[time-nuts] GPS clock to RT-Linux for pulsar machine
dforbes at dakotacom.net
Fri Jul 1 17:19:55 EDT 2005
Matt Ettus wrote:
>>Don't do it that way, you will get in trouble with the interrupt
> I have to echo this advice. While RTLinux and other RTOS'es have
> great interrupt latency performance, using those interrupts to trigger
> sampling on an ADC is a bad idea. It won't work at all if you are
> trying to sample faster than about 1-10 kHz, and the jitter is
> horrible even below that. Even a great RTOS will only get you about
> 10us repeatability. It is always best to use a real oscillator to
> trigger sampling. For some reason, interrupt-based sampling seems
> very popular in the radio astronomy world.
Thanks for the good advice, but I said that I was planning on using
the maser for the ADC sampling clock, and the interrupt only for
getting 1PPS into the computer.
After pondering all the quick replies, I think I'll just run the 1PPS
signal straight into an analog channel on the ADC card. All the $300
ADC cards have a 16-input mux so it's cheap to do that. It's not quite
as memory-efficient as using a digital input, but it's quick and
effective. I can discard the unused bits when I write to disk.
It's funny how the typical middle-aged engineer brain wants a one-bit
digital input for a digital signal, but these days wasting 15 bits of
analog data for one digital bit just isn't a problem.
As for radio astronomers and interrupts, my group is pretty good about
using the appropriate method to get data into the computer. I do shake
my head in wonder occasionally when I read how things used to be done,
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