Tektronix 7834

Theory of Operation-7834
NON-DIGITAL
DEVICES
Not all of the integrated circuit devices
in this instrument
are digital logic devices .
The function of non-digital devices
is described individually,
using operating waveforms or other
techniques to illustrate their function .
CABLING
Diagram 1, in the Diagrams and
Circuit Board Illustrations
section, shows the inter-connections
between major circuits
within
the 7834 .
CALIBRATOR AND MODE SWITCH
Calibrator
The Calibrator circuit provides
voltage outputs of 40 milli-
volts, 0 .4 volt and
4 volts
at
the CALIBRATOR output
connector
. A current output of 40 milliampers is available
from the Calibrator
with an optional current loop adapter .
When the current loop
adapter is used, the Calibrator must
be set to the
4 V switch position .
The Mode Switch circuit includes front-panel switching and
provides the logic for selection of the vertical and horizon-
tal compartments that provide deflection for the crt . The
Mode
Switch circuit operates in conjunction with the Logic
circuit (diagram 4) to develop control signals for use in
other circuits within this instrument, and within
plug-in
units installed in the plug-in compartments .
Figure 3-2 shows a detailed block diagram of the Calibrator
and Mode Switch circuits . A schematic of these circuits is
shown on diagram 2 at the rear of this manual .
Transistors Q376 and Q382 comprise a
1 kilohertz, square-
wave oscillator . Oscillation occurs as
follows : Initially
assume that 0376 is conducting
and Q382 is not conduct-
ing . The voltage at
the emitter of Q382 becomes more
negative as C376 discharges through R381 . Capacitor C376
discharges until the emitter-base junction of Q382 becomes
forward biased . As Q382 begins conducting the oscillator
changes states . Regeneration starts when 0382 conducts
and C376 stops discharging ; this reduces the collector
current of Q376 . Thus, the collector voltage of
Q376 rises
positive which causes the base and emitter of
Q382 to rise
positive . The positive going voltage
is coupled by C376 to
the emitter of Q376,
turning
it
off .
Now, Q382 is conducting
and Q376 is not conducting . The
voltage at the
emitter
of
Q376 goes negative as C376 charges
through R376 . When the emitter-base junction of 0376
becomes
forward biased the oscillator will again change
states to
complete the cycle
.
3- 8
The square wave signal produced at
the collector of Q382
switches 0384 on and off .
When 0384 is on, the current
from R383 and R384 flows to
ground . When Q384 is off,
this current flows through CR386
and R386 into the volt-
age divider network of R387, R392,
R393, R394, R395,
R396 and R397 to produce the
4
volt,
0 .4 volt and 40 milli-
volt Calibrator output voltages . The
accuracy of the Cali-
brator is set by the 0 .4 Volts DC
adjustment, R385 . Both
the 4V and .4V calibrator
push-button switches must be
engaged when adjusting R385 .
The Calibrator frequency is
set by the 1 kHz adjustment, R375 .
Plug-In Single Sweep Logic
The Plug-In Single Sweep Logic stage (consisting of
U322C,
U326C, U326D, 0304 and 0308) developes
the A or B
Single Sweep Logic level
for use by the Storage circuitry on
diagram 18 .
A HI logic level is produced at pin 11 of
U326D
whenever the time-base unit selected by the HORI-
ZONTAL MODE switch
is set for single-sweep operation .
When the HORIZONTAL MODE switch is set to ALT or
CHOP,
pin 11 of U326D will be HI if either the
A or B
time-base unit is set for single-sweep operation
.
Storage Sweep Gate Logic
The Storage Sweep Gate Logic stage
develops the Storage
Sweep Gate ; this gate is used by the
Storage circuitry on
diagram 18, at the rear of this manual . Gate
signals from
the time-base unit(s) selected by
the HORIZONTAL MODE
switch determine when the Storage
Sweep Gate will occur .
STORAGE SWEEP GATE LATCH . The
Storage Sweep
Gate Latch, (consisting of U322D and
U332D), produces
the positive-going Storage Sweep Gate at
pin 11 of U322D .
Once pin 12 of U322D goes LO, the output
pin 11 of
U322D goes HI and remains HI until
pin 13 of U332D goes
LO, at which time the output
will return to its LO state .
STORAGE
SWEEP GATE LATCH SET . The Storage
Sweep Gate Latch Set stage, consisting of U322B,
Q321,
Q325 and U326 (A and B) provides the signal to
set the
output of the Storage Sweep Gate Latch stage to
the HI
state . The HORIZONTAL MODE switch
determines
whether the A Sweep Gate/A Auxiliary Gate or
the BSweep
Gate/B Auxiliary Gate is
used to set the Storage Sweep
Gate Latch
. The HORIZONTAL MODE switch applies logic
levels
to
pin 5
of
U322B and U326B for the gate selection .
When the HORIZONTAL
MODE switch is set to ALT or
CHOP, pin 5 of
U322B and U326B are HI so that the gate
which
occurs
first (from either the A or B time-base unit)
will set the output of the Storage Sweep Gate Latch
H I .
STORAGE SWEEP GATE LATCH RESET . The Storage
Sweep Gate Latch Reset stage, consisting of U330 (A
and
B), U332 (A, B, and C), U334 (A, B, C, and D), 0332,
0336 and 0328, provides the signal to reset the output of