[Synchrodyne]

Thanks much, RW. Yes, it was indeed hidden in full view.

The 2 MHz reference feed into the AFC section must come from the frequency standard section, as nothing else would make sense. Then of course it follows that the same 2 MHz feed was used for the sideband demodulators. And as you say, the AFC system, working back through the frequency control system, served to slew and lock the recovered carrier to the 2 MHz reference.

So the Marconi H2900 system was a bit different to an “independent” PLL demodulator system, in which the demodulator oscillator is locked to the incoming carrier, and to some small extent at least moves with it as it wanders, whether inherently or because of receiver conversion oscillator drift.

Cheers,