[time-nuts] Man with too many clocks.

Bob Camp kb8tq at n1k.org
Sat Nov 5 07:58:32 EDT 2016


Hi

A ten or twenty turn pot on a normal EFC will get you past the point that 
you can reasonably set the oscillator. The typical (not GPS version) EFC is down
around 1 to 2 x 10^-7. A 20 turn pot will be running 1x10^-8 per turn. 100 to 200
points per turn is a pretty typical “set” number for a pot. That gets you into the 
sub 1x10^-10 region. The OCXO’s we are talking about have a temperature,
 pressure and humidity coefficient that each are well above that level.

Bob 

> On Nov 4, 2016, at 2:31 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> I'm not sure if there is a reason counters don't let you digitally
> calibrate beyond that, the 10 MHz ref out on the rear panel would still be
> out of cal.
> 
> On Fri, Nov 4, 2016 at 1:48 PM, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> The only practical way to set the 10811 or 10544 is with a >= 10 turn pot
>> on the EFC. I
>> never have worked out just why there are so many instruments that don’t
>> have a pot on
>> the EFC.
>> 
>> Bob
>> 
>>> On Nov 4, 2016, at 11:35 AM, Peter Reilley <preilley_454 at comcast.net>
>> wrote:
>>> 
>>> I gave up on trying to use the GPS 1 PPS signal to calibrate the 10 MHz
>> OCXO's that
>>> I have.   The reason that others have pointed out is that the
>> uncorrected 1 PPS
>>> signal from the GPS is has just a little too much a jitter to use it for
>> calibration
>>> with your eye using a scope.   If it were sawtooth corrected then it
>> would be better
>>> but you really need a GPS disciplined oscillator.
>>> 
>>> Not to be outdone, I brought out a rubidium oscillator that I had put
>> away because
>>> it did not appear to work properly.   It only put out a 1 PPS signal and
>> nothing else.
>>> I compared that with the GPS PPS and could get a good comparison on the
>> scope.
>>> The rubidium drifted about 40 nS over 12 hours.   So it seemed to be
>> good.
>>> 
>>> With that I could adjust the OCXO's in my 5370's.   The spec for the HP
>> 5370B with
>>> a HP 10811 OCXO is better than 1 X 10^-10 RMS for 1 sec average. That
>> is, it should
>>> take more than 1,000 seconds for one 10 MHz wave to shift by 360
>> degrees.   That
>>> is very hard to do using the screw adjustment in the OCXO.   Even the
>> slightest
>>> movement possible will cause a frequency change greater that is
>> spec'ed.   How
>>> do cal labs do it?
>>> 
>>> My HP 5370A has a 10544 OCXO which is spec'ed for short term stability of
>>> better than 1 X 10^11 for 1 second.   Even better than the 5370B! The
>> adjustment
>>> screw is much coarser and it is not possible to get any better than a
>> few seconds for
>>> one cycle phase shift of the 10 MHz OCXO against the standard.   It
>> seems that I can't
>>> get even close to the spec.
>>> 
>>> These have been running for a few days.   It that enough?
>>> 
>>> Pete.
>>> 
>>> 
>>> 
>>> On 11/3/2016 8:20 AM, Peter Reilley wrote:
>>>> I am the proverbial man with too many clocks and I don't know what time
>> it is.
>>>> To correct this situation I have decided to calibrate everything.
>>>> 
>>>> I have a HP 5370B, a HP 6370A, and a HP 5328A all with the TCXO
>> option.   I also
>>>> have some TCXO modules.   I figured that I would calibrate them against
>> my Trimble
>>>> Resolution T GPS receiver.
>>>> 
>>>> I put the 1 PPS signal in one channel of my scope and one of the 10 MHz
>> TCXO
>>>> signals in the other channel and look at the phase relationship. The
>> TCXO's are
>>>> already close enough that I should not be out by more than a fraction
>> of a waveform.
>>>> I understand that I have to deal with the 1 PPS without sawtooth
>> correction.
>>>> 
>>>> I expected to see the 10 MHz signal bounce around but not move more
>> than 1/2
>>>> of a wave length.   Instead I see the 10 MHz waveform appear steady for
>> a few seconds
>>>> then jump a significant portion of the wave.   The jump is too much to
>> be confident
>>>> that I have not slipped one cycle.
>>>> 
>>>> Can I do what I am trying to do or am I missing something?
>>>> 
>>>> Pete.
>>>> 
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>>> 
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