Circuit
Description-Type
453/R453
provide
an
alternate-trace
sync pulse for dual-trace
opera-
tion.
It
is
also coupled
to
the A GATE
output
connector on
the side panel through
R529.
D528
and
D529 clamp the
gate
signal so it does not
go
more than
about
0.5 volts negative
and
12.5 volts positive.
Disconnect Diode. The Disconnect Diode, D533,
is
quie-
scently conducting current through
R506,
R508,
R509,
R530
and
R531.
The positive-going
gate
signal from
Q504
reverse
biases D533
and
interrupts the quiescent current flow.
Now
the timing current through the Timing Resistor begins to
charge
the Timing Capacitor, C530, so the Sawtooth
Sweep
Generator
stage
can
produce
a sawtooth
output
signal. The
positive-going
gate
signal also reverse biases D547 to dis-
connect the
Sweep
Start Amplifier. The Disconnect Diode
is
a fast turn-off
diode
with low reverse
leakage
to
reduce
switching time
and
improve timing linearity
at
the start
of
the
sweep.
Sawtooth Sweep Generator. The basic
generator
cir-
cuit
is
a Miller Integrator circuit.
8
When
the current flow
through D533
is
interrupted by the Sweep
Gate
signal, the
Timing Capacitor, C530, begins to
charge
through the
Tim-
ing Resistor,
R530,
and
the A
Sweep
Cal Adjustment,
R531.
The Timing
Capacitor
and
Resistor
are
selected by the A
TIME/DIV switch to
change
sweep
rate. The A
Sweep
Cal
adjustment allows calibration for
accurate
sweep
timing. The
A
VARIABLE
control,
R530Y
(see Timing Switch diagram), pro-
vides
variable
sweep
rates by changing the
charge
time of
C530.
The positive-going
voltage
at
the
R530
side of C530
as
it
charges
toward
+75
volts
is
connected to the
gate
of
FET
Q533.
This
produces a positive-going
output
voltage
which
is
connected to the
base
of
Q531 through
R536.
Q531 ampli-
fies
and
inverts the
voltage
change
at
its
base
to produce a
negative-going
sawtooth
output.
To
provide a linear charg-
ing
rate
for the Timing Capacitor, the
sweep
output signal
is
connected to the negative side
of
C530. This feedback pro-
vides a constant charging current for C530 which maintains
a constant
charge
rate
to produce a linear sawtooth output
signal. The
output
voltage
continues to
go
negative until
the circuit
is
reset through the
Sweep
Reset Multivibrator
stage.
The
output
signal from the collector
of
Q531
is
con-
nected to the Horizontal Amplifier circuit through
R538
and
the Delay Pickoff
Comparator
stage
in
the B Sweep
Genera-
tor circuit through
R532.
Sweep Reset Emitter Follower. The negative-going saw-
tooth
voltage
at
the collector of Q531
is
connected to the
base
of
the
Sweep
Reset Emitter Follower stage, Q543. The
negative-going signal
at
the emitter
of
Q543
is
coupled to
the
Sweep
Reset Multivibrator
stage
to determine
sweep
length
and
to the
Sweep
Start Amplifier
stage
to set the start-
ing point for the sweep. D542 connected to
the
base
of
Q543 protects this
stage
during instrument warmup.
Sweep Start Amplifier. The signal
at
the emitter
of
Q543
goes
negative
along
with the
applied
sawtooth signal. This
increases the forward bias on D543 which
in
turn
decreases
the forward bias on D545
as
the sawtooth goes negative.
When
the
anode
of
D543 reaches a level
about
one
volt
more
negative
than
the
level
on
the
base
of
0544,
it
is
reverse biased
to
interrupt
the
current
flow
through
0544.
8
lbid.,
pp.
540-548.
3-16
The circuit remains
in
this condition until
after
the
sweep
retrace
is
complete.
As
the
voltage
at
the emitter
of
Q543
returns to its original
DC
level
at
the end of the sweep, D545
is
again
forward biased
and
Q544
conducts through D547 to
set the quiescent current through the Disconnect Diode, D533.
This establishes the correct starting point for the sweep.
D546 clamps the collector
of
Q544
at
about
+0.5
volt. This
reduces the
voltage
swing
at
the collector of
Q544
and
im-
proves the response time. The
Sweep
Start adjustment,
R758
(in
the B
Sweep
Generator
circuit), sets the
base
voltage
level
of
Q544. The collector of Q531
is
held
at
this
same
voltage
level through the feedback loop comprised of Q533
and
Q53
l,
thereqy setting the starting point
of
the sawtooth out-
put signal. The level established by the Sweep Start adjust-
men~
is
also connected to the B
Sweep
Start Amplifier so the
B
sweep
starts
at
the same
voltage
leve·I
as the A sweep.
Sweep Reset Multivibrator. The negative-going saw-
tooth signal
at
the emitter
of
Q543
is
coupled to the
cathodes
of
D555
and
D556. These
diodes
are
quiescently reverse
biased
at
the start of the sweep.
As
the sawtooth voltage
at
the
cathode
of D555
goes
negative·, D555
is
forward bias-
ed
at
a level
about
0.5 volts more negative than
the
base
level
of
Q575
(A
SWEEP
LENGTH
control
in
FULL
position).
Then the negative-going sawtooth signal from the Sweep
Re-
set Emitter Follower
stage
is
connected to the
base
of Q575.
Q575
and
Q585
are
connected
as
a Schmitt bistable multi-
vibrator9. Quiescently,
at
the
start
of
the sweep, Q585
is
conducting
and
Q575
is
biased
off to produce a negative
level
at
its
collector.
This
negative
level allows the Sweep
Gate
tunnel diode, D505, to
be
switched to produce a sweep
as
discussd previously.
When
the negative-going sweep sig-
nal
is
connected to the
base
of
Q575
through D555, Q575
is
eventually biased on
and
Q585
is
biased off by the emitter
coupling between Q575
and
Q585. The collector of Q575
rises positive
and
D505
is
switched back to its low-voltage
state
through
R502.
D505
is
held
in
its low-voltage
state
so
it
cannot
accept
incoming trigger pulses until
after
the
Sweep
Reset Multivibrator
stage
is
reset. This ends the Sweep
Gate
stage
output
and
the Disconnect Amplifier
stage
is
turned
on to rapidly discharge the Timing
Capacitor
and
pull the
gate
of
0533
rapidly negative to
its
original level to pro-
duce
the retrace portion of the sawtooth signal. The Saw-
tooth
Sweep
Generator
stage
is
now
ready
to produce
another
sweep
as
soon
as
the Swe·ep Reset Multivibrator
stage
is
reset
and
another
trigger pulse
is
received.
When
Q575
is
turned on to
end
the sweep, it remains
in
conduction for a period of time to establish a holdoff period
and
allow all circuits to return to their original conditions
before the next
sweep
is
produced. The holdoff time
is
deter-
mined by the
charge
rate
of
the Holdoff Capacitor, C550. At
the start
of
the sweep, C550
is
completely discharged by the
unblanking
gate
at
the collector
of
Q514.
It
is
held
at
this
level throughout the sweep time.
When
the Sweep
Gate
output ends,
Q514
is
cut off
and
C550 begins to
charge
to-
ward
+75
volts through
R552
and
R551.
The positive-going
voltage
across he Holdoff
Capacitor
as
it
charges
is
con-
nected to the
base
of QS75 through D552
and
D559.
When
the
base
of Q575 rises positive
enough
so
it
is
reverse biased,
its
collector level drops negative
and
Q585
comes back into
conduction. The bias on the
Sweep
Gate
tunnel diode, D505,
returns to a level
that
allows it to
accept
the next trigger
pulse (D505
is
enabled). The Holdoff Capacitor, C550,
is
9
lbid.,
pp.
389-394.
REV.
8,
OCT. 1975
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