Chris Morriss writes:

Many years ago, there was a series published in 'Wireless World' (might
even have been by John Linsley Hood) about the design of an AM
synchrodyne using transistors and linear ICs. It actually stripped off
the carrier from the incoming signal, and then limited and filtered this
to use as the source of the local oscillator. I believe the concept was
called 'Homodyne'.

--
Chris Morriss

Did a "proof-of-concept" bench test of this homodyne.

I took a signal generator set to 1MHz, 400Hz AM at 30%. Took that
output to the RF port of a diode ring double balanced mixer. Also
fed it to Cmos gate to get a "limited" carrier. I feed the square
wave from the Cmos gate to the LO port on the mixer. The IF port
went to a low-pass filter. This is the block diagram of a
"homodyne" receiver.

Okay, I used a big signal from a clean generator, but the results
were perfect. We knew it would work, because I blocked out the real
world.

If Jon allows himself an IC, I suggest using an FM radio chip to
get a limiter, then the audio path can be all tubes. If he puts in
a decent pre-selector, and perhaps a "strong signal" trap, it can
be made to work.

My lash-up is limited by the fact that I need a big signal to get
the square wave from the CMOS.

But as Turner, et al, have said "it is still a lot of work and
engineering."

My new (1980+) radio books say that this will give superior
performance to an envelope detector, but is too complicated for
"consumer" receivers and therefore, never caught on.

I wonder if you can get enough limiting on local signals to make
this work with all tubes.

Steve.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA