Sample and Hold @

The output of the bandswitch drives preamp U5. U5 and UlB together form an op amp with U5 a low-noise, low-temperature drift preamplifier. U5, UlB, and U6 are used in a sample and hold circuit as shown in Figure 2.

Q15, a JFET, is used as the switch for the sample and hold. The switch is opened (that is, the FET is turned off) whenever a bandcrossing occurs. When a bandcrossing occurs, the input voltage at Jl changes as the YTO Loop resets its frequency to start a new band, and the YTX bandswitch changes bands. At this time, the voltage at the input of U5 varies. The sample and hold circuit is used to prevent these variations from reaching the YTX. The sample and hold switch is controlled through Q14 by a signal from the A6All Slope Generator. R44, R45, CRlO, and CR11 keep the output of UlB from saturating when the feedback loop is opened by Q15 being off. This prevents a “glitch” at the output of U6 when the loop closes if UlB suddenly had to recover from a saturated condition.

FROM

BANDSWITCH

TP5

R48

1K

20K

i

TO IF

OFFSET

Figure 2. YTX Driver Sample and Hold, Simplified Schematic

IF Offsets @

In addition to tracking a harmonic of the YTO, the YTX must be offset from this LO harmonic by the first IF (321.4 MHz). T his is done by U7A and its associated components. Q7 to QlO switch in different IF offset adjustments for each band. The YTX peaking signal from A6All Slope Generator is also summed into U7A through R77.

Sweep + Tune @I

Since the U5, UlB, and U6 combination have a gain of 1.05 set by R48 and R75, the voltage at TP5 has an sensitivity of -0.525 V/GHz (YTX). T his signal goes to the A6All Slope Generator, to voltage divider R54 and R55, and to U8. U8 is set to give -1 V/GHz at its output which goes to J2. This signal is cabled to the rear panel of the instrument to provide an analog voltage proportional to input frequency. U2A and its associated circuitry provide an offset voltage for U8 to correct for the first IF (3.6214 GHz) in the 0 to 2.5 GHz band. R98 is adjusted for 0 V at U2 with the spectrum analyzer tuned to 0 Hz.

2 A6A12