HP (Hewlett-Packard) HP 85660B ユーザーズマニュアル

The output of the bandswitch drives preamp U5. U5 and 

 together form an op amp with

U5 a low-noise, low-temperature drift preamplifier. U5, 

 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 

 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 

 Slope

Generator. R44, R45, 

 and CR11 keep the output of 

 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 

 suddenly had to recover from a saturated condition.

Figure 2. YTX Driver Sample and Hold, Simplified Schematic

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 

 and its associated components.

Q7 to 

 switch in different IF offset adjustments for each band. The YTX peaking signal

from 

 Slope Generator is also summed into 

 through R77.

Since the U5, 

 and U6 combination have a gain of 1.05 set by R48 and R75, the voltage

at TP5 has an sensitivity of -0.525 

 (YTX). T his signal goes to the 

 Slope

Generator, to voltage divider R54 and R55, and to U8. U8 is set to give -1 

 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. 

 and its associated circuitry provide an

offset voltage for U8 to correct for the first IF (3.6214 

 in the 0 to 2.5 

 band. R98 is

adjusted for 0 V at U2 with the spectrum analyzer tuned to 0 Hz.

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