Delay Pickoff Circuit; Horizontal Preamp - Tektronix 7B51 Instruction Manual

During sweep inhibit time, 0424 is turned off. This
turns off CR428 and with CR463 off the current from
R428 turns on CR469. C469 now charges to about +1.5
volts, turning on 0468. This does not reset the sweep,
wh
ich is locked out by the conduction of 0416.
When sweep inhibit ends 0416 turns off, making the
base of 0432 more positive. 0424 turns on to open CR469.
C469 holds its charge long enough to allow 0468 to reset
the sweep at the end of inhibit time. This action allows
only one sweep at the end of inhibit time. This action
allows only one sweep after each time the sweep is in-
hibited.
The delay mode control voltage is also connected to the
sweep gate TO, CR299, through CR301 and R303. With
the A time-base B DELAY MODE in B TRIGGERABLE
AFTER DELAY the control voltage is about +3 volts,
which is insufficient to cause CR301 to conduct. The sweep
will wait for the first trigger after the delay interval to run.
With the A time-base B DELAY MODE in B STARTS
AFTER DELAY, the control voltage is about +4.5 volts.
This is sufficient to cause CR301 to conduct and increase
the current through R303 to the sweep gate TO, CR299.
Now the TO will change to its high-level state and start the
sweep as soon as it is enabled by the sweep reset multivibra-
tor at the end of inhibit current to 0410.
When the INDEPENDENT mode is selected by the A
Plug-in DELAY MODE switch, the sweep inhibit is held at
about the 0 mA level. The delay mode control voltage is
clamped to ground. The A time-base has no control over
the B time-base and each operates as an independent Time-
Base.
Delay Pickoff
Delay Comparator. The Delay Comparator stage allows
selection of the amount of delay from the start of the A
Sweep before the B Sweep Generator is turned on. This
stage allows the start of the B Sweep to be delayed from 0
to 10 times the Time-Base A TIME/DIV setting after the A
Sweep starts. Then, the B Sweep Generator is turned on
and operates at a sweep rate determined by the B TIME/
DIV switch setting.
0520A and 0520B operate as a voltage comparator. The
FET with the most positive gate controls conduction.
Reference voltage for the comparator is supplied by the
DELAY TIME MULTIPLIER, R505, a front-panel control.
Delay Stop R500 is provided for calibration of the Delay
Pickoff circuit. R501-C501 filter out the power supply
ripple to provide stability. This instrument is calibrated so
that the major dial markings of the DTM correspond to a
major division of the A Sweep horizontal display. For
example, if the DELAY TIME MULTIPLIER is set to 5.00,
the B Sweep Generator is delayed five divisions of the A
Sweep time before it can produce a sweep.
Circuit Description-Type 7851
Ouiescently, with the DELAY MODE switch set to a
delay mode, 0520A (having the most positive gate) is con-
trolling the Delay Pickoff circuit. The output at the emitter
of 0496 is at about +3.6 volts which inhibits the B sweep.
The sawtooth output signal is applied to the gate of 0520B.
When the sawtooth voltage rises above the level at the gate
of 0520A (established by the DTM) 0520B takes over con-
trol of the comparator current. The increased current
through 0520B makes the base of 0510B less positive.
0510B turns on and the current through R485 gives a
positive-going signal on the base of 0492. 0482-0492 is a
fast switching circuit. The positive-going signal at the base
of 0492 increases current through R489, and the emitter of
0482 moves in a positive direction. This decreases the con-
duction of 0482 and its collector voltage rises. This makes
the base of 0492 more positive, reinforcing the original
signal. The result of this regenerative action is very fast
switching of this circuit.
The negative-going step at the collector of 0492 is
applied to the base of Delay Gate Emitter-Follower 0496.
The negative-going step at the emitter of 0496 is sent to
the Indicator Oscilloscope as the Delay Gate. At the end of
the sweep, the Delay Pickoff circuit reverts to its quiescent
state to produce the positive-going step at the end of the
Delay gate. 0472 aids in quickly returning the Delay
Switching Amplifier to its quiescent state. 0472 is held cut
off by the sweep start gate during sweep time. At the end
of sweep time, 0472 turns on applying a positive-going sig-
nal to the base of 0482.
When the DELAY MODE switch is in INDEPENDENT
the base of 0492 is clamped at about 0 volts. This turns it
on hard and the Delay Gate output is held to its low level.
HORIZONTAL PREAMP
Input Amplifier. ,The Input Amplifier, 0764, is an oper-
ational (feedback) amplifier which provides stable gain and
low input impedance. POSITION control R744 A and B,
varies the DC level at the base of paraphase amplifier 0784
to position the display horizontally. R771-R772-0774
supply a stable DC reference level to the base of 0794.
Sweep Cal adjustment R740 adjusts the sawtooth input cur-
rent. This controls the rate of rise of the sawtooth output
of the Paraphase Amplifier. .
Paraphase Amplifier. The Paraphase Amplifier, 0784-
0794, takes the single-ended input and converts it to a
push-pull output for use in the Indicator Oscilloscope.
The negative-going sawtooth applied to the base of 0784
increases conduction in this half of the Paraphase Ampli-
fier. Through emitter-follower action the negative-going
sawtooth signal is also applied to the emitter of 0794, de-
creasing conduction through this half of the Paraphase
Amplifier. The results of these actions are a positive-going
sawtooth at the collector of 0784 and a negative-going saw-
tooth at the collector of 0794.
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