[time-nuts] Expected 10 MHz offset from a GPSDO?

Tim Lister listertim at gmail.com
Fri Aug 19 17:02:56 EDT 2016


On Fri, Aug 19, 2016 at 12:24 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>
> listertim at gmail.com said:
>> Connecting the 10 MHz output to the Channel A input (on the 5334B) or the
>> frequency input (on the 5370B) and setting 50 Ohm termination and AC signals
>> but without any other input or external reference results in a frequency
>> that is off by about 0.1-0.2 Hz. Is this amount of offset expected or a sign
>> something is not right with the GPSDO?
>
> Your GPSDO is more accurate than your counter.  You are measuring the
> frequency offset of the crystal in the counter rather than the GPSDO.
>
> You may have to stand on your head to understand what I'm trying to say.  It
> will be obvious once you see it.
>
>
> If/when you power cycle the GPSDO (without power cycling the counter) you can
> watch it shift when it locks up to GPS.  If you disconnect the antenna it
> will stay unlocked and you can watch the crystal in the GPSDO drift as it
> warms up.
>
> If you unplug the counter for a while but leave the GPSDO powered on, you can
> watch the crystal in the counter warm up.  (Turning it off isn't good enough,
> it keeps the crystal warm even when the switch says off.)
>
> If you have a GPIB connection to the 5334B, you can get a few more digits by
> asking it to count for 99 seconds.  If you get that working and your lab is
> not air conditioned, you can see the temperature effects on the 5334B.
>
>

Thanks to everyone for the replies, this definitely helps clear things
up. I had my suspicions that I wasn't measuring what I thought I was
as the resulting ADEV curve I was getting in Timelab (I forgot to
mention I have a Prologix GPIB->USB interface and have been using it
for logging/capture) had the '\_/' (down, flat, up) shape  that I
gather you get from OCXO's. I did also have a suspicion that this
could lead down a slippery slope of needing another GPSDO to compare
it with and then and then... ;-) (While I see there is a 5071A
available on ebay right now, those are somewhat out of my budget by
several orders of magnitude...)

Thanks,
Tim


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