UHF MIXER

NAVELEX 0967-LP-544-5010

output of Q3 to 30 MHz. Capacitor C29 and resistor R7

4-103. The following describes individual circuit func-

decouple the amplifier from the +12-V dc line. Resistor

tions of receiver rf module A3. Refer to figure FO-20.

R8 provides bias voltage.

4-104. UHF MIXER. The uhf mixer (U1 of figure FO-20)

4-109. NOISE CHANNEL IF AMPLIFIERS (U2, U3).

functions as an active balanced mixer to generate the

The noise channel if amplifiers (U2 and U3 of figure FO-

30-MHz if signal. Capacitor C1 couples the receive rf

20) consist of two gain-controlled integrated circuit

signal to the signal input port of coupling transformer T1.

amplifiers that produce about 40 dB of open loop gain

Transformer T1 transforms the signal into two balanced,

per stage. Capacitor C30 couples the 30-MHz if signal to

180-degree-phase-related signals and applies them to

the first noise channel if amplifier (U2). Capacitor C34

the source inputs of the two junction field-effect

and inductor L23 tune the output of U2 to 30 MHz.

transistors (JFET) of U1. In a similar manner, capacitor

Variable capacitor C35 couples the output of U2 to the

C11 couples the receive injection signal through the local

input of the second noise channel if amplifier (U3) which

oscillator port of T1 to the source inputs of U1.

is tuned to 30 MHz by capacitor C36 and inductor L24.

Transformer T1 applies the receive injection signal in

Variable capacitor C41, capacitor C42, and inductor L25

phase. The level of the receive injection signal is such

tune the output of U3 to 30 MHz. Variable capacitor C43

that it causes the grounded gate JFET's of U1 to operate

couples the output of U3 to the noise channel detector

in the square law region for mixing action and con-

circuit (CR5). Variable capacitors C35 and C43 are

version gain. The uhf mixer provides about 3 dB of gain

adjusted for maximum rf gain through the noise channel

and, therefore, acts as an rf preamplifier to the receiver rf

if amplifiers. Noise channel AGC voltage developed by

input signal. The 30-MHz difference frequency appears

the noise channel AGC amplifier is applied through

across the secondary of transformer T2.

resistors R9 and R11 to control the gain of the noise

channel if amplifiers for a nearly constant signal level at

4-105. In the uhf mixer circuit, variable capacitors C2

the input to the noise channel detector. Capacitors C32

and C12 provide broadband tuning to the input of

and C104 and resistor R102 form a low-pass filter to

transformer T1. The capacitors are adjustable to provide

decouple the first noise channel if amplifier (U2) from the

maximum flatness of the output signal over the

+12-V dc line. Capacitors C37 and C105 and resistor

frequency range of the uhf mixer. Capacitors C13 and

R103 decouple the second noise channel if amplifier

C19 tune the output of transformer T2 to 30 MHz.

(U3) from the +12-V dc line.

Resistor R2 and capacitor C20 decouple the uhf mixer

from the +12-V dc line. Resistor R1 provides bias

4-110. NOISE CHANNEL DETECTOR. The noise

voltage.

channel detector (CR5 of figure FO-20) demodulates the

30-MHz noise channel if signal and develops the

4-106. 30-MHz FILTER. The 30-MHz filter (FL3 of

detected signal across capacitor C45. The detected

figure FO-20) is a single-tuned, high Q filter that passes

signal contains impulse noise superimposed on a dc

the 30-MHz difference frequency from the uhf mixer to

level that is proportional to the average carrier level.

the first 30-MHz if amplifier. Variable capacitor C10 in

Bias network R14-CR3-CR4-C40 applies a dc voltage

parallel with capacitors C78 and C102 and inductor L7

equivalent to two diode drops through inductor L16 to the

tunes the center frequency of the filter and the output of

anode end of noise detector diode CR5. Inductor L26

transformer T2 to 30 MHz. Capacitor C102 provides

and capacitor C44 form a parallel resonant circuit at 30

temperature compensation.

MHz. This provides a low impedance path for the bias

voltage and a high impedance path for the 30-MHz if

4-107. FIRST 30-MHz IF AMPLIFIER. The first 30- MHz

signal. The bias voltage compensates for the diode drop

if amplifier (Q1 and Q2 of figure FO-20) is a low-noise rf

across CR5 and the emitter-to-base junction voltage of

amplifier that amplifies the 30-MHz if signal by about 14

the emitter follower (Q4) to bias Q4 at the point of

dB to ensure the remaining if stages will not degrade the

conduction. This makes the noise channel AGC voltage

overall receiver noise figure. Capacitor C22 couples the

independent of the voltage drop across detector diode

30-MHz if signal from the 30-MHz filter to the base of

CR5 and the base-to-emitter junction of Q4.

transistor Q1. Bias amplifier Q2 biases transistor Q1 at

approximately 60 milliamperes. Capacitor C23 couples

4-111. EMITTER FOLLOWER. The emitter follower

the output of Q1 to the filter/delay circuit (FL2) in the

(Q4 of figure FO-20) provides a high impedance load to

main receiver if channel and through coupling capacitor

the noise channel detector (CR5). The impulse type

C102 to the noise channel if amplifier.

noise developed across emitter resistor R19 is coupled

through a differentiator formed by capacitor C49 and

4-108. NOISE CHANNEL IF AMPLIFIER (Q3). The

resistor R22 to the noise pulse detector. The dc voltage

noise channel if amplifier (Q3 of figure FO-20) is a self-

developed across R19 is applied to the noise channel

biased FET rf amplifier circuit with a gain of about 4 dB.

AGC amplifier (Q5, Q6).

Variable capacitor C33 and inductor L22 tune the

4-23