[time-nuts] Man with too many clocks.

Scott Stobbe scott.j.stobbe at gmail.com
Thu Nov 3 14:11:59 EDT 2016


Bob has stated this, but perhaps not emphasized that, when you sample the
phase of a 10 MHz clock once a second, you are essentially folding the 20
millionth nyquist band down to baseband. So you can alias any integer
multiple of 1 Hz as if it were 10 MHz, i.e. 10 MHz + 1Hz will hold phase to
1 Hz (sampled once a second) just as well as 10 MHz would.

On Thu, Nov 3, 2016 at 1:00 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> If your TCXO is off by 1 ppm, it will slip 10 cycles per second at 10 MHz.
> If it is off by 0.1 ppm it will slip a full cycle at 10 MHz.
> If it is off by 0.01 ppm *and* uses some sort of digital compensation, it
> will hop around.
> If the GPS is not sawtooth corrected it will hop by a good fraction of a
> cycle at 10 MHz
> If the GPS is not surveyed in and seeing many satellites, it may hop by
> more than a cycle at 10 MHz.
>
> Best bet:
>
> Divide the TCXO down to a much lower frequency (< 100 Hz). Use the 5370 to
> look at the
> delta between the GPS pulse and the TCXO output.
>
> Bob
>
>
> > On Nov 3, 2016, at 9:22 AM, Peter Reilley <preilley_454 at comcast.net>
> wrote:
> >
> > I am using the 1 PPS for the trigger.
> >
> > Pete.
> >
> >
> > On 11/3/2016 8:59 AM, Antonio A. S. Magalhaes wrote:
> >>
> >> Pete,
> >>
> >> Tell us about your trigger: where is it?
> >>
> >> Regards,
> >>
> >> Antonio/CT1TE
> >>
> >> ---
> >>
> >> A 2016-11-03 12:20, Peter Reilley escreveu:
> >>
> >>> 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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