Tektronix WFM601, Руководство по обслуживанию, Страница: 72 / 304

Theory of Operation
WFM 601 - Service Manual
3–15
Diagram 9 Lightning, Vector, & Bowtie Switching
Signal selection for the component displays is accomplished by a series of
four-element switches (U101, U103, & U151) activated by the Controller switch
enable signals SW1 – SW11. Signals to input for the various displays are routed
to the vertical inverting and noninverting amplifier inputs and to the noninvert-
ing horizontal amplifier as shown in Table 3–2. Switch enable signals that are
asserted to close the individual switches are included in parentheses.
Table 3–2: Component Display Output Switching
CRT Display Line
Vertical
Noninverting
Input
Vertical
Inverting
Input
Horizontal
Input
(noninverting)
Component Vector All P
R
(SW 4) Ground (SW 5) P
B
(SW 6)
Lightning (Difference) X Y (SW 3) Ground (SW 5) P
B
(SW 10)
X+1 Ground (SW 2) Y (SW 3) P
R
(SW 11)
Diamond (RGB) X G+B (SW 1) Ground (SW 5) B-G (SW 1)
X+1 Ground (SW 2) G+R (SW 8) R-G (SW 8)
Bowtie (Difference) X Y (SW 3) P
B
(SW 6) none
X+1 Y (SW 3) P
R
(SW 9) none
The vertical amplifer, U127, is a differential amplifier. It is used as a differenc-
ing amplifier in the bowtie mode and on alternating lines, and a noninverting
amplifier in the lightning and diamond modes. In the bowtie mode Y is applied
to the + input as well as the – input; P
B
for one line and P
R
for the second line.
In the lightning mode Y is applied to the + input for the top half of the display
and to the – input, which is inverted, for the bottom half of the display. The
output of U127 drives either an active low-pass filter, Q136, Q137, and Q134, or
is switched past the filter (U102A) for the bowtie display.
The low-pass filter consists of three emitter follower stages with an overall gain
of slightly less than unity. C133 and C136 are adjusted to match the frequency
response and phase of the vertical channel to that of the horizontal channel.
The output of the filter is clamped to re-establish the backporch near 0V. In the
lightning and diamond modes, the offset correction is different depending on
whether the signal was inverted or not; a separate restoring circuit is required for
the top and bottom halves. The sample capacitors, C120 and C121, are charged
whenever the CLAMP PLS (from the Controller on Diagram 10) goes low and
Input Switching
Vertical Amplifier