Tektronix 1S1 Instruction Manual page 48

rent path through Q485 is open. With D472 reverse biased
and D482 conducting, the holdoff capacitors begins the
runup portion of the holdoff cycle, charging toward the
+100-volt supply through D482, R483 and R499. As soon as
the voltage at the junction of R478 and R480 reaches about
+12 volts, D482 becomes reverse biased and the runup
stops. Current is then restored through the emitter-base
junction of Q485, turning on that transistor. Current through
the base-emitter junction of Q495 then turns on Q495 and
both transistors go into saturation. As current through Q495
begins again, Q464 is turned on again and saturates, re-
storing the arming current to the trigger tunnel diodes. The
circuit is then ready for another triggering event.
Minimum duration of the holdoff cycle is approximately
12 µsec on the 50 nsec and 500 nsec time position ranges
with only C479 connected to the circuit. For slower sweep
rates larger holdoff capacitors (C326 A-D) are connected
through the TIME POSITION RANGE switch to retard the
holdoff rundown and runup. The holdoff interval is always
slightly longer than the maximum excursion of the fast ramp.
FAST RAMP
The fast ramp circuit consists of the staircase inverter,
the fast ramp clamp and generator, and the comparator
and comparison-pulse generator. Fig. 3-7 is a detailed block
diagram of the fast ramp circuit.
Staircase Inverter
Feedback voltage from the output of the staircase gen-
Fig. 3-7. Block diagram of the Fast Ramp circuit.
Circuit Description – Type 1S1
erator is connected to the staircase inverter to be used as
a comparison voltage for controlling the amount of sampling
delay with respect to the triggering event. Transistors Q374
and Q384 are connected as an operational amplifier, with
the gain determined by the ratio of feedback resistor R377
to the input resistor selected by the TIME/CM switch. When
the TIME/CM switch is locked to the TIME POSITION RANGE
switch, a 1-2.5-5 magnification sequence occurs for each
time position range as the switches are turned clockwise. This
is accomplished by selecting R530A, R530B or R530C (shown
on the Timing Switch diagram) as the input resistor for the
amplifier. When the switch is turned so that another position
of the TIME POSITION RANGE switch is used, the amplifier
repeats the 1-2.5-5 sequence, but the slope of the fast ramp
has been changed to produce another sequence of equiva-
lent-time sweep rates. When the Time/Cm MAGNIFIER knob
is pulled out and turned clockwise, one of two attenuators
may also be connected to the inverter input to reduce the
signal amplitude and change the equivalent time/cm. The
smaller the amplification factor of the amplifier (R
the greater the attenuation of the input signal, the smaller
will be the output excursion of the amplifier and the shorter
the equivalent time between samples. This produces the
effect of magnifying the display or increasing the sweep rate,
but does not change the display dot density. The Time/Cm
VARIABLE control (R500) can also change the gain of the
amplifier, since it is a series input resistor.
The two time-magnification attenuators are R510 A-C (a
10X attenuator) and R520 A-C (a 100X attenuator). The
six steps of time magnification (produced by turning the
/R ) or
f i
3-11