Detailed Circuit Description; Trigger Preamp And Input Switching; Trigger Generator; Input Stage - Tektronix 7B51 Instruction Manual

The Trigger Generator circuits produce an output pulse
which initiates the sweep signal produced by the Sweep
Generator circuits. It also produces a
+
Gate for use by the
Indicator Oscilloscope. The input signal to the Trigger Gen-
erator can be individually selected from among the fol-
lowing: the internal trigger signal from the Trigger Preamp,
an external signal applied to the EXT TRIG INPUT connec-
tor, or a sample of the line voltage applied to the instru-
ment. Each trigger circuit contains coupling and source con-
trols.
The Sweep Generator circuit produces a linear sawtooth
output signal when initiated by the Trigger Generator cir-
cu it. The slope of the sawtooth produces by the Sweep
Generator is controlled by the TIME/DIV switch. The oper-
ating mode of the Sweep Generator is controlled by the
TRIGGERING MODE switch. In the pop AUTO and AUTO
positions, the absence of an adequate trigger signal causes
the sweep to free run. In the NORM position, a horizontal
sweep is presented only when correctly triggered by an
adequate trigger signal. The SINGLE SWEEP position al-
lows only one sweep to be initiated after the circuit is reset
with the RESET button.
The Horizontal Preamp is a single-ended to push-pull am-
plifier. It uses feedback (operational) amplifiers to achieve a
great degree of stability. A X 1 0 gain feature is incorporated
into this amplifier. The sawtooth signal is received from the
Sweep Generator circuit and changed to a push-pull output
for use by the Indicator Oscilloscope to produce the hori-
zontal display.
DETAILED CIRCUIT DESCRIPTION
Trigger Preamp and Input Switching
Trigger Preamp. The push pull trigger signal from the
Vertical Deflection System is converted to a single-ended
output by the emitter-coupled stage, 0104-0114. The out-
put of 0114 drives current gain stage 0124-0134. The DC
level of the output is set by Internal Trigger Centering con-
trol, R135.
Input Switching. The TRIGGERING SOURCE switch,
S140, selects the source of the trigger signal. Three trigger
sources are available; internal, line and external. The exter-
nal signal may also be passed through a -;- 10 attenuator net-
work.
The TRIGGERING COUPLING switch, S145, offers a
means of attenuating high or low frequency components of
the trigger signal. In addition to AC and DC coupling,
C145A-R 145A can be selected to provide low-frequency
attenuation and R1458-C1458-C145C for high-frequency
attenuation.
®
Circuit Description-Type 7851
Trigger Generator
General. The Trigger Generator circuit produces trigger
pulses to start the Sweep Generator circuit. These pulses are
derived from either the internal trigger signal from the Ver-
tical Deflection System, an external source, or from a
sample of the line voltage applied to the instrument. Con-
trols are provided in this circuit to select trigger LEVEL
and SLOPE, and TRIGGERING MODE. Fig. 3-3 shows a
detailed block diagram of the Trigger Generator circuit.
Refer to the diagrams in Section 8 for complete circuit
components.
Input Stage. The input source-follower, 0213A, pro-
vides a high input impedance for the trigger signal. It also
provides isolation between the Trigger Generator and the
input source. Input protection diodes, CR203 and CR206
protect 0213A from excessive input signals by clamping
the gate of the input FET if the input exceeds about
+
or
-6 volts. The signal at the source of 0213A is passed
through emitter-follower 0214A to the base of 0224 in the
Slope Comparator, and to the pop Auto Trigger Level cir-
cuit.
POp Auto Trigger Level Stage. The triggering level, with
the MODE switch set to Pop AUTO, is controlled by the pop
Auto Trigger Level stage. The trigger input signal is applied
through emitter-follower 0333, to ·reduce loading of the
signal input stage. The signal is applied simultaneously to
the bases of 0353 and 0373.
Assume a + input signal. This signal, coupled through
emitter followers 0353-0356, causes the emitter voltage to
rise and charges C358 to the peak positive level of the input
signal. C378 is charged in a similar manner to the peak neg-
ative level of the input signal. The peak-sensing capacitors,
C358-C378, set the voltage levels applied to the LEVEL
control (R260A). This allows the LEVEL control to be ad-
justed between the two voltage levels representing the most
negative and most positive peaks of the trigger signal. The
unit can be triggered at all positions of the LEVEL control
if the minimum frequency and amplitude specifications are
met. 0366-CR366 and 0386-CR386 provide temperature
compensation for 0364 and 0384 respectively.
Slope Comparator. 0224 and 0234 are connected as a
difference amplifier to provide selection of the slope and
level at which the sweep is triggered. The reference voltage
for the comparator (with the MODE selector at NORM) is
provided by the LEVEL control, R260A, and the Level
Center control, R385. R385 sets the level at the base of
0234 so that the display is triggered at the 0 volt DC level
of the incoming trigger when the LEVEL control is cen-
tered. The LEVEL control varies the voltage at the base of
0234 to select the point on the trigger signal where trigger-
ing occurs.
3-3