[time-nuts] Line Frequency Stability
alw.k9si at gmail.com
Mon Apr 10 19:06:10 EDT 2017
Back in the 1970's I was chief engineer at a couple of local radio
stations. They were automated and needed to carry network programs at
the top of the hour in real time. The switching gear used electrical
motors with cams and roller switches. These motors were locked to the
line frequency and required almost daily adjustment to not get very
sloppy. Often they were off by a few seconds and sounded
unprofessional (at least to the program director who was god).
So we put together our own source of precision 60 cycles. A 10 mhz
oscillator was divided down to 50 hz. A VCO operated at 600 htz and
was divided by 12 and phase locked to the precision 50 htz. The 600
htz was then divided by 10 by a DAC (actually a 7490 and some
resistors). This new 60 htz was low passed and amplified by an old PA
amplifier that drove a filament transformer backwards. This supplied
the AC power to the switching motors and the wall clocks. The 10 mhz
oscillator was checked against WWV from time to time. This solved our
timing motor slop and kept the wall clocks right for months at a time.
We used that system for years. Used all TTL chips.
This was a practical application for me as a time-nut.
Al, retired mostly
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