[time-nuts] Beginners GPS locked frequency counter question

Magnus Danielson magnus at rubidium.dyndns.org
Sat Oct 31 18:29:31 EDT 2015

Hi Chris and Bob,

On 10/31/2015 02:13 PM, Bob Camp wrote:
> Hi
>> On Oct 31, 2015, at 6:50 AM, Chris Wilson <chris at chriswilson.tv> wrote:
>>   31/10/2015 10:46
>>    I have a Racal counter locked to 1 MHz on its rear panel external
>>    input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
>>    from a David Partridge divider board. If I also feed the counter
>>    with the 10 Mhz direct output from the same GPS it reads one or two
>>    Hz out. As I assume the counter is working purely mathematically
>>    why would that be please?
> Is it always out in the same direction ( = broken counter) or does it
> bounce back and forth to each side of “correct” ( = noise) ?

Does not have to be a broken counter, not at all.

For instance, the SR620 displays this kind of behavior if you have not 
calibrated it correctly.

Counters measure elapsed time and elapsed events.
If you divide time with events you get average period measure.
If you divide events with time you get average frequency measure.

While the time-base tries to give you a second worth of measures, in 
actual life it only delays the trigger of the "stop" in relation to the 
"start" trigger that time, and then the "stop" even is recorded and the 
elapsed time occurs.

If "start" and "stop" triggers does not trigger at the same phase (due 
to voltage offsets or time offsets), then there will be a time-bias. 
Some counters calibrate this bias out, where as others try hard to avoid 
it. If you don't calibrate it properly, this bias can cause a shift up 
or down in perceived frequency measure. This is not necessarily a sign 
of a broken counter, it's a sign of an uncalibrated counter.

For instance, the SR620 auto-calibration does not cancel this effect, 
you have to adjust the calibration manually to zero it out.

Another flaw could be missed or added cycles, as it adjust the event 
count in the above formula. Those usually show themselves as larger error.

Some people is very fond of using the frequency measure of counters, 
I've grown more and more sceptic to it for a number of reasons when 
doing ADEV and friends, then I use TI that avoids a number of issues.
But that is a much more involved story.


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