HP 85020A Operating And Service Manual page 52

Service
Model 85020A/B
coaxial imbalances, etc., a voltage will be output to the Network
Analyzer. The ratio of the detected voltage with a perfect
matched-impedance load to the detected voltage with a short is the
directivity.
Source Match is a measure of the impedance lookina into the test
port. It is usuallv given as SWR. It is a function of the bridge
element values, and is independent of directivity: that is, a
bridge with very ciood directivity could have poor source match.
A2 Preamplifier Circuit
The output of the bridge circuit is a low level 27 kHz signal.
Before it is incut to the Network Analyzer, it must be amplified.
This is the function of the preamplifier. Fiqure 8-3 is a
schematic diagram of the preamplifier A2, and Figure 8-2 is a
preamplifier component locations diagram.
The functional center of the preamplifier circuit is a low-noise
operational amplifier Ul wired in the non-inverting configuration.
The qain of the preamplifier is controlled by R3 and R4 {Gain =
1+R4/R3). The gain may be varied from 1 to approximately 17. R4 is
the GAIN adjustment and compensates for the sensitivity of the
bridge diode.
The input impedance adjustment R2 (Z) compensates for the internal
resistance of the diode. The input impedance may be varied from
100 ohms to aproximately 10k ohms.
The 100 ohm resistor R5 and the 1000 pF capacitor C3 form a
decoupling network which keeps RF signals from the input of the
operational amplifier.
R1 is Placed in series with the non-inverting input of the
operational amplifier Ul to prevent the input sinnal from shorting
to ground through the 10k potentiometer R2.