Hi Tom
We use hardware time tagging in some of the spectrometers we've built,
accurate to a few nS, described below:
We've implemented time tagging in spectrometers and correlators (we can
send you some spectrometer
designs if you want), by running the ADC sample clock from a
synthesizer locked to the observatory
maser reference oscillator. The ADC board clocks the FPGA, and the
FPGA design contains a counter
that counts these clock ticks. This counter (really an elapsed time
clock) is read out with each spectrum.
The counter is reset using an external 1 PPS; the 1 PPS signal is fed
into the ADC board
sync input. We issue a command to the FGPA 0.5 seconds before
midnight, which "arms" the
spectrometer's clock counter, telling the counter "on the next 1 PPS,
reset the counter, and then to ignore
future 1 PPS signals until another "arm" command is issued".
Best Wishes,
Dan
Tom Downes wrote:
Hello:
I am new to the list and to ROACH boards so apologies in advance if
this question is misdirected. I am working at Caltech on using a
ROACH(1) board running BORPH via USB. Specifically debian/etch with
kernel 2.6.25-svn1867-dirty1.
Our application requires timestamping FFT data to precision of 10ms -
there is another set of 100Hz data from a separate source with which
we need to align. I imagine that this kind of thing is a fairly
common problem.
My intention is to use linux/BORPH+ntp to get the time-of-day correct
to within 500ms. Then use a 1 pulse-per-second signal (rising edge
guaranteed to be at borders between seconds of time-of-day) to align
the NTP time to the true time as reported by NIST or GPS. This is, in
broad strokes, how an NTP stratum 0 server works.
Getting BORPH right to 500ms is proving to be much harder than
anticipated. The stock installation runs ntpdate at boot and that
manages to get the clock right, but the drift from there is HUGE. On
the order of a second per minute. A standard motherboard clock
(without NTP) will drift by, at most, a few seconds per day.
So I installed the NTP daemon via apt-get. ntpd can't seem to sync to
servers here on campus (ntpq -p does not list a star next to
ntp-XX.caltech.edu <http://ntp-XX.caltech.edu> where XX=01,02). I
have just tried deleting the ntp.drift file to try to force ntpd to
re-calibrate for clock drift, but I think the drift is so huge, and
probably fairly random, that it will give up.
Is there a solution for keeping the ROACH/BORPH time-of-day accurate
over time scales longer than a few seconds? ntpdate is fine for
making sure the logs don't read 1969, but not much else. I can come
up with alternative schemes for my problem, but this seems preferable.
Tom
