[time-nuts] VCXO in a watch timing machine

WB6BNQ wb6bnq at cox.net
Mon Feb 4 20:43:24 EST 2013

Hi John,

First question is do you have a schematic ?

Second question: Is the 1K pot directly across the VCC to ground ?  Or is it in
the middle of a resistor string ?

Normally, the EFC pin has a nominal center point that is around 1/2 of the VCC
going to the VCXO module.  Also, the EFC pin's input resistance is fairly high,
in the order of around 50K.  So it would seem unlikely that the 1K pot is all
there is between VCC and ground.  That would imply 5 ma of current flowing
through the 1K pot, which seems a little high for the needed function.  A typical
current level would be more like 1 ma.

So, before you go crazy changing out the VCXO check to see if the 1K pot is in a
resistive string.  If that is the case, then you could adjust one of the fixed
resistors to put the pot range so that it covers above and below the 18 MHz mark.

As previously mentioned, it would pay to measure some of the circuit parameters
to make sure things seem proper.


john wrote:

> Hi
> I'm hoping to get some information on the VCXO that sits in my watch
> timing machine. The timer is a Chinese clone of a Witschi timer - the
> heart of the thing is an 18MHz VCXO marked 'SCTF 03'. There's a BNC
> socket that outputs the crystal frequency and a 1k pot to adjust the
> frequency.
> The snag is that I can only adjust the frequency between 18,002,359Hz
> and 18.000,958Hz. I have a Hewlett Packard and a Thurlby Thandar
> frequency counter which give me readings that broadly agree - I'm
> quoting the HP because I trust it more! The hardware frequency in my
> scope doesn't have the resolution at 18MHz, but agrees that it's
> fast.These figures translate to a maximum of 11.32s/day fast and a
> minimum of 4.63s/day fast. This is borne out in practice by the
> behaviour of watches - if I adjust them to 0s/day, then they are fast,
> but if I aim for -5s/day then they are about dot-on.
> So if my workings-out are correct, I can 'pull' the VCXO by 78ppm,
> which seems reasonable, but offset away from the nominal 18MHz. Is this
> a common failure of VCXOx? Can anything be done about it? The device is
> about 18.35mm long by 10.8mm wide (and 7.7mm high) - is this a standard
> size, and if so who would sell an 18MHz VCXO (UK)?
> Any advice gratefully received.
> Regards
> John
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