Current Sources; Range Switches; Reference Circuit; Error Feedback - HP 8112A Operating, Programming And Servicing Manual

Error Feedback

10.3-8 Servicing the Timing and
Slope Generators

Current Sources

External constant current sources on the control board A2 provide
I up ref and kown d. 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 I,,
(derived from Iur, ref) t a 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
discharges via the current mirror at a rate set by I&w,, ref, to 0 V.
At this point I&,wn 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 ps to 95 ms are possible with the external
capacitors, slopes less than 100 ps use the internal capacitor in U301.

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;