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Old 29th Dec 2009, 5:32 am   #10
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Homodyne/Direct Conversion

The Linsley-Hood synchrodyne receiver was described in Wireless World (or by then Electronics and Wireless World) issues January, February and March, 1986. As well as the final design, the article discusses some of the practical issues involved in making the synchrodyne concept workable.

A combined synchrodyne/homodyne receiver was described by Slifkin and Dori in Electronics World of November, 1998. And a simple homodyne receiver, based upon the MC1330 television vision demodulator IC, was described by Herbert in Wireless World of September, 1973.

Although true direct conversion synchrodyne and homodyne receivers are quite rare, the use of synchrodyne (fully synchronous) and homodyne (quasi-synchronous) demodulators in conjunction with conventional superhet receivers seems to have been reasonably widespread. The need for amplification ahead of demodulation, and the non-linearities inherent in such indicate that reasonable pre-demodulation selectivity is desirable, and for the homodyne case, suppression of adjacent channel signals is also desirable as these will demodulate directly (although I think to a lesser extent than with envelope demodulators) as well as translate to out-of-AF band signals.

Macario described a homodyne demodulator, using hard-limited carrier as the reference frequency, in Wireless World of April, 1968, and commented upon its performance when used in conjunction with an Eddystone 940 receiver. Brook described an add-on phase-locked loop synchronous demodulator in Electronics and Wireless World of September, 1989, and Hershberger described another in Popular Electronics for April, 1982. The latter also included a homodyne demodulator for “envelope” demodulation, as well as post-demodulator sideband selection by AF phase-shifting.

Use of homodyne demodulators based upon hard-limiting of the IF signal to obtain the reference carrier seems to have become more common from the later 1970s onwards in commercial and consumer receivers. One example was the Eddystone 1570/1590, which used the apparently multipurpose (but mysterious – no information available on the web) TDA1071 IC. As well as IF and AGC functions, it seems that this IC could also be configured for both homodyne AM demodulation and quadrature FM demodulation. The Plessey SL625 IC was similarly configurable in respect of demodulation functions. Homodyne demodulators as used in AM receivers seemed mostly to use hard-limited carrier as the reference without a tank circuit between the limiter and the demodulator itself. However, in the hi-fi world, the second version of the Australian Allen Wright Wideband AM tuner (MF only) included an MC1330-based homodyne demodulator as well as a precision rectifier envelope demodulator. (The original version was designed to match or better, in solid-state form, Quad AMII/AM3 tuner performance.)

However, the use of homodyne demodulation in production receivers predates the solid-state era. The point-to-point professional ISB receivers of the 1950s, such as the Mullard GFR 552 and a similar Marconi model whose designation I do not know. These extracted the pilot carrier in a 60 Hz bandwidth filter (at 100 kHz second IF) prior to its limiting and use as the reference in the sideband demodulators. They also had selectable bandwidths of 3.5 and 6 kHz, the latter allowing their use in SSB and ISB broadcast quality program links. I imagine that these or similar receivers were also used for receiving regular shortwave broadcasts for rebroadcasting purposes. The late 1960s solid-state Marconi Hydrus ISB receiver might have offered similar facilities, but there seems to be little information about it.

A professional HF/ISB receiver with an apparently early synchrodyne demodulator application was the Eddystone EC958/12, circa 1974. This retained separate USB, LSB and carrier filters with their separate IF amplifier strips, but used the recovered carrier to drive a phase-locked loop. I think that other commercial receivers of the period, such as some of the Plessey models, may have been similarly configured, but I am not sure.

For consumer and semi-professional receivers, the use of synchrodyne demodulators, based upon phase-locked loop techniques, and usually with a selectable sideband facility based on AF phase-shifting, seems to date from the later 1970s. The Sansui TU-X1 hi-fi tuner was an early example. Perhaps one of the better known was the Sony ICF2001D/2010 shortwave portable , which used ICs originally developed for AM stereo decoding purposes (and used, for example, in the SRF-A100 AM stereo portable receiver) to provide selectable-sideband synchronous demodulation. An outstanding example was the Liniplex F2 (and preceding F1) with its tracking phase-lock loop demodulator. Having used a Liniplex F2 (feeding a hi-fi system) for extended shortwave program listening, I would say that it was an excellent tool for that purpose. Sherwood in the USA offered (and I think still does) an outboard phase-locked lop synchrodyne demodulator unit, and I have also used one of these in conjunction with a JRC NRD525 receiver. It was not quite as good as the Liniplex F2, though.

One of the most common uses of the homodyne demodulator in superhet circuits was in consumer television receivers, as a vision IF demodulator. The Motorola MC1330 IC, circa 1969, seems to have set the pattern, using hard-limited IF signal to generate the reference carrier, which was then fed via a tank circuit to the demodulator multiplier. That the vision carrier being limited was on the Nyquist slope of the IF curve does not seem to have been much of an issue until the advent of stereo sound systems. Still, that said, the BBC RC1/511 TV receiver, described briefly in Electronics & Wireless World of July, 1984, used a more elegant approach with the carrier IF separated after amplification and then conditioned by filtering and phase-shifting before being used for vision demodulation and intercarrier generation. There was also some use of synchrodyne demodulators for vision IF, as in the RCA CA3136E and national LM1823 ICs.

Clearly the advent of integrated circuitry made it much easier to include homodyne and synchrodyne demodulators in consumer receivers. Still, some early American colour TV receivers included phase-locked loop techniques in their chroma demodulator sections, based totally on valve technology. (Carnt and Townsend Volume I describes some interesting examples.) And if one allows that quadrature-type FM demodulators are within the synchronous family, then these go back well into the valve era. The gated beam valve (6BN6) and nonode (EQ80) would appear to lean more to being of the homodyne type, whereas the later locked-oscillator (EH90) type looks more like a synchrodyne. Insofar as single valves did the limiting and demodulation jobs, they were early examples of “integration”. And FM quadrature demodulation was an early function to be integrated. Back in the 1970s, I had a Bang & Olufsen 24 inch monochrome TV receiver whose circuitry was all discrete solid-state except for the TAA570 FM quadrature demodulator. Returning to the valve circuits, one assumes that for FM demodulation, close phase-locking of the reference carrier was not required, as it seems that such simple circuits would not have worked well for AM demodulation. Perhaps an interesting experiment could be done with one of the Eddystone receivers, such as a 940, equipped with a self-oscillating SSB demodulator (EK90), but tuned to an AM signal. Maybe the oscillator would lock well enough to the incoming carrier to allow synchronous demodulation, but I suspect that it would be more a demonstration of the inherent difficulties in getting the synchrodyne system to work, particularly with fading and Doppler-shifting HF signals. Still, RF injection to synchronize the oscillator was what was proposed in an August, 1948 Wireless World article on the synchrodyne by “Cathode Ray”.

Cheers,
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