[Synchrodyne]

In the valve era, locked oscillator type synchronous (quadrature) demodulation does seem to have been used more for FM than for AM.

Possibly the first such circuit used in production receivers was that developed by Bradley of Philco c.1946. For this Sylvania developed a purpose-designed heptode valve, namely the FM1000. Philco used this demodulator for a few years, but it did not appear to have sustained success.

In part it was the outcome of a quest for a single-valve circuit that combined limiting and FM demodulation, and preferably with AF gain as well, and was one of several approaches that appeared following WWII.

Another was the gated beam-valve circuit, a joint Zenith and GE effort, and for which a special valve, the 6BN6 was developed. This was a non-oscillating self-limiting quadrature demodulator, founded upon the earlier work by Kalmus and Zakarias. I think that it was used in some American TV receivers through the 1950s.

And yet another was the Philips enneode circuit, also of the non-oscillating, self-limiting quadrature type, based upon the earlier work of Okrent. For this the EQ40/EQ80 enneode valve (misnamed by Mullard as a nonode) was developed. It was not used long-term.

In 1954 RCA introduced its own locked-oscillator FM demodulator, based upon a dual-control pentode valve, the 6DT6. This was actually a hybrid circuit, acting as a locked-oscillator demodulator with lower level input signals, but as a quadrature demodulator, with oscillation suppressed, at higher signal levels. I think one of the objectives was to do the job with a lower cost and more conventional valve than the 6BN6. As best I can determine this was used through to the end of the valve era, and improved dual-control pentode valves were developed, such as the 6GX6 in 1961.

Quite late on, early 1960s I think, Philips/Mullard introduced a locked-oscillator FM demodulator based upon the EH90 heptode valve, essentially for TV sound applications. To some extent this appears to have been a reincarnation of the Philco circuit. The EH90 was the Pro-Electron designation for the American 6CS6. This, a sharp cutoff short grid-base heptode, had been introduced c.1983-54 for use as a noise-gated sync separator in TV receivers, as had the similar 6BY6. Zenith had originally developed the basic noise-gated sync separator circuit around the 6BN6 gated bean valve FM demodulator, but as this was an expensive valve, had looked at alternatives and found that the 6BE6 radio pentagrid worked reasonably well. The US valve industry must have seen that a purpose-designed valve was justified, hence the 6BY6 and 6CS6. So it’s somewhat ironic that the 6CS6 ended up in FM demodulator service.

The Korting FM demodulator appears to have been another resurrection of the locked-oscillator type, this time with a sub-harmonic oscillator using a separate valve section. Possibly there were other implementations as well. But these may not have been well enough known to be mentioned in the available histories. I had not previously seen anything on the Korting circuit.


Cheers,