Thanks Steve. Now I get the idea of quieting curves. Quieting is a kind of like a squelch function in HF transceivers; a circuit detects the condition when there is no signal and it filters out the hiss. Attached is the SNR versus Rf input for different receivers from the Ofcom report on page 28 . It is essentially the same graph:
https://www.ofcom.org.uk/__data/asse...10/annex-f.pdf
This is the first time I heard of WBRD. You are right that Radford FMT1. It has a WBRD as there is a 8.2K resistor across the primary tuned circuit of the ratio detector (see attachment 2). I built a few cheap DIY FM radio kits with IC chips using quadrature detectors or PLL (e.g. sc3610d, cxa1691, SC1088 ).
I didn't know that 6CS6 heptode ended up in FM demodulation application. In fact, I built my first MW superhet using a 6CS6 as a frequency convertor.
PS: BTW it is possible to measure the gain, one db compression point, 3rd order intermodulation intercept point, noise figure of an amplitude using a TinySA:
https://youtu.be/HJKjrEZRAKg
Even it is nothing like the real deal, this is something I will try next.