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

Bob Camp kb8tq at n1k.org
Fri Sep 12 07:10:47 EDT 2014


Hi

If this is an RTC, it’s probably running off of a battery when the machine is powered down. It is far more likely that the oscillator is dropping out (stopping) rather than shifting frequency. One way it might do this is to stop for a relatively brief period, battery recovers, and then start back up again. Another way it might do this is to go into a squedge mode (audio speed blocking) at low voltage levels.

Bob

On Sep 12, 2014, at 4:09 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:

> 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
>> KJ5UHN
> 
> 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
> analyzer.
> 
> 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
> pointless.
> 
> 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
> oven.
> 
> Dave, G8WRB
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