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TM 11-5825-270-23
3-6. If. Amplifier Assembly
the switch bracket assembly and provides a gain of 20
(2) Up-coverts the rf input to the receiver if (4.4
a. The if. amplifier assembly performs the following
MHz), using a crystal-controlled local oscillator.
b. The rf input from tuning circuit is applied,
(1) Amplifies the 4.4 MHz if. input from the RF
through P1-7 and C1, to the grounded base amplifier
PCB assembly.
Q1. This amplifier provides a fixed gain of 20 dB and is
(2) Detects the amplified if. signal to extract the
followed by a low pass filter comprising C5, L2, and C6.
modulation and carrier level information.
The low pass filter rejects the induced if. and image
(3) Provides a 600 ohm balanced output of the
c. Transistor Q2 is a common emitter amplifier and
detected modulation (audio output).
(4) Continuously  checks  the  carrier  and
its output is taken through the SENSITIVITY control R7
modulation levels and triggers the alarm circuits if the
which determines the signal level applied to the
carrier and/or modulation fall below a predetermined
balanced mixer formed by Q3, Q4, T1, T2 and
associated components. Note that capacitor C7 in the
(5) Generates and resets alarm conditions
collector circuit of Q2 serves to block the dc and permits
according to a predetermined logical sequence of
diode CR1 to provide a bias pedestal for transistors Q3
and Q4. For rf signal, capacitor C7 may be considered
b. The 4.4 MHz if. signal from the RF PCB is
as a short circuit.
d. Transformer T1 is a 1:1 transformer which
applied, through P1-17 and C1, to a fixed gain if
amplifier formed by Q1 through Q4 and associated
provides an out-of-phase rf signal at Q4 base, with
components. Basically, the if. amplifier comprises four
respect to the signal at Q3 base. The emitters of Q3 and
cascaded, low-noise, ground-base amplifiers, providing
Q4 are fed with the out-of-phase local oscillator signal
on overall gain of 80 dB. The gain of the first stage Q1
from transformer T2. At the output of the balanced mixer
is determined by the Q of the tuned circuit, L1, C4 and
(the common collector point of Q3 and Q4), the sum as
C2.  This tuned circuit has a Q of 10, therefore Q1
well as difference of the incoming frequencies are
provides a gain of 20 dB. The subsequent stages of the
However, only the sum of the rf signal
if. amplifier, Q2 through Q4, operate in an identical
frequency and the local oscillator frequency is of interest
manner, each providing a 20 dB gain.
and this frequency is selected by the crystal filter FL1.
c. The 4.4 MHz if. signal at the collector of Q4 is
Note that local oscillator frequency is always 4.4 MHz
detected by CR2 and filtered by R10 and C16. As a
minus the signal (carrier) frequency. Thus, the signal
result, the signal at the base of Q8 is a dc signal
selected by FL1 is always at 4.4 MHz, the receiver if. All
proportional to the if. signal (hence the carrier level).
other signals, image as well as harmonic products, that
Superimposed on this dc signal is the keying signal
may be present at the input of crystal filter FL1 are
(modulation audio).
d. The purpose of diode CR1 is to provide a dc
e. Resistor R12 in the collector of Q4 defines the
pedestal of approximately 0.7 Vdc at the anode of CR2
output impedance of the balanced mixer at 600 ohms.
and thus eliminates the threshold limit of CR2.  As a
As the input impedance of crystal filter FL1 is also 600
result the detector circuit (CR2, R10, C16) can operate
ohms, the balanced mixer and FL1 are properly
satisfactorily at very low input levels.
matched. The output impedance of FL1 (600 ohms) is
e. Transistor Q8 is an emitter follower. Therefore,
stepped-down by transformer T3 with the result that the
the signal at Q8 emitter is identical to the signal at its
4.4 MHz if output from RF PCB is at 50 ohms
base and contains both the carrier derived dc and the
f. The local oscillator circuit consists of a crystal-
modulation audio. This signal is fed to the monitoring
circuits and the line amplifier.
controlled  oscillator  (Q5,  Y1  and  associated
f. Transistors Q14 through Q17, transformer T1
components) and a driver stage Q6.  The operating
and associated components form the line amplifier which
frequency of the local oscillator, determined by Y1, can
provides a balanced 600 ohm output of the modulation
be fine tuned by trimmer capacitor C13. The range of
audio from Q8. Input to the line amplifier is provided
thisnfine tuning is limited to 200 Hz of the nominal
through potentiometer R14; capacitor C25 blocks the
crystal frequency. Under normal operation, C13 requires
carrier derived dc.
slight readjustment only if crystal Y1 is replaced.
g. The line amplifier has a gain of 25 dB; it is a
class B, complementary push-pull amplifier accoupled

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