[Vetroc_daq_sbs] 16-channel tdc 'dropout' at high-rate

[R. Evan McClellan]

I should also note that this was all done using a 10 kHz pulser as the trigger.

Cheers,
Evan

----- Original Message -----
From: "R. Evan McClellan" <randallm at jlab.org>
To: "Benjamin Raydo" <braydo at jlab.org>
Cc: "Vetroc_daq_sbs" <vetroc_daq_sbs at jlab.org>
Sent: Monday, May 15, 2017 3:40:31 PM
Subject: [Vetroc_daq_sbs] 16-channel tdc 'dropout' at high-rate

Hey Ben,

I'm seeing a loss of TDC hits from the VETROC, in 16-channel blocks, when I run the PMTs at very high-rate (~1 to ~10 MHz).

I replaced one PMT signal with a pulser signal, and checked that channel as I took many runs, slowly increasing the average PMT rate. At some 'threshold' average rate, the number of hits suddenly plummets, and drops to zero. The channels seem to drop out in independent groups of 16 (0-15, 16-31, etc.).

I see the same loss (at the same average rate-per-channel) whether all inputs are connected, or only a single NINO is connected (16 channels). Therefore, I guess the loss must come somewhere in processing that is done separately for each 16-channel group.

I have checked that the NINO output shows no losses up through 10MHz per channel.

I also checked that it's not simply a decoder problem. The hits are missing in the raw data.



Perhaps I have asked you this before, but what is the expected rate capability of the VETROC TDC firmware? And, perhaps more importantly, what is the expected failure mode when the rate is too high?

I will be available this week (except for meetings Tuesday afternoon). I can meet you at ESB if it would be helpful.

(It's possible that these rates are already above the experimental requirements, but I want to make sure to understand the mechanism for the loss of hits.)

Cheers,
Evan



R. Evan McClellan, PhD
Hall A Postdoctoral Fellow
Jefferson Lab
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