Electronic – Identify discriminator circuit


The Dynaco FM-3 tuner uses what appears to be a Foster-Seeley discriminator with one diode reversed, or, if you prefer, a ratio detector without the third coil and stabilizing capacitor. The Dynaco marketing material and user manual uses the term 'discriminator' consistently throughout.

I don't find this variant discussed in the RDH, or Cook & Liffey, which are my only FM texts of enough depth to cover this sort of thing. Does it have a name? How does it work? And what are its salient properties?

Schematic (also here):

enter image description here

Best Answer

Answering my own question here.

It is indeed a Foster-Seeley discriminator, by the arrangement of the coils and the inductive and capacitative coupling between them, which produces the characteristic voltage-difference AM, but the method of demodulating the AM is different by the reversal of one of the diodes.