(4) The varactor bias voltage (J1 terminals 4
CRYOGENIC TEMP meter 1M1. An integrated circuit
and 5) is set in Unit 1 during gain-bandwidth alignment.
operational amplifier (U1) is used as the active element.
The presence of the voltage, not its value, is the
This operational amplifier requires + 12 V on terminal 7
important factor. Because the bias circuit cannot tolerate
and - 12 V on terminal 4, supplied through card (P1)
any appreciable dc current (10 microamp, maximum) no
terminals C and E, respectively. The input signal (2.5 to
impedance measurements can be made. Current will
7.5 mV) is applied to U1 terminals 2 and 3 via P1
cause irreparable damage to the varactor diode mounted
terminals F and B, respectively. A bridge circuit (R1, R2,
R7, R8, R6) is used to balance the input voltage with
device that provides parametric amplification. Bias
respect to circuit ground and center the output meter.
voltage can be measured on Unit 1 at 1J5, FIRST
Variable resistor R12 is a feedback resistor used to set
STAGE VARACTOR VOLTAGE TEST.
the overall gain of the amplifier. Meter 1M1 is connected
b. Temperature Monitor 1A1 (fig. 2-9). Circuit card
directly across P1 terminals J and H. R4, in the meter
circuit, is a limiting resistor used to protect the meter.
assembly 1A1, Temperature Monitor, is a dc amplifier
Assembly 1A1 provides a gain of about 13 dB.
used to amplify the thermocouple output voltage (a
above) to a level sufficient to drive a 1-mil meter,
Figure 2-9. Temperature Monitor Card 1A1 (Sheet 1 of 2).