or:
https://ntrs.nasa.gov/citations/19870019361
from Charlie Greenhall
On 2020-12-01 15:52, Tom Van Baak wrote:
Hi Skip,
You are exactly right. And, surprisingly, this is correct and normal
for most TI counters.
The short answer is -- for your use case, a *zero deadtime* time
interval counter (TIC) or a timestamping counter (TSC), is what you
want.
----
The long answer is -- your problems is due to counter dead time. That
is, in TI mode, the counter waits for chA, and then waits for chB, and
then calculates and reports the chB-chA time interval measurement. The
latter takes finite time and depending on the counter it may consume
milliseconds or even tens of milliseconds. Part of it is CPU time,
part of it is I/O time (GPIB, RS232/USB, even LAN takes time).
So as chB drifts far away from chA, the next chA may occur during the
short busy measurement report window while the counter is "blind". The
counter then has to wait for the *next* next chA and this results in a
reduction in measurement rate from exactly 10 sps to exactly 5 sps.
Annoying as it is, it's actually a nice way to measure the counter's
deadtime ;-)
Note that this problem occurs with any TI measurement, but it is much
worse with a DMTD, so that's why you ran into already! If you were to
compare a OCXO/1PPS against a GPS/1PPS you might have to wait *years*
before a 1 s phase wrap, a counter deadtime issue, occurred.
You see, a DMTD is a huge magnifier. The good news is that it
magnifies phase difference and magnifies short-term instability by a
factor of, say, 500,000. The bad news is that it also magnifies phase
wrapping by 500,000. So instead of having an annoying 1 s phase wrap
every couple of years, or even once a lifetime, you're going to see it
happen every couple of hours
----
Ok, that said, one solution to this mess is to use a timestamping
counter (TSC) instead of a time interval counter (TIC). Yes, your
TAPR/TICC will work perfectly for this. In fact, this is the main
reason that the TICC was designed to be a flexible dual, independent,
simultaneous, zero-deadtime, chA-*and*-chB timestamping counter
instead of a traditional hardcoded chA-*to*-chB time interval counter.
Another solution is to use a microcontroller-based timer that can do
multi-channel timestamp captures. I have used synchronized picPET's
for this in the past. But more recently, for Corby's new DMTD, I
created a six channel timer that can do timestamping or *zero
deadtime* time interval measurements. Right now this solution works
better than a 5370B or 53132A at 1/100th the price.
Still, if you have a TAPR/TICC use that instead of your 5370B.
/tvb
On 12/1/2020 12:04 PM, Skip Withrow wrote:
Hello Time-Nuts,
I have been having fun playing with the DMTD, but have a question
regarding data collection.
Obviously with a 10Hz offset oscillator I am feeding square waves with
a 100ms period to the START and STOP of the 5370B. The trigger lights
are happily blinking away at 10Hz. However, I seem to have two
different operating modes depending on the difference between the
START and STOP signals.
If the difference is small, no problem. I get 10 samples per second
when TimeLab is run (TimeLab puts the 5370B in Talk Only mode, I
believe). When the difference between START and STOP approaches
something larger than 50ms I start getting only 5 samples per second.
I believe this is because the re-arm time misses the next START
edge,
such that I get every other sample.
This definitely gives bad data for TimeLab. So, the question is, how
do I straighten out this mess? For long-term measurements with two
oscillators that have any kind offset/aging difference you are toast.
I have thought about maybe adding a 10Hz EXT ARM signal, but if it is
not at exactly the 10Hz difference frequency that could create
problems too (and it is a pain in the butt).
I would like to use the 5370B, but maybe it's time to switch to the
TICC.
Any suggestions would be appreciated.
Thanks,
Skip Withrow
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
--
Dr. Don Latham AJ7LL
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
