This method, while close enough for HF work, is not anywhere near as close
as a GPSDO. With the prices of Trimble Thunderbolts in the $100 range, you
really should consider getting one.
I have two Trimbles, and two Truetime GPSDO's. I also found over on the
"evil empire" a box containing three phase-locked oscillators (72,5, 725 and
7,975 MHz) that require a 10 MHz reference. That gives me calibration
points over a wide range.
I have one Trimble that is used as the reference input (through an SRS FS710
Splitter) that feeds all 4 counters.
The three phase-locked oscillators are fed from a HB GPSDO.
A bit of over-kill, but I get anal about frequency measurement !!
73, Dick, W1KSZ
On Thu, Oct 11, 2012 at 9:02 AM, Chuck Lewis
<clewis@...> wrote:
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I¡¯ve had some success coupling the counter¡¯s 10Mz. signal to the SW receiver¡¯s input (just a wire in close proximity ¨C minimum coupling to the counter to avoid pulling the oscillator). If you can get the counter¡¯s signal and the WWV signal reasonably close in amplitude, you should be able to tune the counter¡¯s trimmer to zero-beat with the WWV carrier. Is this what you¡¯re doing? When I tried it, I could hear the WWV signal fluctuate at the beat frequency well below a few Hertz.
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Chuck Lewis
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I have three frequency counters. Two have oven standards. They are old and
purchased from ebay. They all disagreed about what frequency was what. They all
have 10 mhz frequency oscillators. I have tried calibrating them with WWV before
and been only moderately successful. Short wave receivers simply do not produce
audio below 100 hz. I needed to get much closer than that. What I had going was
three freq counters that might be as much as 45 hz off from each other on 10
mhz.
Then I had an inspiration. I realized that at the receiver detector, there is
nothing to limit frequency response. I got out my old DX160 receiver and a
schematic. I found the diode (D3) that did the detecting. I put a scope probe on
this. I then tuned in WWV. I put a little antenna on the BNC output of the
counters 10 mhz clock. Presto, I had a hetrodyne sine wave from the detector
that gave me the clock error. I moved the counter freq adjust up and down past
the null. I had to do this to be sure I was actually finding a null. I had to
slow down the scope horizontal sweep speed a whole lot to see the resulting sine
wave. I could actually see a 1 hz sine wave. I let the oven warm up for a half
an hour. I set it for a "perfect" null. You still cannot be perfect at this. I
had my scope on DC input. I could watch the error signal drift up and down as
much a 1 hz as the oven and crystal drifted. I then set the rest of counters to
my new standard.
BTW there was no sound of this mixed signal coming out of my DX160. It was just
too low to be reproduced.
This procedure is much cheaper than sending your counter into a lab to be
calibrated and finally I can trust the readings on my frequency counters. This
is pretty important when you are trying to receive and transmit SSB. A 30 hz
error is pretty apparent when you are listening to it. One of my counters was 45
hz off at 10 mhz! Yikes.
