TM 11-5805-356-34-1
back-biased and non-conducting, the capacitors
C7 to lower the output frequency of the tuned
are isolated from the circuit. The anode of A1-
circuit.
CR2 is connected to a potential of approximately
(4) A break-in signal is generated by the
+9 volts developed by voltage divider A1R15
operation of BREAK IN switch S2. Operation of
and A1R16. The cathode of A1CR1 is connected
the switch opens the local send teletypewriter
to a voltage divider made up of transistor A1Q3
circuit (contacts 7 and 8), equivalent to a space
and resistor A1R12. With a space input, transis-
input, defeating the carrier-suppression circuit
tor A1Q3 is biased to cutoff and the diodes are
and causing the modulator to connect capacitors
forward-biased through resistor A1R12, which is
A1C5; A1C6, and A1C7 across the oscillator
returned to 0 volt dc. With a mark input, tran-
tank. In addition, operation of the switch con-
sistor A1Q3 is forward-biased and conducts,
nects capacitors A1C12, A1C13, and A1C14
making the cathode of diode A1CR1 approxi-
across the tank (contacts 10 and 12), causing
mately +18 volts and back-biasing the diodes.
the break-in frequency to be generated.
(3) Capacitor A1C5, one of the capacitors
added to the tank circuit to obtain the lower
f. Gated Amplifier A1Q6. The vf signals ap-
space frequency, is a variable capacitor to permit
plied to this circuit are amplified or inhibited,
exact adjustment of the space frequency to 1232.5
according to the gating signal applied to the
Hz. As the resistance of back-biased diodes can
emitter of transistor A1Q6 by the carrier-suppres-
vary significantly between different samples, re-
sion circuit. The input signal is developed across
sistors A1R13 and A1R14 are placed across the
base resistor A1R26 and applied to transistor A1-
diodes to assure an approximately equal division
Q6, where it is amplified and developed across
of the back-biasing voltage.
collector load resistor A1R27. The output signal
e. Oscillator A1Q4, Emitter Follower A1Q5.
is direct-coupled to amplified A1Q7. Resistor A1-
(1) The oscillator circuit uses a two-stage
R28 provides emitter bias. The gating signal con-
feedback amplified arrangement to produce three
sists of a biasing voltage that reverse-biases the
output frequencies: 1180 Hz for break-in signals,
emitter-base junction of transistor A1Q6. With
1232.5 Hz for space signals, and 1317.5 Hz for
transistor A1Q6 cut off, the signals applied to
mark signals.
its base are inhibited. Resistor A1R25 limits the
(2) The signal generated in the basic tuned
base current of transistor A1Q6.
circuit (inductor A1L1 and capacitors A1C9, A1-
g. Amplifier A1Q7. This circuit amplifies the
C10, and A1C11) is amplified by transistor A1-
vf signals direct-coupled to the base of transistor
Q4 and direct-coupled to transistor A1Q5. The
A1Q7. The amplified output signal is developed
output signal of transistor A1Q5 is developed
across the primary of output transformer T3. Re-
across resistors A1R23 and A1R24. Part of this
sistor A1R29 is a collector load resistor, and re-
signal voltage is fed back through capacitor A1-
sistor A1R30 provides emitter bias. Capacitors
C16 and resistor A1R22 to maintain the tuned
A1C17 and A1C18, together with resistors A1R-
circuit in oscillation. In addition, the voltage de-
31, A1R32, and A1R33, match the impedance of
veloped across emitter resistor A1R24 is fed back
the output of transistor A1Q7 to that of the in-
to the base of transistor A1Q4, through the
put of transformer T3, and provide a constant
parallel combination of resistor A1R17 and ca-
600-ohm output impedance. Resistors R18, R21,
pacitor A1C8, for maintaining the output of tran-
and R22 form a pi-type attenuation network,
sistor A1Q4 at a constant level, independent of
which is switch out of the signal path (for in-
frequency. Resistors A1R20 and A1R21 are emit-
creased break-in signal strength) by BOOST
ter biasing resistors. Capacitor A1C15 bypasses
switch S6.
emitter resistor A1R21, leaving emitter resistor
A1R20 unbypassed to provide local feedback. Re-
2-6. Receive Circuit
sistor A1R19 is the collector load for transistor
A1Q4.
(3) The output frequency is determined by
a. Emitter Follower A3Q1. Emitter follower
the tuned circuit with capacitors added to lower
A3Q1 matches the output impedance of trans-
the frequency of the basic circuit. When a mark
former T2 to the input `impedance of limiter-am-
frequency is to be generated, the modulator pre-
plifier A3Q2. The incoming vf signals from the
vents the addition of any capacitors by presenting
transmission line are impressed across the pri-
a high impedance. When a space frequency is to
mary of input transformer T2 and a correspond-
be generated, the modulator, by presenting a low
impedance, adds capacitors A1C5, A1C6, and A1-
ing output signal is developed across its second-
2-8
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