Tektronix WFM601, Service Manual

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Theory of Operation
WFM 601 - Service Manual
3–11
and R92, which reduces the rise time on the drain of Q1 which lowers the supply
radiated noise.
CR1 is the supply rectifier. C46, L8, and C47 form the output pi filter.
Diagram 5 Y Delay, Half-Band Filter, & DAC
The luminance channel elements are clocked at twice the clock frequency of the
color difference channels. In order to properly time the luminance and color
difference signals, a solid state delay is used. U94 is clocked at the luminance
rate by the Clock PAL to delay the Y signal prior to digital half-band filtering.
In order to simplify the analog filtering at the output of the Digital-to-Analog
Converters (DACs), the Digital Half-Band Filter, U29, clocks data out at twice
the rate of the input data. The clock signal is the buffered, regenerated clock
from Diagram 4. The clock synchronizing signal (which is at luminance
frequency, 13.5 MHz) is from the Clock PAL, which is also on Diagram 4.
The Y channel DAC is U9, which converts the 10-bit digital input data to an
output analog current. The enable signal, from the regenerated clock, is at twice
the luminance frequency (27 MHz). R271 is a 75
terminating resistance that
converts the output analog current to a voltage.
Gain of the DAC is controlled by a signal fed back from the gain and control
circuitry on Component circuit board (A7) on Diagram 10.
Diagram 6 P
B
& P
R
Half-Band Filters & DACs
The Digital Half-Band Filters, U17 and U20, clock data out at twice the rate of
the input data. The clock signal is from the Clock PAL on Diagram 4, and is
13.5 MHz, which is twice the color difference frequency. The clock synchroniz-
ing signal (6.75 MHz) is also from the Clock PAL.
The P
R
channel DAC is U18, which converts the 10-bit digital input data to an
output analog current. The enable signal, from the Clock PAL, is at twice the
color difference frequency (13.5 MHz). R255 is a 75
terminating resistance
that converts the output analog current to a voltage.
Gain of the DAC is controlled by a signal fed back from the gain and control
circuitry on Component circuit board (A7) on Diagram 10.
Y Delay
Digital Half-Band Filter
Y DAC
Digital Half-Band Filters
P
R
DAC & P
B
DAC