[time-nuts] Couple of questions for Racal 1992 owners

Ed Palmer ed_palmer at sasktel.net
Thu Aug 27 20:46:43 UTC 2009



Lux, Jim (337C) wrote:
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>> Behalf Of Ed Palmer
>> Sent: Thursday, August 27, 2009 12:06 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Couple of questions for Racal 1992 owners
>>
>> Thanks for the info Antonio.  Racal didn't spec the warmup
>> characteristics of the counter as a whole and it seemed a little odd
>> that the warmup drift for the counter was as large as 10 days of
>> oscillator drift.
>>
>> Ed
>>     
>
> Intuitively, this wouldn't surprise me.  You set the OCXO up so that the temperature is somewhat higher than the highest expected ambient (so, say, it's at 60C) and pick a crystal that has a flat freq/temp characteristic near there.  So, when you turn on, your crystal is at, say, 25C, where the slope of the freq/temp curve is pretty steep.  I'll bet someone has some curves out there that could give a quick order of magnitude estimate...
>
> I have a poor quality chart here that shows an AT cut with about 40 ppm change for a 50 degree C change, and an SC changes about 5 ppm over the same span.  Since a decent oscillator has aging of small ppm/year, that's orders of magnitude bigger for the temperature effect.
>
> Is this a SC or an AT cut?
>
>   
I agree with what you've said, but on the 1991/1992 counter, there's a 
standby function which keeps the oscillator hot, to avoid the issue 
you've stated above.  So the 10 days worth of drift when you turn the 
counter on is just due to the counter warming up, not the oscillator.

The oscillators that I have (opt. 4A and 4E) are both AT crystals.  This 
isn't stated anywhere that I can find, but the frequency at startup is 
in the range of 20ppm high which is consistent with an AT cut crystal.

Ed





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