Some items that may help or hinder – I’m not sure which.
Firstly, a generalized quieting curve, which shows the expected initial increase in noise as signal level increases, followed by decline.
Secondly, the quieting curve for the Marconi HR92, HR93 series of point-to-point SSB/ISB receivers.
Here the noise curve shows no initial increase, just a steady decline. My initial thought that was this might have been because these receivers had only coherent demodulation, not an envelope demodulator of any kind.
That was confirmed by the quieting curve for the GPO SSB-ISB receiver (I think Radio Receiver #22, or a derivative thereof), which was designed to a similar target to the Marconi HR92, HR93.
But then this from the same source as the first, generalized curve.
Here the shape of the noise curve depends on the number of RF amplifiers. With two, there is no hump at all. The same chapter of the book, written by a Marconi staffer, goes on to describe what is surely a Marconi communications receiver, which I think might be the CR150/5. I imagine that the curves were obtained via the AM demodulator, a diode. The described receiver has a typical Marconi RF amplifier section, with a Z77 (EF91) followed by a W77 (EF92). As best I can work out, the HR92,93 would have been similar. Anyway, it suggests that the type of demodulator might not be the key factor as I first thought.
That all leaves me as confused as ever, although with one thought that the mixer might be the guilty party as far as causing the “noise hump” is concerned, and that where it is preceded by RF amplification, the hump is pushed down the scale relative to input signal strength until perhaps like the Oozlum bird, it just disappears…
Quieting curves for FM receivers seem not to show a noise hump.
Cheers,