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Old 1st Jan 2010, 6:52 am   #13
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Homodyne/Direct Conversion

I suspect that the homodyne works well for NBTV for essentially the same reasons that homodyne (quasi-synchronous) demodulators become very commonplace, and eventually more-or-less standard in domestic colour TV receivers once suitable IC technology was available and economic.

The PLL approach would seem to work with VSB signals as evidenced by the existence of TV IF/demodulator ICs such as the LM1823. In the latter case, the carrier is not filtered prior to feeding the PLL control circuitry. Rather, the PLL itself sets the effective carrier channel bandwidth, and at a much lower level than would likely be possible with a filter. I am not sure if the fact that the incoming carrier is on the Nyquist slope of the IF bandwidth curve has much effect on the quality of the reference carrier generated by the PLL. But my understanding is that National developed the LM1823 in part to allow production of a sufficiently “clean” intercarrier signal for stereo sound purposes, something not achieved with the standard homodyne demodulators, and which more frequently has resulted in the use of quasi-split sound techniques.

Many years ago I had the good fortune to have some dialogue with Ed Forster, the designer of and patent holder for the Liniplex HF receivers. In summary, the use of appropriate PLL circuits (in this case a tracking type) was seen as a better choice than the use of filters, although it did require accurate AF phase-shifting circuits to derive the USB and LSB signals post-demodulation. In connection with an item early in this thread, Ed noted that the Liniplex (and similar) receivers could not follow any phase modulation that might be on AM signal, such as with the BBC Droitwich LF transmitter, so its presence would result in some low-level distortion.

When homodyne demodulators are used with AM signals whose negative modulation depth can approach 100%, really hard limiting is required to develop a sufficiently clean reference signal. But it seems to be possible. As far as I know, Motorola used a homodyne demodulator – with unfiltered carrier - to obtain the envelope signal in its original C-QUAM AM Stereo station monitor chain, so one assumes that it worked well enough with the heavily modulated (and heavily compressed) signals that were/are characteristic of North American broadcasting practice. The Hershberger/Popular Electronics homodyne/synchrodyne demodulator design I mentioned in my earlier posting probably also had effective limiting for the homodyne section – I think that Hershberger was the main architect of the Harris AM Stereo system, so presumably knew what he was doing. It is reasonable to assume that ICs such as the SL625 and TDA1071, both having HF receiver applications, had adequate limiting. From what I can deduce about the TDA1071 configuration, it might make the basis for a simple TRF receiver with homodyne demodulator.

Homodyne demodulators have also been used for P-type television systems with positive vision modulation and AM sound, where negative modulation depth can approach 100%. The TDA2542 IC is a vision IF amplifier and demodulator, I think the P-system counterpart to the TDA2541, and incorporates the customary tank circuit between the limiter and the demodulator multiplier. The TDA2543 is a sound IF amplifier and demodulator for AM TV sound, and likewise incorporates a reference carrier tank circuit. On the other hand, the “multistandard” TDA3845, with both FM (quasi-split) and AM channels, does not include a tank circuit on the AM side. So I remain somewhat mystified as to the reasons why the tank circuit is used in some cases, but not in others.

Where limiting is not quite adequate for heavily modulated signals, the homodyne demodulator would tend to act as a squarer, which would introduce distortion. Adjacent channels also cause some distortion, as they modify the parameters of the limited reference carrier. Hence the need for adequate pre-demodulator selectivity in the homodyne case.

Cheers,
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