Tektronix 3B5, Инструкция по эксплуатации

Страница: 65 / 187

Circuit Description —Type 3B5
to run up at this sweep rate. The Advance Pulse Generator
is not turned on by the trigger pulse which starts the sweep,
due
to
normal delay time designed into this circuit.
However,
if the next trigger pulse is applied to Q665 before the saw ­
tooth turns the Window Gate circuit on (i.e., collector of
Q664 still negative), the Advance Pulse Generator, Q665
and Q675, is turned on. When Q665 turns on, its collector
rises positive to bias Q675 on. The resulting negative-going
change at the collector of Q675 holds Q665 in conduction
through C671-R671 until the charge on C671 equalizes.
When C671 is charged, the base of Q665 rises positive and
it turns off. The signal at the collector of Q665 provides
the advance pulse to the Advance Pulse Output
stage.
It
also provides a sweep reset pulse to the Sweep Generator
circuit through C681-D681 so the sweep restarts with each
advance pulse. Each time a trigger signal produces an
advance pulse during the window-gate signal (before the
sawtooth biases Q653 on), the sawtoooth starts to run up
again at the next faster sweep rate (advance pulse increases
sweep rate; see Counter Circuit discussion). This action con ­
tinues each time Q665 is triggered until the sweep rate is
fast enough to turn Q653 on (to end window-gate signal)
before a trigger can produce an advance pulse.
The amount that the sawtooth must run up before it turns
Q653 on is set by the CYCLES/SWEEP adjustment. Since
voltage level on the sawtooth is related to divisions on the
CRT, the setting of this control determines the number of
cycles in the CRT display. For example, assume a constant
input signal produces a three-cycle display in the Seek Mode
when R650 is set to midrange. Then, when R650 is set clock ­
wise, the base of Q653 is less negative and the sawtooth
must overcome less of the sweep gate current to turn Q653
on. Therefore, fewer advance pulses are produced before
the sweep rate is advanced enough so that Q653 turns on
before Q655 can be triggered. With fewer advance pulses
to the Counter Circuit, a slower sweep rate is produced to
display more cycles of the signal. On the other hand, if the
CYCLES/SWEEP adjustment is set counterclockwise, the saw­
tooth must overcome more sweep-gate current and run up
farther to turn Q653 on. More advance pulses are produced
before the sawtooth reaches the level where it turns Q653
on before Q665 can be triggered. More advance pulses
to the Counter Circuit produce a faster sweeo rate which
results in less cycles in the display.
Advance Pulse Output
Q684 provides amplification for the advance pulses pro­
duced by the Advance Pulse Generator stage and couples
them to the Counter Circuit. This stage only produces an
output when enabled by the Advance Gate stage through
D682. Locking this stage out in addition to locking out the
Advance Pulse Generator circuit insures that all advance
pulses are locked out after the 0.1 microsecond sweep rate
has been reached. It also prevents the Counter Circuit from
being advanced by noise or other extraneous pulses in the
Seek Circuit when a seek command is not present.
Fig. 3-23 shows operating waveforms from the Seek Circuit.
The first set of waveforms shows circuit conditions with a
1-kHz sine wave applied (CYCLES/SWEEP adjustment set
according to Calibration Procedure). The second set of wave­
forms shows circuit conditions with a 1.3 kHz sine-wave
signal.
Counter Circuit
General
The Counter Circuit produces the multiplier and decade
logic levels for the Seek and External modes of operation.
In the Seek Mode, the output of this circuit is controlled
by the seek mono pulse and advance pulses
produced
by
the Seek Circuit. For external Mode operation, the output of
this circuit is controlled by external program logic levels
applied through the front-panel PROGRAM connector. Fig.
3-24 shows a logic block diagram of the Counter Circuit.
A diagram of this circuit is shown on diagram 7 at the
rear of this manual.
Multiplier Counter
The three-state Multiplier Counter produces the 1-2-5
logic to select the sweep-rate multiplier within the timing
decade selected by the Decade Counter stage. Fig. 3-25
shows circuit conditions for "5 ” output. The positive-going
seek mono pulse from the Seek Circuit turns on Q705 at
the start of each seek cycle through C731, D731 and R731.
The collector level of Q705 is coupled to Q715 and Q725
through D713 and D723 to hold them off. The first advance
pulse produced by the Seek Circuit is coupled to the base of
Q705 through C704 and D704. Q705 turns off and its col­
lector rises positive. This level change turns Q715 on through
C709.
Fig. 3-26 shows circuit conditions for "2" multiplier output
Q715 turns on when Q705 is turned off by the advance pulse.
Then, the collector level of Q715 holds Q705 and Q725 off
through D703 and D723. The second advance pulse pro ­
duced by the Seek Circuit is coupled to the base of Q715
through C714 and D714. This negative-going pulse turns
Q715 off and its collector rises positive. C719 couples this
change to the base of Q725 to turn it on.
"1" multiplier logic is produced by Q725 as shown in
Fig. 3-27. Q725 is turned on by Q715. As it turns on, the
collector level of Q725 goes negative to hold Q705 and
Q715 off through D702 and D712. The third advance pulse
produced by the Seek Circuit is connected to the base of
Q725 through C724 and D724. Q725 turns off and its col­
lector rises positive. This positive level change turns Q705
back on through C729. As Q705 comes back on, the nega­
tive-going change at its collector produces an advance com ­
mand level to the Enable Multivibrator stage. The action
described starts over and repeats until the Seek Circuit stops
producing advance pulses. One advance command to the
Enable Multivibrator is produced for each three advance
pulses produced by the Seek Circuit.
For External Mode operation grounding (zero-volt) external
multiplier program logic is applied to the Multiplier Counter
circuit through either terminal 2, 3 or 5 of J30. This external
program logic biases on the associated transistor to produce
the desired multiplier logic output. The transistor which is
turned on by the external program logic holds the other
transistors off and this transistor remains on until the external
program logic is removed.
For Manual Mode operation, — 12.2-volt Seek/External
Power to the Counter Circuit is disconnected. This enables
the entire circuit in this mode and the output levels of this
circuit rise to about zero volts.
3-37