About 2 years ago, I picked up a Rubidium oscillator on ebay for around $60. Put it in a box with power and convection ventilation. Picked up a Garmin miniature hockey puck sized GPS receiver for $59 (I think it was either direct or an Amazon purchase). Only problem is the Garmin put out a 1pps signal and I was not in the mood to get fancy with this.
I ran the 10MHz Rubidium into an HP 3336B and generated a much lower frequency at the output of the synthesizer. Both the lower frequency (can't recall, maybe 10kHz) and 1PPS were run into a scope, triggered on one and the timebase adjusted for a shorter and shorter interval as my adjustments of the Rubidium got closer.
After a few hours, I think I had it between 10^-9 and 10^-10 (an estimate by observing the rate of drift of the two).
At that point I called it a day and haven't checked since.
Den
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--- In TekScopes@..., "amxcoder" <micoderup@...> wrote:
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.
