Trigger Holdoff - Tektronix 1S1 Instruction Manual

Circuit Description – Type 1S1
control tunnel diode D449. The front-panel TRIGGER SEN-
SITIVITY control sets the sensitivity of the trigger recognition
circuit by adjusting the current division between D430 and
R435 (TRIGGER SENSITIVITY). The INT TRIG LEVEL adjust-
ment, R420, sets the dc current through Q424 to adjust the
operating range of the TRIGGER SENSITIVITY control.
When the trigger circuit is set for triggered operation, the
quiescent circuit conditions following reset and before the
arrival of another triggering excursion are as follows: Q424
is conducting moderately; Q454 is cut off and Q464 is held
in saturation by the trigger holdoff circuit; D430 and D449
are biased in the low-voltage state near the switching point;
D456 is reverse biased and D470 is forward biased. Back
diode D432, which is part of the high-frequency synchro-
nizing circuit, is non-conducting.
Normal Triggered Operation. A positive-going signal
at the emitter of Q424 (corresponding to a positive-going
input signal excursion when the TRIGGER SOURCE switch
is set to one of the + positions) causes current to increase
through Q424 and thus through D430. When the triggering
signal excursion raises the diode to its switching level, D430
sends an output pulse through C448, R449 and L449 to con-
trol tunnel diode D449.
Since D449 is set near its switching point by current
through R460 (CONTROL TD BIAS), the positive-going pulse
from D430 causes D449 to switch to its high-voltage state.
The resulting positive-going pulse is applied through L302
to the fast ramp circuit and through L449, R449 and R448
to the base of Q454, turning on that transistor. As Q454
turns on, the negative-going voltage at its collector reverse
biases D470 to start the operation of the trigger holdoff
circuit. (A detailed description of the Trigger Holdoff opera-
tion will be given later). As soon as recognition tunnel diode
D430 has switched to its high-voltage state and has pulsed
D449, D430 is immediately reset to its low-voltage state by
back diode D432. The rate at which D430 is reset is de-
termined by the L/R time constant of L428, D432 and D430.
Then, as Q454 becomes saturated, D456 turns on and diverts
enough current to keep D430 in its low-voltage state, pre-
venting another trigger recognition from being made. How-
ever, current remains sufficient to hold D449 in its high-
voltage state.
At the half-way point of the trigger holdoff interval,
Q464 is turned off by the trigger holdoff circuit. With the
current source removed, D449 resets to its low-voltage state
and Q454 turns off, but D430 is still not triggerable. At
the end of the holdoff period when Q464 is turned on again,
current is restored through Q464 and the trigger recognition
circuit becomes rearmed and able to be triggered by the
input signal.
Synchronized Operation. When the RECOVERY TIME
control is switched to the SYNC position, the rearming rate
of control tunnel diode D449, following holdoff, is retarded
by connecting C461 to ground. During the time that D449 is
recovering, D430, L428 and D432 operate as a free-running
multivibrator. Diode D432 sets the load on D430 so that it
will turn on and stay on momentarily, then turn off and
stay off momentarily, and repeat the cycle until D449 has
reached the triggerable level. If a triggering signal between
20 Mc and 1 Gc is applied to the input of the circuit, the fre-
quency of the free-running multivibrator can be adjusted
with the TRIGGER SENSITIVITY control to synchronize with
the signal. Each time D430 switches to its high state, a
small pulse is applied to D449; however, D449 cannot be
3-10
triggered until it has reached the proper bias level. As
the bias on D449 reaches the triggerable level, the first of
the small triggers from D430 that is able to raise D449
above its switching point causes D449 to switch and produce
a normal output pulse to the fast ramp. The normal holdoff
interval follows, then another triggering cycle begins as
D430 and D432 again synchronize on the input triggering
signal. In this way, D449 becomes synchronized (at a much
lower frequency) to the input signal.
Free Run. When the TRIGGER SOURCE switch is set to
FREE RUN position, R460 (CONTROL TD BIAS) is shorted
out, providing enough bias current for D449 so it will switch
to its high-voltage state following the holdoff interval with-
out waiting for the recognition tunnel diode to produce a
pulse.
Free run operation also occurs when the TRIGGER
SOURCE switch is set to one of the INT or EXT positions
and the TRIGGER SENSITIVITY control is turned toward the
clockwise end of its range of rotation. In this case, how-
ever, the bias level on D430, rather than the bias on D449,
is set to the free run level. Following the holdoff interval,
D430 automatically switches to its high-voltage state, trig-
gering control tunnel diode D449 to produce an output
pulse. In this method of free run, the trigger circuit may also
synchronize with input triggering signals above 20Mc, just
as with the RECOVERY TIME control at SYNC position. Since
the rearming of D449 is faster with the RECOVERY TIME
control switched out of SYNC position, this synchronization
may not be quite as stable (at some frequencies) as it is
with the control set to SYNC position.

Trigger Holdoff

Before the arrival of an input triggering event, transistors
Q485 and Q495, making up the trigger holdoff multivibrator,
are in a saturated condition. Diodes D472, D482 and D490 are
reverse biased, and diodes D470 and D492 are forward
biased. Trigger holdoff capacitors C479 and C480 are
charged to a value established by their total capacitance,
as selected by the TIME POSITION RANGE switch, and by
the voltage at the junction of R478 and R480 (approximately
+12 volts). Current through R495 and Q495 keeps transis-
tor Q464 in saturation to provide bias for the trigger tunnel
diodes.
When D470 is reverse biased as Q454 turns on, current
flow switches from D470 to D472, initiating the rundown
portion of the holdoff cycle. The trigger holdoff capacitors
begin to discharge toward the -139-volt supply through
D472, R470, R473 and R475 (RECOVERY TIME). The rate
of discharge is determined primarily by the holdoff capaci-
tors and can be adjusted over about a 30% range by the
RECOVERY TIME control. Minimum recovery time is obtained
with the control set fully counterclockwise, but not switched
to SYNC position.
When the voltage at the junction of R470 and R473
reaches approximately ground level, D490 becomes forward
biased, diverting current from the base circuit of Q495. This
turns off Q495, which turns off Q464 and Q485, forward
biasing D482. With Q464 turned off, control tunnel diode
D449 resets, turning off Q454 and forward biasing D470.
This is the halfway point of the holdoff cycle.
As D470 turns on, current is again diverted from D472 and
both D472 and D490 become reverse biased. Transistor
Q495 does not turn on at this time, however, since its cur