[Lowfer] frequency counter calibration

[Bill Cromwell]

On Fri, 2013-01-25 at 23:10 -0800, KD7JYK DM09 wrote:
> : > ways (shouldn't be very much). I will set it to agree with WWV at 10 MHz
> : > and send it back to you.
> 
> How is this done?  optoelectronics told me years ago people do this, they 
> said they didn't know how.
> 
> Kurt 

Hi Kurt,

It seems incredible that the folks at Opto "don't know how". You need
some kind of receiver that can detect zero beat. Zero beat is not the
tone that matches your sidetone! Many newer (and not so new any more)
receivers have a severe rolloff in response below 500 or 300 cycles per
second. Even if the lowest frequencies are severely suppressed you might
still be able to detect zero beat. If not...you will have to tap into
the detector output before the audio stage (perhaps at the top of the
volume control). You will be using AM of course and the carrier must be
somewhere in the IF passband.

Use the heterodyne frequency measurement techniques. There are a number
of ways to approach it. I would most likely lightly couple the counter's
clock generator at 10 MHz to the receiver set on 10 MHz WWV and then
measure the zero with a scope. Human hearing will NOT be able to hear
anything at all near zero beat so some means has to be used to detect it
other than by ear. On some receivers you can watch the S-meter wobble
near zero beat or even hear the AGC pumping. The response will slow to
nothing (zero) and then increase again after zero is passed. I would use
my O'scope. It will be the counter's clock that is tuned and NOT the
radio. It should go without saying but all of the gear used should have
a substantial warmup time - an hour of more. Overnight is *good*.

There are other known signals that can be used. If you are unfamiliar
with heterodyne frequency measurement look up the the LM or BC-221
manuals. The method is very accurate for setting an oscillator to a
known standard. WWV *IS* a known standard. Once the frequency counter
clock is zeroed it serves as a secondary standard with the nice feature
of automatically interpolating between multiples of the secondary
standard and displaying the results on the digital readout. An AM
broadcast station can be used (the tolerances may not be as tight as
WWV). Drive the counter with the signal generator. Set the signal
generator to zero with BC transmitter. Then set the counter's clock so
that it reads the transmitters (known) frequency. You can use any
acceptable standard including GPS, WWVB, your rubidium or cesium
standard. How accurate you want your Opto to be is up to you. Optos are
NOT temp compensated unless you modify them. I'm an amateur and my
license even says so. WWV is probably good enough for most of what I
will be doing. I check my secondary standards against WWV often. They
seem to stay spot on of the time...sometimes for years.

On those LMs .. heterodyne frequency meters can do several very handy
tricks that your frequency counter can NEVER EVER do. It's good to have
those tricks available in addition to a digital frequency counter. Maybe
the technical people at Opto are all very young and missed the boat for
alternative means and the advantages of alternatives. I could use one of
my LMs to zero against WWV and then zero the counter's clock against the
LM. So many choices.

Did this answer your questions? Did it raise more questions?

73,

Bill  KU8H