Atec Tektronix-1502 - Manual (Page 31)

TM 9-4935-601-14-3&P
A negative-going signal from the sampler comparator is amplified by Q1553 and the collector output is
fed to a fast-rise one-shot multivibrator. A positive-going signal to Q1544 produces a negative-going signal on
the base of Q1535, which results in a more positive-going signal on the base of Q1544. This action continues
until both transistors are completely saturated, at which point TP1534 (violet test point) is at ground level. It will
stay in this state as long as a charge remains on C1543. When this charge is depleted, the circuit resets and
TP1534 returns to -5 volts. The fast positive edge at TP1534 is fed through C1534 into transformer T1538,
which drives the avalanche circuit. Avalanche transistor Q1537 goes into its avalanche mode and the voltage
across the transistor collapses very rapidly (300 ps). The collector voltage drops and the emitter voltage rises.
These opposite signals are fed to the SNAP-OFF circuit by way of C1631 and C1632. The SNAP-OFF diode
CR1632 is normally forward biased by a current from R1625 and R1633. This current is established by the
emitter voltage of Q1539. The emitter voltage is controlled by R1538 (SNAP-OFF CURRENT). The negative-
going signal of the avalanche circuit is applied to the anode of the SNAP-OFF diode and the positive-going
signal is applied to the cathode to reverse bias this diode. After a small delay, the SNAP-OFF diode will
become reverse biased. The reverse biasing occurs in less than 50 nanoseconds. The fast rising step from
CR1632 is applied to the shorted strip lines. The width of the strobes is determined by the physical length of
the shorted strip lines. The generated strobe is differentiated by capacitors C1635 and C1636 and then strobes
the sampling diodes.
The avalanche voltage amplitude is controlled by R1525 with Q1529 acting as an emitter follower.
Blowby Compensation. The Blowby compensation network, Q1656, R1657, R1752, and R1656,
cancels any signal component passing the gate due to diode shunt capacitance. This is accomplished by
inverting the transmission line signal and adding it to the input of the preamplifier through C1648. The amount
of compensation is set by R1657 and R1659 (LO FREQ COMP).
Comparators
There are two comparators; the Pulser Comparator and the Sampling Comparator. The Pulser
Comparator provides the TD trigger signal to the Pulse Generator and the Sampler Comparator provides the
Sampling Trigger signal to the Strobe Generator. Each comparator consists of a differential amplifier; Q1336
and Q1325 in the Sampler Comparator; and Q1347, Q1348 in the Pulser Comparator. The two comparators
operate the same, therefore only the Sampler Comparator component numbers will be used in this description
(see circuit diagram 1B).
The two inputs for the Sampler Comparator are the bases of Q1336 and Q1325. Depending on which
input is higher, the voltage at TP1324 (green test point) will be +5 V or negative. If the base of Q1336 is high,
Q1336 conducts and Q1325 is turned off. The current for Q1336 comes from Q1329 which turns Q1328 on.
This puts the voltage at TP1324 high (+5 V). When the base of Q1336 is below that of Q1325, Q1336, Q1329
and Q1328 are turned off. Q1325 is conducting. The output voltage drops to the level of a conducting diode
junction.
Q1425 and Q1431 form a temperature-compensated current source for the differential amplifier. The
collector current of Q1431 is determined by the value of R1431 and R1422. It is approximately equal to the
current of Q1425.
The second input of the Sampler Comparator is connected to the attenuated and inverted slow ramp
signal. (The second input for the Pulser Comparator is a dc voltage that is controlled by the ZERO REF SET,
R0151. The X.1 POSITION CAL control, R1132, compensates for unequal delays in the comparator when the
speed of the Fast Ramp is changed (X1 to X.1).)
Ramp Generators
There are two ramp generators, the Fast Ramp Generator and the Slow Ramp Generator. The Fast
Ramp is used for setting the X1 and X.1 distance timing. The Slow Ramp is used for the crt sweep and, when
combined with the Fast Ramp, to create slewed strobes for the sampler.
Fast Ramp. The Fast Ramp Generator consists of amplifier Q1357 and Q1358; fast ramp clamp Q1338;
current source Q1337 and Q1339; and timing capacitors C1325, C1237, and C1332.
Q1337 provides a constant current source for the timing capacitors to develop a linear voltage ramp
across them. The magnitude of current can be precisely set with the FEET/DIV CAL, R1435. Q1339 is needed
for thermal tracking. The timing capacitors are reset, at the end of time slot 6, by high speed ramp clamp
Q1338. The ramp clamp is controlled by the pulse generator programmer through 01338 and Q1357. C1332 is
a timing adjustment for the X.1 ramp. The Fast Ramp signal is supplied to the Sampler and Pulser
Comparator.
Slow Ramp (Schematic 3). The Slow Ramp Generator consists of an integrator, a bi-level comparator,
a positive current source, a negative current source, a retrace switch, and a retrace signal amplifier. A
functional block diagram of the Slow Ramp Generator is shown in Fig. 3-5.
REV A FEB 1980
3-6