[time-nuts] Man with too many clocks.

Chris Albertson albertson.chris at gmail.com
Thu Nov 3 12:19:14 EDT 2016


There is a problem with you plan.    You are looking at the relative phase
of a reference pule and an oscillator, just once.    The pulse moves
around, even on a good GPS receive.  So what you really have to do is
compare the phase of the oscillator to the AVERAGE phase of the reference.
  You have to look at many pulses.  This is hard to do what what many
people do is build/buy a GPSDO that sets an oscillator to the running
average GPS PPS pulse then compare the sine wave of the GPS to the
oscillator under test.

Now if working only by eyeball and screwdriver you can still make a GPSDO.
You are the controller.  Every second you try and adjust (say) 10% or the
error out of the oscillator and just keep doing this for hours.
Eventually you thing "A $4 micro controller could do this better than I an
and it will not fall asleep after 10 or 20 hours of work."   So you spend
the $4 and have a real GPSDO.   But you could continue by hand.

Now you have a known-good sine wave use that to compare with your others

But then you start thinking,   Is the known good sine wave really good.
How good? Can I measure it?    Yes but you will need to build/buy a two
more GPSDOs and run a three-way compare.

Then you see that at least one of the GPSDOs is not as good as the others
so you upgrade it.   Then of course one of the other two is now the poorest
performer, so you upgrade it

Then you do all this and likely you forget and have to ask "Why was it I
needed a good clock?"

In theory is is simple:  You can't calibrate anything without bringing an
external standard into you lab.  GPS is the best one for this use.    But
even the GPS PPS is not 100% stable so you need to average tens, or
thousands of them.   The number of them you can average depends on the
stability of your oscillator.   That's it the rest is just math.

On Thu, Nov 3, 2016 at 5:20 AM, Peter Reilley <preilley_454 at comcast.net>
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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-- 

Chris Albertson
Redondo Beach, California


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