[time-nuts] VCXO in a watch timing machine

Ed Breya eb at telight.com
Tue Feb 5 01:47:09 EST 2013

How do you know that the frequency range is incorrect for the function? 
Assuming this is for calibration of wristwatches, maybe the idea is to 
set them to some nominal value at room temperature, then expect them to 
drift to the "right" frequency at skin temperature or so. There would 
have to then be some relationship between the approximately 18 MHz and 
the ideal 32,768 Hz watch crystal frequency.

If the idea is to divide the 18 MHz down to an accurate version of 50/60 
Hz or 1 Hz, then yes, it looks like something's wrong. But, does it need 
to be a VCXO, or just settable to the right frequency? I believe that 
18.000000 MHz is a standard frequency for VCXOs and TCXOs, so readily 
available from typical vendors, or you could build an oscillator as good 
as needed. The package dimensions seem like a standard 14 pin (only 4 
are used) DIP style - very common.

I think most of those modules use a limited tuning voltage range like 
0-5 V if the supply is +5 V. If you haven't already, check to be sure 
the full tuning voltage range is covered at the module. You can usually 
get a little higher frequency by exceeding the tuning voltage - 
especially if there's nothing to lose anyway if it's damaged - but 
unfortunately it's hard to get lower frequency since the varicap diode 
will reach zero or forward bias if you go very far the other way.

Also, if the supply voltage is off, it could have drastic effects on 
operation. Changing the supply voltage a little can affect the frequency 
and tuning range, so this is another option for slight adjustment - but 
may give unpredictable results.


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