[time-nuts] pps vs. 10 MHz timing

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 11 21:54:46 EST 2008


For those GPSDOs which also use the OCXO clock to generate the LO for
the GPS receiver (eg Thunderbolt) the zerocrossing transitions of the
10MHz output are accurately aligned to the leading edge of the 1PPS output.

For other GPSDOs there may be an offset between the 10MHz zero crossing
and the receiver 1 PPS output.

If the PPS output is derived from the OCXO via a divider then the
leading edge of the PPS output will be aligned (within a gate
propagation delay or 2) with the PPS output.

One can always use a dual phase synchroniser to detect the 1Hz
transition using the 10Mhz output.


Matt Ettus wrote:
> On my Oscilloquartz GPSDO, the 10 MHz output goes low at very close to
> the same time as the 1 PPS output goes high.
> On my Fury, the 10 MHz sine wave is just dropping off of its max high
> voltage as the 1 PPS goes high.  The 10 MHz CMOS output goes high just
> shortly before the 1 PPS.
> What do other GPSDOs do?
> Clearly, any device trying to latch the 1 PPS signal using the 10 MHz
> clock will need to choose which clock edge to use, depending on which
> type of GPSDO is used.  How is this normally handled?
> Also, I noticed that the TADD-1 has some weird distortion of a 10 MHz
> sine wave.  It turns it into a sort of a 3 valued square wave.
> Essentially, it stops and dwells at the midpoint for a while instead
> of making a sharp transition.  I think this would be bad if that dwell
> point is close to the threshold for the receiving device.  Has anyone
> else noticed this?
> Thanks,
> Matt
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