On Tue, 2008-05-20 at 09:17 -0700, Ron Castro wrote:
> Rob:
>
> The frequency drift problem on Orions and Orion II's is well documented.
> Since every TCXO is slightly different, replacing the unit might improve
> overall stability, but it might also make it worse. Adjusting the TCXO will
> get it closer on frequency but it won't improve the drift problem.
>
> If you go to my web site, www.n6ie.com, you see two pages that deal with the
> problem. One is a simple mod that involves placing a small fan over the
> TCXO to slow down its heat-up rate and allow the temperature compensation in
> the oscillator module time do its job, which improves overall stability.
> The other shows how to adjust the TCXO so that after a reasonable warm-up,
> which is reduced to about 30 minutes with the fan mod, the frequency ends up
> pretty close to WWV, generally within +- 3 Hz at 15 MHz.
>
>
> Ron N6IE
> www.N6IE.com
>
>
Such frequency drift is a constant problem at VHF through microwave ham
activities running SSB and CW. 3 ppm at 10 GHz is 30 KHz. The first help
is isolation of the oscillator by hiding it in a block of styrofoam.
With radiant heat sources around (like tubes in my 75S-3B) I found it
helped to put some aluminum foil on the outside of the foam around the
calibrator crystal. The next step is to find a positive temperature
coefficient thermistor with a turn off temperature of 40 degrees C and
put that in an enclosure around the oscillator (or crystal alone)
surrounded by foam. That simple device (with 12 volts applied, Digi-Key
used to carry them in a piece about a quarter inch diameter) holds the
oscillator temperature within a few degrees which makes the crystal or
the TCXO significantly more stable than ordinary room and radio interior
ambients.
Those that are particularly critical, use a frequency locking circuit,
often from Luis, CT1DMK described at:
http://www.ve1alq.com/clpd_pll/clpd_pll.htm and:
http://w3ref.cfn.ist.utl.pt/cupido
which locks the VHF oscillator to a local frequency standard and a
similar circuit locks the local frequency standard to GPS signals
getting stability on the order of one part in 10^12. One millionth part
per million. By using a long time constant the phase noise of the
controlled oscillator is not disturbed in the way it would with phase
lock.
But is a 60 Hz error more than most user's tuning error on SSB and has
it any significance in ham operations which are not channelized? I
suppose it means one has to stay at least 60 Hz INSIDE a ham band. But
most CW needs more than that because the CW speed contributes bandwidth
and SSB is much wider.
73, Jerry, K0CQ
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