[Milsurplus] Frequency Counter

[WA5CAB at cs.com]

Joe,

The suggestion about using a receiver with digital readout to track and align 
the local oscillator has some merit.  However, two howevers.  (a) the digital 
readout on the receiver is probably a counter tracking its LO and if the IF 
alignment is off in the digital receiver, then all measurements will be in 
error.  (b) you can do the same thing with a counter if it has enough sensitivity 
by putting a sniffer pickup lead into the oscillator coil compartment.  In 
fact, you will probably remember that the alignment instructions for the BC-312 
family use this method to align the LO, although of course the freq meter 
referenced is one from the period.

I don't recall (if I knew) what signal generator you have, but hooking up a 
counter to continuously read its output frequency actually isn't a trivial 
excercise.  You can't just hang the counter across the output of the sig gen 
that's also going to the injection point in the receiver for two reasons.  (a) the 
counter may load the generator causing you to get different results with it 
connected than with it disconnected. (b) when you are aligning the receiver RF 
and probably Mixer the output of the sig gen may be turned down too low for the 
counter to accurately lock onto.  

My sig gen for MF and HF receivers is an SG-85B/URM-25D (i.e., AN/URM-25H).  
I solved both problems with a minor (reversable) mod to the SG-85.  These sig 
gens have a 4-position coarse Range/Output switch.  Two of the position are 
VLF and the other two LF and above.  One of each of these position pairs 
bypasses the output attenuator and output only appears on the BNC jack marked X200K.  
The other two positions route output through the attenuator.  What I did was 
to install a jumper across the switch such that output always appears on the 
X200K jack and connected it to the counter.  Output to the set under test is 
taken from the attenuator jack.  As there seemed to be some loading effect 
caused by either the coax or the counter and the counter did not need the 1 volt 
RMS normally present on the X200K jack, I added a 10K resistor in series (value 
experimentally determined) mounted in one of the little attenuator boxes often 
found with military signal generators and attached to the X200K jack with a 
right angle BNC adaptor.  (the box was NOT one supplied with the AN/URM-25 but 
from a 60's vintage set no one every wants).

In a message dated 5/11/2007 5:37:52 PM Central Standard Time, 
n4fs at eozinc.com writes: 
> -----Original Message-----
> From: milsurplus-bounces at mailman.qth.net
> [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Carole White-Connor
> Sent: Friday, May 11, 2007 5:18 PM
> Cc: boatanchors at mailman.qth.net; milsurplus at mailman.qth.net
> Subject: [Milsurplus] Frequency Counter
> 
> Guys, I'm looking for a cheap frequency counter to use to check my
> signal-generator
> frequency when conducing IF and RF alignments.
> 
> What do you suggest? I see that some of the Radio Shack digital mulimeters
> have a
> frequency counting function. Are they any good?
> 
> As always, thanks
> 

Robert & Susan Downs - Houston
<http://www.wa5cab.com> (Web Store)
MVPA 9480
<wa5cab at cs.com> (Primary email)
<wa5cab at houston.rr.com> (Backup email)
   
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