Tektronix WFM601, Service Manual

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Theory of Operation
WFM 601 - Service Manual
3–13
the input amplifiers drive the Transcoder and inputs to the horizontal and vertical
signal processing amplifiers on Diagram 9.
Driven by Input Amplifiers, the Color Difference to RGB Transcoder consists of
three operational amplifiers (U41, U51, and U61) and resistance matrices. The
operational amplifiers and the resistance matrices are signal mixers. Subtraction
and addition of signals is accomplished by presenting the color difference
components to either the inverting or noninverting inputs of the amplifiers. The
resultant signal proportions are shown in Table 3–1. Note that Y is input to the
noninverting input of all three amplifiers (U41, U51, and U61), and that both P
B
and P
R
are input to the inverting input of U41 (the G transcoder). This makes
the output mix for SMPTE format G = Y – 0.3441 P
B
– 0.7141 P
R
.
Table 3–1: Transcoder Signal Mixing
GBR SMPTE
Green Y – 0.3441 P
B
– 0.7141 P
R
Blue Y + 1.772 P
B
Red Y + 1.402 P
R
The GBR outputs of the operational amplifiers drive clamped amplifiers (U45,
U55, and U65) whose output is always GBR. Offset levels are from the DAC on
Diagram 10. The clamping pulse is from the Coprocessor.
The Transcoder is in operation under all circumstances; however, the Monitor
Output can display either color difference or RGB. When the desired monitor
output is color difference, the Monitor output signal bypasses the Transcoder.
U43, U53, and U63 are switchable input amplifiers driven by either the
Transcoder output or color difference signals output by the Input Selection
amplifiers.
These three identical switch/buffer amplifiers (U81, U82, and U83) drive the
Vertical Input Multiplexer with either GBR or color difference signals. GBR
input is driven by the output of the Transcoder, while the color difference input is
driven directly by the Reconstruction Filters on Diagram 7. Switching signal is
from the Microprocessor on Diagram 14.
Each of the components of the GBR signal, output by the Transcoder, is
compared against limits from the DAC on Diagram 10. The plus limit is set at
735 mV (700 mV + 5%) and the minus limit at –35 mV (0
5% of the 700 mV
total amplitude). When one of the GBR signals exceeds a limit, the comparator
output becomes the Gamut Error signal, which is monitored by the Coprocessor.
In addition, it also drives a brightup circuit (Q76 driving U76), that causes the
RGB Transcoder
Parade Output
Gamut Limit & Sync
Insertion