L Nput Stage; Peak Detector - Tektronix 7B70 Time Base Instruction Manual

Circuit Description-7B70 (SN B205000 and Up)
Fig. 3-2. Simplified block diagram of trigger amplifier.
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L
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EXTERNAL
HORIZONTAL TO HORIZONTAL
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AMPLIFIER PREAMP
l n
pu t Stage
The trigger signal (internal or external a s selected by the
SOURCE switch) is applied push-pull to the bases of 0107
and 0108. Impedance compensating networks R 100, C100,
L100and R101 at the base of 0107 and R104, C104, L104
and R103 at the base of Q108 provide a 50 ohm input for
the trigger signal. Q126 with a voltage divider network
R 126, R 127, R 128 provides a DC reference level for 0 1 16.
0120, R119, R 120, R121, R122 and the output of the
peak detector circuit provides a variable voltage source for
Q110. R 121 i s adjustable to DC balance this stage with the
LEVEL control set to 9:00 o'clock (centered on the posi-
tive slope). 0110-R111 and 0 1 16431 17 provide the current
source for 0107 and 0108 respectively.
li
The AC HF REJ control, located in the output of the
cascode Input Stage, engages C138 to provide attenuation
of signals above 50 kHz.
U192
A
p i q
/
A 4 /
/
/
/
/
-
Peak Detector
The triggering level, with the MODE switch set to P-P
AUTO, is controlled by the Peak Detector stage. The signal
from the trigger amplifier input stage i s applied to the Peak
Detector peak sensing network. The output of the Peak
.
Detector sets the voltage applied to the LEVEL control
(R146A) so that the range of the LEVEL control corres-
ponds to the peak-to-peak amplitude or less of the input
signal.
The positive portion of the input signal i s inverted by
the trigger input stage and applied through CR210 to the
peak sensing network, C210 charges to the peak value of
the input signal. The signal is amplified by 0220-0224 and
applied to the gate of source follower 0248. Current
through 0248 decreases and the voltage at the wiper of
R251 becomes more negative. This negative-going signal is
inverted by operational amplifier U260, and applied to the
top of the LEVEL control, R146A.
>,
4
SLOPE
COMPARATOR
0150,0156,0158,
0159. 0161. 0168
0169
+
TRIGGER
'
INPUT
r
-
A t the end of the positive input signal the voltage at the
source of 0248 rises at
a
rate deterrr~ined by C248 and
R241. The feedback from the source of 0248 aids in hold-
ing the charge on C210. For signals with
a
repetition rate of
about 200 hertz or greater, the peak sensing network will
remain charged to the peak level of the input signal.
+
TO
OUTPUT STAGE
:
TRIGGER
0170. Q171.0181
-
GENERATOR
0182
INPUT STAGE
-
0107,0108,0110
0116, 0120.0126
Q131, Q134
+ - +
The negative portion of the input signal is applied in-
phase to the Peak Detector. The operation of the peak
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REV. MAR 1974
Scans by ArtekMedia O 2006
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U267
I
PEAK DETECTOR
0220, 0221, 0224
0235, 0236.0238
0248,0255, U260
,
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