Hi, Jason,
I hope you're not reading this on your mobile while riding your
motorcycle!
On Aug 28, 2009, at 2:58 , Jason Manley wrote:
If you trigger off the external 1PPS, then you are assured of
absolute timing, rather than the internal sync, which may or may
not be aligned (it is precisely this that you need to test).
Bearing in mind that the exact trigger point of the 1PPS on each
IBOB may jitter by up to 1 clock period (hence your snap's counter
values might be misaligned by up to 2 values). I have seen this
with low amplitude or slewed (poor distribution network) 1PPS pulses.
At the ATA, each ibob uses an internal counter to generate its sync
pulse internally. At power-up and whenever "re-armed" via software,
the internal counter is reset by the next external sync pulse (1 PPS
in our case). We have found that to work very well. It keeps the
exact same number of clock cycles (and samples!) per sync pulse
without having to worry about the exact capturing of every external
sync pulse. There is +/- a cycle of ambiguity from ibob to ibob, but
that's fixed between "re-arms" (i.e. for weeks at a time) so it gets
calibrated out with the fixed delays anyway. Because this scheme
uses the external sync pulse only occasionally (i.e. on power ups and
software re-arms), it is also very robust to intermittent "problems"
with the external sync distribution network such as someone
accidentally unplugging an active pulse distributor (not that
anything like that has ever happened at the observatory of course! :-)).
We even had a period of time where our 1 PPS generator had some
stability "issues" so we added a register that captures the value of
the internal sync's counter on each rising edge of the external sync
signal. Based on this register, we can tell not only whether the
internal and external sync signals are out of sync, but also how far
out of sync they are. This has been very helpful. FWIW, our ibobs
stay exactly in sync most of the time, but sometimes we observe (when
we bother to look) a variation of +/- one cycle.
Dave
