Theory of Operation

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Figure 4-1: 1705A L- Band comb display showing the areas each of the six sweep shaper variable resistors adjust

Bright- Up Generator 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 [Rf], R84 [Ri] 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 [Rf] and R87 [Ri]). Again under these

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1705A Spectrum Monitor

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Tektronix 1705A instruction manual Start Display