Specifications
SR400 Circuit Description
77
by the processor. When the digital portion of the
delay is complete, !DLYA_TIMEOUT goes low
which causes the differential pair, Q702 and Q703,
to switch the current away from the operational
transconductance amplifier (OTA) 1/2 U704 and
to C706. The voltage on C706 then ramps up until
it reaches -5 V at which point the differential
comparator, Q704 and Q705, switches state and
generates the EOD_A (end of delay) signals. The
OTA is used to precharge the capacitor C706 to a
voltage programmed by D/A output DLYA_ADJ.
Changing this voltage will change the duration of
the analog delay. The current source is calibrated
to charge C706 at a rate of 2 mV/ns.
DELAY RESET (sheet 8)
When both gate delays have finished, EOD_A and
EOD_B will both be high. This triggers the one-
shot 1/2 U806 to provide an 800 ns reset pulse.
This pulse resets the trigger latch and switches the
analog charging current from C706 back to the
OTA. The OTA then discharges C706 back to the
initial programmed voltage. Q801 provides an
additional current kick to speed up this recharging
process. The reset pulse also reloads the digital
delay counters. Any trigger received during a
delay or a delay reset is flagged as a rate error.
GATES (sheet 9)
The EOD_A signal clocks the width latch 1/2
U902. The width latch in turn clocks the gate
latch 2/2 U902 thereby starting the A GATE. The
width latch also starts the width delay. The width
delay is similar to the gate delay described above.
The A width delay has its own phase coherent
oscillator which is started by the start of the
respective gate. This allows both the delay and
width to be active at the same time and allows a
gate delay and width to cover nearly all of the
trigger period simultaneously.
The EOW_A (end of width) signal resets the gate
latch 2/2 U902 and terminates the A gate. The
gate latch drives the A GATE output and enables
counter A.
POWER SUPPLY
The power supply is a switchmode power supply
operating directly from the rectified line voltage.
POTENTIALLY LETHAL VOLTAGES ARE
PRESENT IN THIS CIRCUIT. DO NOT
ATTEMPT TO SERVICE THE POWER
SUPPLY. ALWAYS LEAVE THE COVERS
ON WHEN OPERATING.
The power supply is over-Voltage, under-Voltage,
and short circuit protected. and switching
frequency is approximately 50 KHz. The fan
control circuit and the LCD backlight power are
also in the power supply enclosure.