[BC] Frequency Measuring

[Johnson, Richard]

On Tue, 20 Mar 2007, Tom Dimeo wrote:

> During my very limited experience in broadcasting back in the
> sixties, the station I worked for in central Pennsylvania
> used a person in Massachusetts to measure the frequency.
>
> I always wondered how he could hear the station well enough
> to make the measurement?
>
> The station was on 920 kHz with five hundred watts
> directional at night and one thousand watts nondirectional
> during the day.  When the station went off the air at
> midnight he would call and we would put the transmitter back
> on with the one thousand watt nondirectional signal with a
> one thousand cycle tone and he'd give the frequency measurement.
>
> How in the world did he do this?
>
> Tom

In the good old days it was with a transfer oscillator. The
operator would zero-beat your very-weak, barely-audible, signal
and measure the transfer oscillator frequency, not your actual
station frequency. This, in the days before frequency counters,
was measured with an interpolation oscillator and marker generator!

Basically, you set up a bunch of calibrations. You would zero-beat
your 100 kHz marker generator's Nth harmonic to a frequency standard
which was kept on all the while and periodically adjusted to WWV or
WWVB. Then you would set your interpolation oscillator to, perhaps,
10 Hz and 100 Hz. You would set an index at these two points where
its harmonics would zero-beat your 100 kHz crystal-controlled
marker generator. Now you would have beats at 10 Hz intervals, with
10 Hz accuracy, in addition to stronger beats at 100 kHz intervals.
You could interpolate in between. It was an art, counting those beats. 
Further, it was an art to determine if you were a few Hz above or a
few Hz below your assigned frequency. This was found by altering the 
frequency of the marker generator slightly by pressing a button which 
lowered the frequency (it added a capacitor across the crystal). If
the beat-frequency increased, your signal was above its assigned, if
it decreased, it was below.

If you think this is radical, note that this is how microwave
frequencies, higher than can be directly measured, are still
measured although the logic is done with a microprocessor! The
accuracy depends upon being able to complete all the operations
before the transfer oscillator can drift out of spec. With a
microprocessor, its trivial. With a human operator one needs to
go though the same motions several times to get some
repeatability. Also, we didn't have kHz in those days. We had
kilocycles and they were much better because they didn't hertz ;)!

Cheers,

Richard B. Johnson
Project Engineer
Analogic Corporation


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