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Last modified: 12:45 Friday 01-OCT-1999 |
A single crystal on this card produces all clocks for the readout boards; 9.4345 MHz is produced by dividing a standard 18.869 MHz crystal.
In MiniDAQ, the lasers and the ADC sampling clock are both free running. (For that matter, the cosmics are free running, too.) The laser sends a trigger signal, which sends the 0x04 command to the readout boards. It's a very simple circuit. To a good approximation, the trigger time is probably more-or-less smeared uniformly over one clock period by this. It is not measured.
The laser trigger itself does not contribute significantly to the smearing. It is created from a fast photodiode with ~2 ns rise time. This signal comes to the laser trigger board, and a TTL signal is sent to MiniDAQ with a total delay ~150-200 ns (including cables).
For RHIC running---The ADC sampling clock will be phase-locked to the RHIC cycle, taking advantage of the fact that the accelerator pulses are not free running. Spencer Klein notes that, for RHIC data taken in the final configuration, laser events will also be phase-locked to the RHIC strobe through a feedback loop that adjusts when the Q-switch fires. Mikhail Aluyshin (MEPhI) is building this circuitry. Until it is installed, the offset will may vary from event to event.
The switch positions must exactly correspond to enabled/disabled states of the Rosies set in MiniDAQ at configuration time.