Re: Quasi-Synchronous Demodulation
That phase noise issue is nicely illustrated in the block schematic for the BBC RC1/511 TV receiver, copy attached.
This had a consumer-type front-end module (Mullard U321) and used quasi-synchronous vision demodulation in which the carrier IF channel was separated from the main IF channel after amplification and processed through a narrower bandwidth filter.
The highlighted part of the schematic includes the carrier filter, with the caption “B.W. ½ MHz centred on vision carrier (allows Ø noise through).”
Other important QS demodulation features shown are the carrier channel phase adjustment ahead of the filter and limiter, the filter ahead of type limiter, and the limiter being of the low-phase shift type.
Not shown, but I imagine very likely included in the carrier channel, perhaps simply by shaping/offsetting of the bandpass filter, would have been an “anti-Nyquist” filter that cancelled the Nyquist slope imparted by the main IF filter and so eliminated spurious PM from that source. The 500 kHz filter bandwidth was well within the DSB portion of the vision signal (±1.25 MHz) so there was no PM from inherent sideband asymmetry.
The carrier channel filter bandwidth chosen by the BBC, 500 kHz, was that same as that suggested by National Semiconductor for the PLL bandwidth of its LM1823 IC to enable it to better deal with ICPM.
Cheers,
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