In article le.org,
Michael Black wrote:
On Wed, 28 Jan 2015, Dave Platt wrote:


In article ,
nothermark wrote:

I need a simple poor ham's deviation meter. That got me looking at
the IC-7100 receiver sitting in my shack. I could get to the
discriminators but it would be a bit messy. OTOH it has a 10.7 MHz IF
output I could add an amplifier and detector to and feed that to my
O'scope to do what I want for long enough to sort out some problems.
That got me looking for a simple detector circuit. No joy so far.

There was a pretty simple circuit shown in the old ARRL VHF Manual.
I'll see if I can pull out copy and scan it.


Except they don't give much output at 10.7MHz, a pulse counting circuit
would be simple. Those seemed to get a lot of travel for novelty forty
years ago. They'd use logic ICs to amplify and limit the IF signal, then
a divider to get it down to a lower frequency where the pulse counting
could happen (the logic being kind of slow back then so it didn't work
well at 10.7MHz).

Pulse-counting discriminators were fairly popular in some of the
high-end FM broadcast-music tuners in the 80s and 90s, I believe,
due to their high linearity and low distortion.

I found that circuit from the ARRL "FM and Repeaters" manual (1972
edition) I remembered. Unfortunately it doesn't include the FM
detector... it assumes that output is available directly from the
discriminator. It's just a fairly simple peak detector and meter,
which they suggest to calibrate via the Bessel method. Nothermark,
email me directly if you'd like a copy.

As to doing the FM detection/discrimination: there are probably still
"FM detector on a chip" ICs available today: OnSemi LA1225, NJM
2549/2550, and so forth. Hardest part is probably chasing down a
coil.

You could try using a ham radio which has a "packet data" jack... the
audio signal which comes out of this is often a fairly direct,
non-equalized version of the discriminator output (since this is what
a 9600-baud packet decoder wants to see).