[GPS_Standard] Xtal Oven coming up to temperature from cold?

[Bert, VE2ZAZ]

Hi Dick,

It is unlikely that the system will have time to go out of range before your
XO warms up, that is unless your Vtune sweet spot is already really close to
the higher or lower limit (0 or 5 volts). There are 1024 coarse steps
availabe to tune the XO. Assumung your XO's Vtune sweet spot is somewhere in
the middle of the range, this leaves 512 steps to adjust in one direction.
Assuming a averaging sample size S=10 (which is realistically the shortest
you will use), this means a one step adjustment will happen every 10x16 secs
or 160 seconds. 30 minutes is 1800 seconds. So the system might adjust 11
steps, which is far from the possible 512 steps before railing to the end of
the range. These 11 spurious steps will obvioulsy be compensated in the
following 30 minutes after the XO is warm.

That being said, it won't hurt to hold the reset line down. If you want to
have fun designing something automated, then go ahead!

bert, VE2ZAZ


----- Original Message -----
From: "Dick" <dick at zs6ro.co.za>
To: <gps_standard at mailman.qth.net>
Sent: Friday, December 15, 2006 12:47 PM
Subject: [GPS_Standard] Xtal Oven coming up to temperature from cold?


I'm starting to build this project and am wondering if its a good idea to
hold pin-1 of the PiC (reset-pin) low for such time until the xtal oven has
stabilised ??

>From cold my oven will be at it's correct temperature after approximately
one half hour ... In the meantime the frequency is still slowly coming to
the correct 5MHz frequency.

Would the PIC not be trying to correct this, all in vain ?? .. My xtal oven
is one of those with a 1Hz end to end range for 5 volts .. Would the PIC
possibly go out of lock range ? .. Would the error correction voltage look
like a damped oscillation with the compensation, though I would assume that
the voltage would 'bottom' or hit the 'ceiling' and stay there until the
frequency came close to nominal ... ?

I was contemplating using a half-hour monostable which would bring pin-1
(reset) of the PIC high after the half hour delay .. Then the PIC would be
dealing with a stable frequency within range ..

Is this an issue or can I leave the PIC to it's own devices and let it sort
out the frequency correction from cold ? .. I hope I can leave the PIC to do
the job because I haven't worked out a 'simple' half-hour monostable yet !!
( I am using a 12 to 24 volt convertor to feed the xtal-oven ... From cold
the voltage to the oven is 18 volts and reaches 24 volts once the oven is at
it's correct temperature and is drawing minimum current .. I was going to
sense this 18 - 24 volts change and when the oven gets to say 23 volts
change pin-1 of the PIC from low to high - a bit more elegant I think) ..


73, Dick ZS6RO\
http://www.zs6ro.co.za

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