Tektronix 485, Инструкция по эксплуатации

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Circuit Description—4 8 5 /R 4 8 5 Service
A TR IG G E R G EN ER A TO R
The trigger amplifier drives a Schmitt Trigger circuit
based around CR751 and a constant resistance load. This
circuit provides a standardized output pulse that is time
related to the trigger event. This standardized pulse and a
holdoff signal drive a two tunnel diode circuit to provide
stable triggering from DC to at least 350 MHz with very low
trigger jitter.
CR762 is the arming tunnel diode and CR772 is the
output tunnel diode. After CR762 is fu lly armed and
CR772 is partially armed (at the end of holdoff), a pulse
from CR751 causes CR762 to go to its high state. This
increases the current through R764 to fu lly arm CR772,
which goes to its high state when the same pulse that fired
CR762 is received through the short delay line. The output
of CR772 drives differential pair Q792 and Q794. When
CR772 is in its high state pin 1 of U780 is high, turning on
the light DS781 (TRIG'D). Q792 provides the A GATE
through emitter follower Q810. The output of Q794
couples to a differential pair Q802 and Q804, which
provide the sweep gates.
When the sweep is completed, a positive signal from A
sweep turns on Q816 and Q822 to provide a signal to pin
16 if U780. This initiates the holdoff portion of U780 and
pin 17 goes positive. Pin 17 stays positive as determined by
timing R's and C's on the timing switch to pin 8. The
holdoff signal on pin 17 of U780 drives Q768 and Q778.
Q768 and Q778 gate the current to CR762 and CR772.
When Q768 and Q778 are off, CR762 and CR772 are
receiving the appropriate current as set by R765 (A arming
T.D.) and R775 (A output T.D.). When Q768 and Q778 are
on they reset CR762 and CR772 to their low states.
In AUTO TRIG pin 19 of U780 is grounded. The trigger
signals must recur at least every 50 ms, which is the lowest
rate the circuit will provide a triggered sweep in AUTO
MODE. The timing of the AUTO MODE is determined by
the time constant of R787 and C787.
In SINGLE SWEEP mode +5.5 V is applied to pin 12 of
U780. There will be only one sweep for each time U780 is
armed by pushing the RESET switch. The sweep will fire
when a trigger signal is received while or after the RESET
switch is depressed. The READY light on pin 11 of U780
indicates when the sweep is armed.
B TR IG G E R G EN ER A TO R
Is similar to A Trigger Generator except it has RUNS
AFTER DELAY TIME mode that disconnects the trigger
amplifier. In addition, it uses A Trigger Gate and delay
pickoff gate as hold-off.
In the triggerable mode, the B Trigger Generator triggers
only after receiving both the delay pickoff and A Gate
signals. Delay pickoff gate drives comparator Q1092 and
Q1096. Both A Gate and delay pickoff gate are required to
drive Q1082. The output of Q1074 provides arming current
for CR1062 and CR1104. When the triggering signal causes
CR1052 to go to its high state, CR1062 is properly armed,
and will fire arming CR1104, which fires approximately
1.5 ns after CR1062.
In B RUNS AFTER DELAY TIME mode, the trigger
amplifier is turned o ff via R1038 and enough current is
flowing in the base of Q1068 so that it is in saturation. This
saturation routes enough current to CR1104 to turn it on
when A Gate and delay pickoff gate arrive.
In both cases when CR1104 goes to its high state,
comparator Q1106 and Q1110 changes state giving a B
GATE out thru Q1114. Q1106 is connected to comparator
Q1121 and Q1124 which changes state, giving plus and
minus sweep gate out.
In A 1, 2, and 5 ns, B is in B RUNS AFTER DELAY
TIME. In addition in 1 and 2 ns the voltage to which C l 121
charges is varied by the HORIZONTAL POSITION control.
This gives a time position to these two sweep speeds when
using the HORIZONTAL POSITION control.
A and B SWEEPS
A and B sweep generators each produce, upon receipt of
a gate, a negative going sweep of 1.0 V/div of horizontal
excursion, starting at +13 V.
In A Sweep, Q884 and Q888 are the miller amplifier.
Timing resistors R873 and R1409 to R1422 return to
+50 V in calibrated settings. The values of the timing
resistors and capacitors C877 to C882, C l404 and Cl 405,
at various sweep speeds may be determined on the Timing
Switch and Generator schematics. Q892 compares the
sweep output to +13 V and sends the error signal to the
miller input via R866 and Q854. Receipt of a + gate at
TP851 turns o ff Q854, allowing the sweep to run. CR867
must not conduct during baseline, so Q866 sinks the higher
timing currents. Q934 turns on at the end of the sweep,
producing a + going stop pulse that terminates the sweep
gate. Q856 and Q858 feed a current to the miller input at
the end of the gate to prevent large negative voltages and
long recovery times at low timing currents. R863 affects
linearity at 10 and 20 ns/div.
The Delay Comparator Q902 enables B Sweep when the
sweep voltage becomes less than the voltage on the Delay
Time position control, where U910 is a voltage follower.
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