[Synchrodyne]

The origins of the infinite impedance demodulator were recorded in a Wireless World article in the 1937 January 01 issue, page 08ff, entitled “New Detector Circuit – Applying Feedback to the Anode Bend Rectifier”, by W.N. Weeden.

Evidently the circuit was announced by Hygrade Sylvania after development in the RCA Licensee Laboratories. It was described thus:

“It is actually a triode anode-bend detector having a high resistance load in its cathode circuit. This load resistance is by-passed for carrier frequencies, and as it is common to both anode and grid circuits, degeneration of negative feed-back plays a large part in its freedom from harmonic distortion.”

The preamble discussed the diode demodulator and in particular the AC-to-DC load ratio issue, including the comment “The ideal condition is obtained when this ratio is unity, which can be realised only with direct coupling.” So quite possibly the idea of direct coupling a diode to a cathode follower was known in those days.

One of the concluding comments was: “..the new detector should prove superior to any but the direct-coupled diode, and even then, should win out because of the freedom from damping of its input circuit.” I think that overlooked the fact that freedom from damping was not necessarily a virtue. In high quality receivers, the final IFT often had the role of infilling the gap of a double-humped IF response developed by the preceding IFT(s), for which a modest loaded Q was often required.

More recently, WW 1978 September carried an article “The F.E.T. as Detector – Improvements Over the Diode Detector”, by Amos the Younger (Roger B.) This discussed various configurations, but most emphasis was placed upon the fet version of the infinite impedance circuit, including mention of its tendency to oscillate if not carefully designed and built. A more general discussion of AM demodulators was provided by Amos the Elder (S.W.) in WW 1980 April page 68ff.


Cheers,