Tektronix RTD 710A Service Manual page 144

Theory of Operation
TV TRIGGER OPTION
The TV trigger option (Figure 1-93) adds hardware
to allow triggering on and viewing of television sig-
nals. it consists of an analog and phase locked loop
section, and a digital section.
The analog section contains composite video signal
processing circuitry including signal amplification,
automatic gain control, back-porch clamping, sync
pickoff, and syne separation. Clocks at the horizon-
tal (line) rate and a field indicator signal are sent to
the digital section.
The digital section contains the line number detect
counter and delay counter.
A trigger signal is connected to the Trigger Slope
Select circuit on the A22 Trigger board. The trigger
is generated when the selected horizontal sync pulse
(line) occurs.
Waveforms produced by the TV trigger option are
shown in Figure 1-94.
ANALOG AND PHASE LOCKED LOOP
CIRCUITRY (DIAGRAM 73 & 74)
The TV option analog and phase locked loop cir-
cuitry (refer to schematics 73 and 74) processes the
composite video to produce the back-porch level con-
trol, horizontal clock, and vertical field signais for
other circuitry in the instrument.
TV Trigger Source Selector
The TV trigger source selector (U100A) switches be-
tween the Channel input amplifier and the composite
video signal from the EXT TRIG !N connector, and
applies it to the variable gain amplifier.
When the SSA SEL signal is low, U100A-10 be-
comes low selecting the SSA signal from the CHi
Input Amplifier. When the SSA SEL signal is high,
the EXT SIG signal is selected. Diodes CR101 and
CR103 provide input protection for U100A.
Variable Gain Amplifier
The variable gain amplifier (U110, U120B, and Q100)
amplifies the input composite video signal. The front
panel SLOPE switch determines whether the amplifier
is inverting or noninverting.
Differential amplifier U110, which amplifies the com-
posite video signal, contains two pairs of switching
transistors that provide signal inversion when needed.
The Sync Tip Clamp and Automatic Gain Control cir-
cuitry controls the channel resistance of Q100, which
determines the gain of the amplifier. The gain is
automatically adjusted to maintain proper sync-tip
level. With no input signal, the gain is maximum.
The composite video signal is applied to one input
of the differential amplifier U110-3 and to De Offset
amplifier U120B. The input to U1208 is fitered by
low pass filter A112 and C100 making its output the
dc component of the composite video signal. This
filtered output is applied to U110-11.
Four transistors in U110 are controlled by the SLOPE
signal from J240-35. When SLOPE is high, the tran-
sistors connected to U110-2 and U110-9 are biased
on, causing the collector signal U110-8 to drive Q102.
When SLOPE is low, the transistors connected to
U110-13 and U110-6 are on causing the collector
signal at U110-14 to be inverted with respect to both
the input signal and the signal at U110-8 to drive
Level shifter Q102 shifts the signal from Ut10 to
provide the voltage gain to drive U130D. For stable
triggering, the cornposite video signal that drives
U130D must be sync-negative; if the displayed input
signal is sync-positive, the SLOPE switch. must be
selected to invert the signai.
Fixed Gain Amplifier and Back-Porch
The fixed gain amplifier and back-porch clamp provide
additional gain to the video signal and additionai
start-up circuitry to set amplifier parameters when a
TV signal is first applied.
RTD 710A Service Manual