[time-nuts] GPS vagaries and binary interface
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Oct 29 00:12:19 EDT 2006
Didier Juges wrote:
> That's the impression I am getting. I do not know if any of the GPSDO
> that I have seen described in recent literature take care of this properly.
> It seems when the GPS goes nuts, the 1 PPS goes quite a bit out of
> normal range, so it should not take too much processing power to
> determine if it's in range or not.
> Of course, an analog solution would require many more parts to do that
> determination, filtering and switching, so it seems the most *practical*
> way to implement a GPSDO is with a uC of some sort.
> The uC could even monitor what's coming out of the GPS receiver's serial
> port and open the loop if there are not enough satellites in range.
> Didier KO4BB
> time-nuts mailing list
> time-nuts at febo.com
On further reflection a slower 2 channel simultaneous sampling ADC (eg
AD7862 200ps aperture delay mismatch, 100ps sampling jitter ) with
matched ADC gains and aperture delays is a better fit when sampling the
nominally quadrature phased sinewaves.
To assist in filtering out spurious data a coarse (1us resolution??)
phase derived by sampling a digital counter can be used to detect when
the GPS PPS pulse timing deteriorates.
When using a timing GPS receiver with TRAIM enabled this elaboration is
It only remains to measure the quadrature phase error and amplitude
mismatch errors of the nominally quadrature phased sinewaves.
This can be done by taking a burst of calibration samples triggered by
the PPS input (after synchronising it to the reference clock).
Equivalent time sampling techniques can be used to take samples with an
effective spacing of one reference oscillator period (100ns) throughout
the sinewave cycle.
A total of 64/128 samples is a sufficient number to allow the dc offset,
the amplitude and phase offset of the fundamental, as well as the and
amplitudes and phases of the at least the first 16/32 harmonics for
each of the 2 nominally quadrature phased waveforms to be determined.
Since the processor only need process a few (~100??) samples each second
almost anything that can read and process the ADC samples fast enough (1
sample burst /sec)
Residual ADC distortion, gain mismatch and aperture delay mismatch can
be combined with the equivalent sinewave errors, the actual source of
these errors doesn't really matter too much as long as their effect on
the phase measurements can be corrected.
The required logic can easily be implemented using a small programmable
gate array or equivalent device.
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