Calibrating and Gain Monitoring the Chorus PM's
The Flasher System consists of 10 flasher boxes, each having 24 LED's, and a central control box, containing the 220 to 24 Volt supply, and the Lemo to BNC conversion vor the signal cables.
At the top-left side we find the signal input (J2) NIM-level (0 Volts, going to -1 Volts when active, terminated in 50 Ohms). This signal is first passing a rate-limiter circuit (U5 & U4), to limit the pulse rate to a frequency that avoids destroying the LED's, when someone would put several Mhz on the pulser by accident.
DO NOT TOUCH the potmeter R49. The max. repetition rate is controlled by R52, R53 and C68.
The rate-limiter consists of a discriminator (AD9696) that starts a one-shot (74HCT123). The one-shot blocks the discriminator for a period that is selectable with jumper J1. When the boards are delivered, the Jumper J1 is in the low-rate position. For short periods one may use the high-rate position, to get a better image on an oscilloscope. But put the jumper back on the low position, to avoid unnecessary ageing of the LED's.
Next, the pulse is split into 24 identical pulses, via a tree of bus-drivers (74F244, U6, U1, U2, U3), and routed to the avalanche transistors (BFR96), that act as Fast Switches, discharging the 100 nF capacitors over the LED`s.
The amplitudes of the light-pulses have to be individually tuned with 24 potmeters (R9 to R46), to compensate for differences between the LED's and the PM gains.