Video Option - Tektronix 2432 Service Manual

Theory of Operation-2432 Service
1. Selection of trigger field (Field1 or Field2).
2. Choice of triggering on positive- or negative-sync
input signals (NEG-SYNC).
3. Selection of correct polarity of the offset signal via
the CH2 INV signal.
4. Control of the display functions (VIDEO CLAMP and
FAST CLAMP).
5. Enabling the Video Trigger Circuit to trigger
the scope.
6. Selection of TV Line Coupling-allowing all lines to
produce a trigger signal to the main Trigger circuit of
the scope.
VIDEO OPTION
The Video Option (diagram 21) consists of additional
hardware and firmware installed in the host instrument to
enhance triggering on and viewing of composite video sig-
nals. The Video Option block diagram located in the
tabbed foldout pages in the rear of the manual may be an
aid in following the Video Option circuit descriptions.
The Video Option circuitry contains both video-signal
processing and trigger-generation circuits. The video-signal
processing circuits stabilize the input signal and separat~
the television synchronization signals (horizontal and verti-
cal sync pulses) from the composite video signal. The
trigger-generation circuits then count these separated sync
pulses to determine when a TV Trigger signal is to be
produced.
In the video-signal processing circuits, the gain of the
AGC (automatic gain control) Amplifier is automatically
adjusted to produce the correct signal amplitude to the
Sync Pickoff Comparator for proper sync separation over
a wide range of input signal levels. The Trigger Back-
Porch Clamp adjusts the back-porch level of the input sig-
nal through the Fixed-Gain Amplifier on each sync pulse.
The feedback to the Fixed Gain Amplifier compensates for
level shifting caused by any power-line ripple riding on the
composite video signal. The Sync-Tip Clamp circuit moni-
tors the horizontal-sync pulse amplitude and produces the
automatic-gain-control voltage that sets the gain of the
AGC Amplifier. Sync pulses are separated from the com-
posite video signal by the Sync Pickoff Comparator. The
horizontal- and vertical-sync pulses are further separated
by the Pulse Stretcher and Field Generator circuits for use
in producing the horizontal clock and field-sync signals
needed by the Trigger Generation circuitry.
To set up the Video Option operating modes,. the
System Processor writes control settings to the V1d~o
Mode Option Register (diagram 20) in the System 1/0 cir-
3-84
cuitry. The latched setting in the register is held until a
different mode is needed. Programmable counters, also
under System processor control, count the extracted hor-
izontal sync pulses (lines) until the line number for the
selected trigger point is reached. At that point, if the main
trigger circuit is finished with holdoff, the TV Trigger Gen-
erator circuit produces a TV Trigger to the A/B Trigger
Generator to trigger the next storage acquisition.
An additional display function added to Channel 2 is the
TV CLAMP feature. When enabled, the circuitry holds the
back-porch level of the displayed signal on Channel 2 at
ground level. The Channel 2 Vertical Display Clamp circuit
checks the back-porch levels of the incoming TV signal on
Channel 2 and produces offsetting voltages to the Chan-
nel 2 Preamplifier to bring those levels back to ground
reference. The circuit action produces a stable vertical sig-
nal display of a TV signal by removing power supply ripple
that may be present. Either inverted or noninverted signals
may be displayed with the TV CLAMP feature.
Video Signal Processing Circuitry
AGC AMPLIFIER. The AGC (automatic gain control)
Amplifier, 0514, U612, and U71 OB, amplifies the
composite-video input signal from the selected trigger
channel. Stage gain is controlled by feedback that is
derived from the amplitude of the incoming horizontal sync
pulses. The amplifier itself is formed by two cross-
connected differential amplifier pairs in U612 that permit
normal or inverted amplification of the signal. The front-
panel SLOPE/SYNC switch selects whether the amplifier is
inverting or noninverting to match the required signal
polarity for the sync-separation circuits. For correct opera-
tion of the sync separation circuit, the composite-video sig-
nal must be sync-negative; therefore, if a "noninverted"
signal display has positive sync, the SLOPE/SYNC switch
may be pressed to invert the signal (
+
SLOPE LED is on
for positive-sync input display). Inversion only occurs in the
trigger Sync Separator path; the display polarity remains
unaffected.
Gain of the AGC Amplifier is controlled by the action of
the Trigger Back-Porch Clamp, the Sync-Tip Clamp, and
the Automatic Gain-Control circuitry working together to
set the channel resistance of FET 0514 and thereby the
gain of AGC Amplifier U612. Amplifier gain is automatically
adjusted to maintain the sync-tip level at a known point
relative to the back-porch amplitude of the signal. This
action provides an accurate and stable pickoff point on the
signal to the Sync Pickoff Comparator circuit (0504 and
0510) with input video signals of different or varying ampli-
tudes. The minimum gain of the circuit is decreased (to
permits the application of higher amplitude signals) by the
use of constant-current diodes CR526 and CR620 as the
current sources for the differential amplifiers.
When power is first applied, the operating level of the
AGC Amplifier is established by feedback only. With no