[time-nuts] Is this measurement for real?
Charles Steinmetz
csteinmetz at yandex.com
Sun Dec 8 16:17:22 EST 2013
Dave wrote:
>I also measured some AM stations and I had drifts of 56 mHz for a
>1030 KHz station, and I have measured the DSP drift as around 14
>mHz. Do the AM stations really drift around that much? I thought
>they were pretty stable.
The FCC requirement for AM stations is +/- 20 Hz. It is not uncommon
to find stations 10 Hz or more off frequency. I just checked one of
the major stations here, and it is more than 16 Hz high with hundreds
of mHz variation over 15 minutes.
Some stations are locked to GPS these days, but far fewer than I
expect. And some that are "locked" to GPS are rather loosely
coupled, so they have offsets of as much as a Hz or even more
(synthesizer steps, I presume) and/or drift of +/- tens to even
hundreds of mHz.
Most AM stations still use crystals in bang-bang ovens. It is often
very easy to watch the oven heater cycle if you are using something
with enough resolution (I generally use a spectrum analyzer with
resolution in the few mHz range for such work).
This afternoon, I can see 5 carriers on 1030 +/- 20 Hz. The station
closest to you (WWGB) has been about 1 Hz low over the last few
hours, with about 300 mHz variation (peak to peak). The variation
shows the familiar asymmetrical ramp of an oven cycling with a period
of ~29 minutes. It takes ~6.8 minutes to fall from its positive peak
to its negative peak (heater on), and 21.9 minutes to climb back to
its positive peak (heater off).
Best regards,
Charles
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