[GreenKeys] DT600 replacement op amps

[Jim Haynes]

On Thu, 9 Jul 2020, Gil Smith wrote:
> So, anyone on GK have suggestions on some way to test performance of a
> demodulator?  I was thinking some canned audio test files to play, at

I guess the professionals do it with propagation simulators that allow
for varying amounts of fading, multipath echoes, noise, interfering
signals, etc.  Although it is open to question whether the simulated
propagation is similar to what we encounter in real life.

I recorded some off-the-air signals under somewhat poor conditions
intending to use them to compare performance of various demodulators,
but I never did the work.  I've also run some tests with the radio
feeding two demodulators and comparing the copy from each.

In doing this kind of demodulator testing it is worthwhile to use an
electronic terminal rather than a mechanical printer, because the
shortcomings of the mechanical printer tend to overshadow the finer
points of demodulator performance.  Which is why I always advocate
an electronic regenerative repeater if one is serious about using a
mechanical printer in competition with electronic terminals.

There is a wealth of good stuff on the web site w7ay.net

It's interesting to look at the evolution of the Dovetron demodulators,
since it is there to be looked at.  First there is the patent 4,013,965
which seems to be the way early MPC demodulators were built.  Then
a bunch of stuff was removed from the big mother board and a "binary
bit processor" was installed as a daughter board.  And the binary bit
processor went through at least two editions.  And I've often mentioned
a manufacturing error in at least some of the late binary bit processor
boards that seriously degrades performance.

Now we have DSP which can do some things that just can't be done with
real hardware circuits.  And there are some unturned stones in the
fields of both hardware demodulators and presumably DSP implementations.

Jim W6JVE