Tektronix WFM601, Service Manual

Page: 69 / 304

Theory of Operation
3–12
WFM 601 - Service Manual
The P
B
channel DAC is U19, 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). R256 is a 75
terminating resistance
that converts the output analog current to a voltage.
Diagram 7 Y, P
B
, & P
R
Reconstruction Filters
The input to the Y Filter is band pass filtered by a 5th order elliptical filter with
its 60 dB stop point at
[
16 MHz. Filter bandwidth is approximately twice that
of the color difference filters P
R
and P
B
. U300 is an active all pass filter that
also provides a delay compensation, C308, to match the propagating delay of the
filter to the filter delay in the color difference channels.
U301 and U302 provide amplitude and group delay equalization. U301 is a 2nd
order band pass filter. R312 is the frequency response adjustment for the
luminance channel (Y). DC level offset, from the DAC on Diagram 10, is added
to meet the signal display requirements. R325 and C325, in the feedback loop
around U301, are the SIN X/X equalization. J300 is a 75
compensated output
(not normally used).
These color difference filters are identical. The input is band pass filtered by a
5th order elliptical filter with a 60 dB stop point at
[
10 MHz. Filter bandwidth
is approximately half that of the luminance (Y) filters. U400 and U500 are
active all pass filters that also provide a delay compensation, C407 and C507, to
match the propagating delay of the filter to the filter delay of the other color
difference and luminance channels.
U401 and U402 (P
B
), and U501 and U502 (P
R
) provide amplitude and group
delay equalization. U401 and U501 are 2nd order band pass filters. R410 (P
B
)
and R510 (P
R
) are the frequency response adjustments for the color difference
channels. DC level offset, from the DAC on diagram 10, is added to meet the
signal display requirements. R425 and C425 (P
B
) and R525 and C525 (P
R
), in
the feedback loops around U401 and U501, are the SIN X/X equalization. J400
and J500 are 75
compensated outputs (not normally used).
Diagram 8 Transcoders & Picture Monitor Outputs
Luminance and color difference signals (Y, P
B
, and P
R
) are input to three
identical input amplifiers, which are switched between the component signals
and blanking levels. Switching is controlled by the Coprocessor, Diagram 7.
Blanking levels for P
B
and P
R
are set by the DAC on Diagram 10. Blanking
level for Y is at ground. Output signals, Y/Blank, P
B
/Blank, and P
R
/Blank, from
Y Filter
P
B
& P
R
Filters
Input Selection