Thought this might be of some interest. I've recalibrated my Orion,
and in the process learned a thing or two about how the (or at least
my) TCXO drifts over time.
I originally followed the usual practice of turning on the radio and
letting it warm up for 30-40 minutes. Then using the CW mode and
20Hmz WWV I beat the WWV signal against the side tone, and
adjusted the 45Mhz TCXO to match the tones. Simple, and the
zero-beat point is easy to hear.
Later in the day, lo and behold, the calibration was off by 10Hz
(normalized to 10 Mhz) .. the dial reading was 10 hz low. Now that
isn't exactly bad, but I wondered what was going on, so I took some
long term stability measurements. Here's what I found over a period
of some hours, readings are dial vs. WWV, normalized to 10Mhz:
Power on - cold -10Hz
5 minutes -2 Hz
10 minutes +1 Hz
15 minutes +2Hz
20 minutes +2-3Hz
25 minutes +2Hz
35 minutes +1Hz
40 minutes +0Hz
50 minutes -1hZ
6 hours -10Hz couldn't sit there all day long :-)
8 hours -10Hz
15 hours -10 Hz
22 hours -9Hz
23 hours -9Hz
Do you see a pattern here? The radio appears to drift up a bit and
then settles back (plus or minus a hz or two) to it's frequency
calibration setting cold over a long time period. Since I only know
what happens to my radio in my environment, I'm hesitant to
generalize. And I need to take more data points in the 1-6 hour time
range, to see how fast it settles out. All I can say at this point is that
it takes more than an hour and less than 6.
Which only means that if you're going to try to recalibrate your
TXCO, let the radio warm up with the bottom panel in place for a
few hours first, and then make the adjustment fairly quickly so the
thermal changes caused by slipping the bottom panel off don't have
a significant effect in the outcome.
I don't find the 1Hz step rate useful for actual radio operation, so
with a 10Hz step rate, these small calibration errors aren't going to
be noticeable. I might argue it would be nice to have an oven
stabilized option to reach equilibrium more quickly, but that's a nit in
any practical sense.
Grant/NQ5T
|