting and matrixing circuitry used in the Sony ICF2010 was in fact taken direct from that which Sony developed, inclusive of dedicated ICs, for AM stereo hi-fi tuners with wideband audio (and which was also used in the SRF-A100 portable radio receiver). Phase-shifting and matrixing was required for full decoding of the Kahn-Hazeltine ISB AM stereo system. Sansui was amongst others who developed multiple system AM Stereo decoders inclusive of wideband phase-shifting and matrixing. It also used the same techniques in its landmark TU-X1 tuner for AM sideband selection. This predated its AM Stereo models, and evidently remains a much sought after model in the second-hand market. To a large extent that may be a reflection of its FM performance, although it is usually noted as having exceptional AM performance. Sansui TU-X1 original literature is available at several places on-line, such as at: http://www.goodsoundclub.com/Forums/...px?postID=4649

PLL/phase-shifting/matrixing circuits for AM sideband separation have also appeared in home construction magazine articles, including by Hershberger (Popular Electronics April, 1982) and Brook (Electronics & Wireless World September, 1989), both authors having, I think, good credentials. The Hershberger design included relatively elaborate three-stage phase-shifters that were said to be accurate to plus/minus 3 degrees over the band 50 to 12 000 Hz, and provided at least 31 dB of unwanted sideband rejection. The need for appropriate component selection and matching – including the avoidance of carbon composition resistors – was heavily stressed. The Brook design was simpler in this aspect, being of medium bandwidth, namely 7 kHz.

How well any of these systems worked or how well they maintained their performance over time I am not in a position to know. But it seems not unreasonable to suppose that those equipment makers who had developed what they believed, however erroneously, were adequate AM stereo decoding circuits inclusive of phase-shifting and matrixing following quadrature PLL synchronous demodulation of a (typically) 455 kHz carrier would not, had they so chosen, have applied the same techniques to similar treatment of the 38 kHz FM subcarrier with the objective of extracting the lower sideband. In fact PLL synchronous demodulation of the subcarrier had become pretty much normal from the early 1970s, following the availability of the RCA CA3090 IC and more so the Motorola MC1310 IC and its many derivatives. So moving to quadrature demodulation would not have been such a big step. In the FM stereo case, where the objective would be to reduce the influence of the noisier upper sideband, the crosstalk target might even have been milder than in the AM case, where either stereo separation was directly affected, or it was desired to suppress a sideband suffering from interference. Here’s an interesting question – clearly random and unequal phase-shifting errors in the respective I and Q demodulated signal paths would affect the relative amounts of the lower and upper sidebands that found their way into the nominally lower sideband (L-R) signal after matrixing, but would the net signal then have major phase errors that in turn would compromise the final matrixing with the baseband (L+R) signal? Or would it be simply the case that the LSB/USB mix in (L-R) varied somewhat over the audio band, but basic (L-R) integrity was retained.
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I still have not been able to trace the make and model of the FM tuner that did use SSB subcarrier demodulation, but surely it will come to light sooner or later.

Cheers,
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