Tektronix 7623, Инструкция по эксплуатации

Страница: 54 / 266

Circuit Description— 7623/R7623 Service
C59 cannot change its charge instantaneously, the sudden
change in voltage at the emitter of Q1 pulls the emitter of
Q2 positive also, to reverse-bias it. With Q2 reverse biased,
its collector rises positive to produce a positive output level
at pin 14 (see time T1 on the waveforms}.
Now, conditions are reversed. Since Q2 is reverse biased,
there is no current through it. Therefore, C59 can begin to
discharge through R59. The emitter level of Q2 follows the
discharge of C59 until it reaches a level about 0.6 volt more
negative than its base. Then Q2 is forward biased and its
collector drops negative to reverse-bias QI. The level at pin
14 drops negative also, to complete the cycle. Once again,
C59 begins to charge through R56-R57 to start the second
cycle.
Two outputs are provided from this oscillator. The
Delay Ramp signal from the junction of R56-R57 is
connected to the Vertical Chopped Blanking stage. This
signal has the same waveshape as shown by the waveform at
pin 13, with its slope determined by the divider ratio
between R56-R57. A square-wave output is provided at pin
14. The frequency of this square wave is determined by the
RC relationship between C59 and R1. The duty cycle is
determined by the ratio of R56-R57 to R59.
The square wave at pin 14 is connected to pin 16
through C60. C60, along with the internal resistance of
U55A, differentiates the square wave at pin 14 to produce a
negative-going pulse coincident with the falling edge of the
square wave (positive-going pulse coincident with rising
edge has no effect on circuit operation). This negative-going
pulse is connected to pin 15 through an inverter-shaper
which is also part of U55A. The output at pin 15 is a
positive-going Clock pulse at a repetition rate of about two
megahertz.
Vertical Chopped Blanking
The Vertical Chopped Blanking stage is made up of the
remaining half of integrated circuit U55B, Fig. 3-5A. This
stage determines if Vertical Chopped Blanking pulses are re­
quired, based upon the operating mode of the vertical system
or the plug-in units (dual trace units only). Vertical Chopped
Blanking pulses are produced if: (1) VERT MODE switch is
set to CHOP; (2) dual-trace vertical unit is operating in the
chopped mode and that unit is being displayed; (3)
dual-trace vertical unit is operating in the chopped mode
with the VERT MODE switch set to ADD. The repetition
rate of the negative-going Vertical Chopped Blanking pulse
output at pin 4 is always two megahertz as determined by
the Clock Generator stage.
The Delay Ramp signal from the Clock Generator stage
determines the repetition rate and pulse width of the
Vertical Chopped Blanking pulses. The Delay Ramp applied
to pin 10 starts to go negative from a level of about +1.1
volts coincident with the leading edge of the Clock pulse
(see waveforms in Fig. 3-5B). This results in a HI quiescent
condition for the Vertical Chopped Blanking pulse. The
slope of the negative-going Delay Ramp is determined by
the Clock Generator stage. As it reaches a level slightly
negative from ground, the Vertical Chopped Blanking pulse
output level changes to the LO state. This signal remains
LO until the Delay Ramp goes HI again. Notice the delay
between the leading edge of the Clock pulse generated by
U55A and the leading edge of the Vertical Chopped
Blanking pulses (see Fig. 3-5B). The amount of delay
between the leading edges of these pulses is determined by
the slope of the Delay Ramp applied to pin 10. This delay
is necessary due to the delay line in the vertical deflection
system. Otherwise, the trace blanking resulting from the
Vertical Chopped Blanking pulse would not coincide with
the switching between the displayed traces. The duty cycle
of the square wave produced in the Clock Generator stage
determines the pulse width of the Vertical Chopped
Blanking pulses (see Clock Generator discussion for more
information).
Fig. 3-5. (A) Input and output pins for Vertical Chopped Blanking stage, (B) Idealized waveforms for Vertical Chopped Blanking stage.
3-10