[time-nuts] Linux PPS clues?

[Ilia Platone]

On 10/20/16 22:08, Hal Murray wrote:
> info at iliaplatone.com said:
>> These events are random photon arrivals (converted to 5vTTL pulses),  their
>> rate was measured using the pulse width of the smaller detected,  which was
>> 5~10 uS during an observation in low-light environment. The photon arrival
>> and pulse width were random with a minimum pulse  width of 10uS. What I want
>> to do is measuring the photon arrival  precisely (low to high transition -
>> interrupt I guess), consider that  the maximum rate would be 100Kcps because
>> the photon events would  overlap if higher. If the 3130 controller can
>> handle such rate it would  be great :)
> I think your 100K samples per second is going to be exciting.
>
> You can collect some data by writing some code and connecting up a signal
> generator.  Start with a slow frequency to debug the software than turn it up
> and see how fast you can go.
>
> I assume it's OK if some (but not many) samples are lost.
>
> You (or I) may be confusing the data rate.  There are two issues.  One is the
> peak rate.  That sounds like your 100K above.  The other is the average.  How
> many samples do you expect in a typical second?  If that is low enough, you
> can solve the peak problem with enough buffering.
>
> My straw man would be a FPGA.  The idea is to collect a batch of pulse
> arrival times and DMA them into memory.  When the buffer fills up or a timer
> expires, it would finish that block and switch to the next one.  You could
> feed a PPS to the FPGA if you want timing linked to the outside world rather
> than just relative times.
>
This was the very first approach. But.. err.. I'm trying to determine 
more precisely the pulse width right now, so the maximum event rate. The 
real problem is my analogue oscilloscope that seems to lose time at peak 
detection: it shows a ramp at each pulse raise.
If this can help, I attach a photo with the reading of my oscilloscope 
(2uS/div, 1V/div). not all pulses saturate, but can be detected by a 
CMOS input. It's difficult to sync because of the high entropy of this 
signal.
Let me know if you can give me any hint on how to setup the oscilloscope..



Best Regards,
Ilia.

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Ilia Platone
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