[Collins] R-390A frequency measurement - in service

[Dr. Gerald N. Johnson]

On Sun, 2009-11-22 at 11:29 -0500, frankshughes at aim.com wrote:
> Hi,
> I am searching for a way to connect my HP 5316A frequency counter to the R-390A so I can get an accurate 
> measurement of what frequency it is tuned to while in operation.

You need to measure three frequencies, and add or subtract them
according to the band. You have to take account also if a crystal
injection is used on the second harmonic. You have to measure the PTO,
the BFO and the crystal injection frequencies, taking sums or
difference. If on AM, you have to add the IF frequency to the count even
if the BFO isn't running.

Some vintage receivers do all that by using band data to preset the
V/PTO counter to the negative of the appropriate sum or difference
frequency of the crystal and the BFO. And they are right once in a
while.

There have been mixers made for that purpose, maybe AAST electronics was
one, probably intended more for the S-line, but it might be customized
for the 390 except the S-line always has the crystal on the high side.
And on some bands the 390 has an extra mixing, like the 1/2 to 2 MHz
ranges and that LO has to be included in the sum or difference to the
counter.

It might be easiest to program a PIC chip that had four built in
counters to count each oscillator, check the band switch position by
some other means, then to compute and display the appropriate sums and
differences. Its not a trivial project.
> 
> I searched around and it looks like I need to connect to the PTO output. 
> If this is the best way, what should I use to make the connection w/o effecting the circuit operation? 
> 
> 73, Frank
> KJ4OLL

For coupling you want a small coupling capacitor to a high impedance
buffer amp near the sample point. For each oscillator. Four of them.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer