Tektronix 5T1A Instrument Reference Book page 118

Page 3-12 , Type ST1A
drive the sampler in the '4' Series sampling unit. A pulse
from the comparator circuit also drives the staircase gene-
rator, causing it to advance one step. The fast-ramp cir-
cuit operates each time a trigger output signal is fed to
it from the trigger circuit. As the staircase voltage ad-
vances, step by step, the fast-ramp voltage must become
greater, and thus will run a little longer with each succeed~
ing run-down before the comparator circuit generates a
pulse. The fast-ramp voltage represents the real time on
which the system reconstructs the signal.
Ramp Generator
Q261 is a constant-current supply. Before a trigger
signal is received from the trigger circuit, D255 is in the high
state and holds Q254 on. Current from Q261 passes through
Q254 and holds C260 near ground voltage. D255 is held in
the high state by current through R244. Current through
R244 also provides base current for Q254, and Q254 conducts
the current supplied by 9261.
9261 receives a constant base voltage from the -19-volt
supply through decoupling filter R260 and C260. Q261 emit-
ter current, which comes from the -100-volt supply, passes
through R265 and R267 which drop about 81 volts. This deter-
mines the current through Q261. current supplied by the
transistor is determined by emitter current, and is little
affected by any change in collector voltage.
The trigger cirmit sends a positive pulse which causes
Q2hk to conduct more current. The collector voltage of qehh
rises to ground, and switches D255 to its low state, turning