Amplifier Assembly

TM 11-5825-270-23

3-6. If. Amplifier Assembly

the switch bracket assembly and provides a gain of 20

(fig. FO-2 and FO-3)

dB.

(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,

functions:

(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

signals.

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

threshold.

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

events.

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

present.

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).

rejected.

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

impedance.

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

3-5