[time-nuts] Is a crystal likely to change frequency by 3% ?

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Fri Sep 12 04:09:43 EDT 2014

On 12 Sep 2014 03:35, "Alex Pummer" <alex at pcscons.com> wrote:
> No that is to much, except if you overdrive it and you are so lucky that
after it broke it is still working on a different frequency, but I would
suggest check your frequency counter too, because 3% off of a clock
frequency wold make the clock almost unusable not just for time nuts...
> 73

This was a follow up post to something I wrote a year ago. I think you have
misunderstood me.

1) The clock is in a commercial instrument - HP 8720D vector network

2) When I got the instrument the internal clock which gives the date and
time was loosing about a day per month, which is roughly 3% from a quick
mental calculation.

That was an unacceptable

3) Now it keeps within a few seconds per month. I have not bothered
checking the actual frequency of the oscillator with a counter or logging
the time reported by the clock on a regular basis,  but it is now
sufficiently accurate for my usage. I only use it to record the date and
time I take a measurement with the network analyzer. Given a typical set of
measurements takes a minute or so, worrying about the exact time is

There must be at least 3 independent oscillators in this machine.

1) Real time clock.

2) 10 MHz standard oscillator.

3) Optional high stability 10 MHz oscillator.  I don't know if that is an
oven or not. I know last time it was calibrated by Agilent is was off about
0.25 Hz.

When the VNA was calibrated by Agilent a year ago, the accuracy of the RTC
was not checked.  There is no published specification for it.

The standard 10 MHz oscillator was within spec. I don't recall the error.

The optional 10 MHz high stability oscillator was in error by about 0.25
Hz. The specification is +/- 1 Hz. I don't know if that is an oven or not.
There's nothing to indicate the oven is cold, but given the specifications
of the instrument are based on a one hour warmup period, maybe HP thought
there was no point in indicating if the oven is cold. Or maybe there's no

Dave, G8WRB

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