HP 8112A Operating, Programming And Servicing Manual page 208

Error Feedback
1 0.3-8 Servicing the Timing and
Slope Generators
Current Sources
External constant current sources on the control board A2 provide
re f re f ·
Iup and Idown When a rising edge is detected at TRIG IN,
Q303 is switched on via the internal buffer to enable one of the
internal or external ramp capacitors to charge up linearly by Iup
re f
(derived from Iup ) at a rate set by the LEE DAC on the control
board. Once the charge threshold of 2 V has been reached, charging
current is diverted via internal diodes into the current mirror circuit,
and the capacitor voltage remains constant.
When a falling edge is detected at pin 1 , Q303 is switched off and
Q304 is switched on, dumping Iup to ground. The capacitor now
discharges via the current mirror at a rate set by Idown
At this point Idown is diverted via the internal diodes and the current
mirror circuit, and the capacitor voltage remains at 0 V until the
next trigger is detected.
Range Switches
The ramp timing capacitance is selected from range latch U300.
Slopes from 100 µs to 95 ms are possible with the external
capacitors, slopes less than 100 µs use the internal capacitor in U30 1 .
Reference Circuit
An external reference source U320 and associated components,
provides the reference voltage (via the Schmidt trigger) required by
the internal switching diodes which clamp the ramp voltage and the
output current sources.
The four ECL error feedback outputs from the Timing and Slope ICs
are converted to TTL levels by transistors Q140 to Q143 and their
associated components. The error outputs are fed via latches and
drivers to the Data Bus when RSA is active,
re f ,
to 0 V.