[time-nuts] VCXO in a watch timing machine

john john at ic0n.org.uk
Mon Feb 4 18:10:35 EST 2013


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 

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.


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