TM 11-5805-424-15/NAVELEX 0967-220-9010/TO 31W2-2G-41
signals when the stage is changing operating
5-16. Demodulator Circuits
levels.
Resistor R11 is the collector load for transistor
limiting amplifier circuits, each comprising an
Q3, resistor R12 is the base grounding
operational amplifier, a feedback circuit and a shaper,
resistance, and resistor R15 is the collector load
several feedback amplifiers used for impedance
for transistor Q4. Resistor R16 is a collector dc
matching, nonsaturating amplifier NSA-1, a 6-volt
limiting resistor, and resistor R17 is the emitter
clamp, and a discriminator circuit. Input fsk signals are
load resistor for transistor Q5.
applied from the receiver filter to amplifier AM-5, an
(2) Feedback circuit. Diodes CR1 and CR2 are
emitter follower. Resistors R1 and R3 form an input
negative breakdown diodes that conduct when
signal voltage divider, resistor R4 is a de collector
the output voltage of transistor Q5 reaches a
limiting resistor, resistor R5 is the emitter load resistor,
sufficiently negative level; diodes CR3 and CR4
and resistors R68 and R69 are supply-voltage limiting
are positive breakdown diodes that conduct
resistors. Capacitors C14 and C15 are high-frequency
when the output of transistor Q5 reaches a
bypass capacitors for the +15 volts power supply
sufficiently positive level.
Capacitor C3
circuits. The output of amplifier AM-5 is coupled to an
provides a high-frequency feedback to transistor
operating amplifier through a coupling network
Q2, and resistor R70 provides a discharge path
composed of resistor R6 and capacitor C1, c below, and
for the capacitor. Resistors R13 and R14, with
also through coupling resistor R29 to amplifier AM-6, b
capacitor C4, provide for low-frequency
below.
feedback to transistor Q2.
The feedback
circuits supply equal amounts of feedback for
b. Input fsk signals from emitter follower Q9 are
the low-frequency mark signal and the high-
coupled through coupling resistor R33 to the receive
frequency space signal, so that the average dc
discriminator, which provides a phase shift of 90for the
output applied to the shaper circuit (through
carrier center frequency of more than 90for the space
coupling resistor R18 and coupling capacitor
frequency, and of less than 90for the mark frequency.
C6) is as close to 0 volt as possible. The
The received discriminator output is applied to cascade
shaper, a high gain amplifier, provides sharp
impedance-matching amplifier AM-7 and AM-8. The
leading and trailing edges of the input signal.
output from AM8 is fed through a coupling network
Resistor R20 is a collector load resistor, resistor
(resistor R39 and capacitor C7) to the operational
R19 is the base resistor, and diode CR5 acts as
amplifier (c below).
a shunt limiter. The output of each shaper is
c. The limiting amplifier circuits are identical;
applied to the phase detector (d below).
therefore, only one (using transistors Q2 and Q3) is
d. The phase detector, a nonlinear exclusive OR
described below.
circuit, combines the original fsk input signals with the
(1) Operational amplifier. Transistors Q2 and Q3
phase-shifted input.
signals from the receive
form a nonsaturating differential amplifier. Input
discriminator. The two carrier signal frequencies are
signals applied to transistor Q2 cannot overdrive
identical, but 90 out of phase; therefore, the phase
the stage, because the voltage developed
detector provides an output frequency that is twice the
across common resistor R9 (the input signal
carrier frequency, with a 50-percent duty cycle.
through transistor Q3) provides a degenerative
Because mark frequencies occur with less than 90
effect on both transistors. The output from
phase shift between them, the output duty cycle is
transistor Q3, inverted by transistor Q4, is
greater than 51 percent. Space frequencies occur with
applied to emitter follower Q5, which supplies
more than 90phase shift between the output; therefore,
enough drive-signal to the feedback circuit ((2)
the duty cycle is less than 50 percent. This changing
below). Resistor R7 and capacitor C2, and
duty cycle produces an output sinusoidal voltage that
resistor R10 and capacitor C5 form a shift
varies above and below the average dc level
network for transistors Q2 and Q5, respectively;
each network prevents a phase shift in the
Change 5
5-15
