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
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
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