Receiver Frequency Mixer Stage 2A8 Circuit Functioning

TM 11-5820-695-35

Figure 2-25. 70 MHz band pass filter (B=5.0 MHz) 2A6 (684447), schematic diagram.

stage Q2, and capacitor C16 is the if signal bypass for

RT1 and potentiometer R12. Thermistor RT1 also

2-72. Receiver Frequency Mixer Stage 2A8 Circuit

provides temperature compensation for the gain of this

Functioning (fig. S:6)

stage over a wide temperature range. Potentiometer

Receiver frequency mixer stage 2A8 produces the 4.33

R12 sets the operating current through second

to 5.07 GHz local oscillator signal by mixing the 220

preamplifier stage 02, and, therefore, sets the gain of

MHz signal from the 220 MHz frequency multiplier-osc

second preamplifier stage Q2.

2A13 with the 4.55 to 4.85 GHz signal from bandpass

filter 2FL21. Receiver frequency mixer stage 2A8

consists of 220 MHz bandpass filter FL1, crystal mixer

d. Third Preamplifier. Third preamplifier Q3 also

Z1, a crystal current metering circuit, and coaxial

amplifies the 70 MHz if signal. Its output is coupled by

circulator HY1.

C17 to 70 MHz bandpass filter 2A6 through output plug

P2 designated IF OUT. Resistors R17, R19, R20, and

capacitor C22, form an impedance matching network to

a. 220 MHz Bandpass Filter 2A8FL1. 220 MHz

provide a 75 ohm output impedance. Capacitor C22 is

bandpass filter 2A8FL1 is a three-pole Chebyshev type

variable in this network to permit matching the load.

bandpass filter. Maximum insertion loss is 8 db;

Resistors R13 and R14 provide fixed dc bias for third

bandwidth is 11.5 MHz at the 3 db reference points.

preamplifier stage Q3, and inductor L9 is its collector

The input is filtered by 2A8FL1 for application to crystal

load.

Emitter resistor R15 provides negative ac

mixer 2A8Z1.

feedback to stabilize this stage, and capacitor C19 is the

ac bypass. Resistors R15 and R16 determine the

b. Crystal Mixer 2A8Z1. Crystal mixer 2A8Z1 is

current through third preamplifier stage Q3. The 70

similar to a conventional balanced coaxial mixer but

MHz if signal out of third preamplifier stage Q3 is also

uses two forward diodes CR1 and CR2. The unit

coupled by C18 to a diode detector circuit formed by

accepts a 220 MHz input signal at a level of +11 dbm to

CR3, R18, and C21. This provides a positive dc voltage

+13 dbm and an input signal in the frequency range

which is proportional to the level of the detected if signal

4550 to 4850 MHz at a level of + 16 dbm to +22 dbm.

at jack J4 designated TP1 IF MON. Test point TP1 IF

The resultant rf output

MON can be connected to the meter on meter panel

assembly 2A15A2 with a test lead.

2-114

Change 6