[time-nuts] VCXO in a watch timing machine

WB6BNQ wb6bnq at cox.net
Tue Feb 5 03:12:41 EST 2013


Interesting point, I hadn't considered that the frequency should be off !  {32768
x 550 = 18,022,400.000}  So, maybe it has drifted the other way ?

However, looking at the Witschi web site suggest that their instruments sense by
picking up audio or vibration via some kind of sensor as they talk about checking
motors and buzzer noise from the watch.  That being the case it is more likely
the 18 MHz should be right on frequency.

Well, unless we hear back from the guy and he answers questions put to him, we
really won't know whats up.


Ed Breya wrote:

> 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.
> Ed
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