Tektronix 1705A, Instruction Manual

Page: 81 / 223

Theory of Operation
4- 8
1705A Spectrum Monitor
START
OF
DISPLAY
R727 R729
R737 R733
R731 R735
Figure 4-1: 1705A L- Band comb display showing the areas each of the six sweep
shaper variable resistors adjust
The Marker Generator is U14. The front-panel CENTER FREQUENCY control
provides the voltage from which the digital frequency readout is obtained. In
addition, the CENTER FREQUENCY (through U14) positions the bright zone
in FULL SPAN/DIV and tunes the center frequency in magnified SPAN/DIV
settings. The gain and offset of U14A are adjustable with R37 and R38. When
the CENTER FREQUENCY control is turned from one end to the other, the
output of U14A will move from about -- to +2 V, which is approximately the
range of the ramp at U24B. In any SPAN/DIV setting other than FULL, analog
switch U17A is closed and the output of U14A provides an offset voltage to
move the attenuated ramp, at the output of U18A, over the range of the large
ramp in FULL SPAN/DIV
When the SPAN/DIV is set at FULL, the output of U14A positions the bright-up
zone to correspond to the frequency displayed by the readout. The output of
U14B provides a voltage that offsets the ramp at the output of U14C.
In the FULL setting of SPAN/DIV, U17D (an analog switch) is closed and U14C
provides a gain of 10 to both the ramp from U24B and the offset from U14B.
The bright-up zone is generated when the ramp, at the output of U14C, passes
through 0 V. The offset voltage from U14B can offset the ramp from almost tip
to tip, thus moving the bright-up zone over almost the entire length of the sweep.
U14D is an absolute value circuit. Its output will be near the positive power
supply unless the input, from U14C, is within
±
0.7 V of ground. If the input
goes above +0.7 V, CR8 conducts and U14D becomes a non-inverting amplifier
with a gain of about 20 (set by R83 [R
f
], R84 [R
i
] and the divider R87, R88).
Under these conditions the output goes towards the positive power supply. If the
input to U14D goes below --0.7 V, CR12 conducts and becomes an inverting
amplifier with a gain of
¶
20 (set by R83 [R
f
] and R87 [R
i
]). Again under these
Bright- Up Generator