A 2,805-Hz ± 42.5 (or 28,050-Hz ± 425) signal is applied to sine wave pass filter A3A2C1, A3A2C2, and A3A2L1.
The sine wave pass filter is a tuned circuit which converts the square wave input signal to a sine wave for amplification
by isolation amplifier A3A2Q1. Capacitor A3A2C2 and inductor A3A2L1 are parallel-resonant at the input frequency of
28,050 Hz. In 850-Hz mode of operation, capacitor A3A2C1 is ungrounded and has no effect. During 85-Hz mode of
operation, capacitor A3A2C1 is grounded through contacts of front panel MODE SELECTOR switch S4G. This changes
the parallel-resonant frequency of the sine wave pass filter to 2,805 Hz. The resulting sine wave output of the filter is fed
to the base of isolation amplifier A3A2Q1 through coupling resistor A3A2R1.
Isolation amplifier A3A2Q1 is a conventional common-emitter amplifier. Capacitor A3A2C6 bypasses only a
portion of the emitter resistance of A3A2R48 and A3A2R6 in order to retain a high input impedance to the stage to
prevent loading of the sine wave pass filter. The amplified output of the transistor is fed through potentiometer
A3A2R4,,coupling capacitor A3A2C25, voltage divider A3A2R62, R63, and pin 3 of connector A3J1 to 2,805-Hz nsk filter
The signal at the collector of isolation amplifier A3A2Q1 is also fed through coupling capacitor A3A2C5 and
isolating resistor A3A2R7 to the primary of mixer transformer A3A2T1. There the signal is mixed with one from mixer
oscillator output divide-by-10 circuit A3A2Z1 (85-Hz diversity mode) or from mixer oscillator output divide-by-10 bypass
switch A3A2CR10 (850-Hz mode). Diodes A3A2CR1 and A3A2CR2 provide the nonlinear circuit elements necessary for
mixing. The resulting difference frequency is coupled through mixer transformer A3A2T1 to potentiometer A3A2R9. A
portion of the signal that appears across A3A2R9 is fed to mixer amplifier A3A2Q2 (para 1-49).
1-49. MIXER AMPLIFIER AND TRANSMIT FSK/NSK FILTER SWITCHING
The mixer amplifier and fsk/nsk filter switching circuit amplify the 2,000-Hz ± 425 or 425-Hz ± 42.5 signal from
mixer transformer A3A2T1 (para 1-48). The circuit also switches the amplified signal to either the fsk or nsk filter,
depending on the mode of operation of the modem.
The 2,000-Hz fsk (or 425-Hz nsk) signal is fed through coupling capacitor A3A2C8 to the base of amplifier
A3A2Q2. The amplified signal is developed across collector load resistor A3A2R12 and applied through coupling
capacitor A3A2C9 to the anodes of switching diodes A3A2CR3 and A3A2CR4.
Voltage divider A3A2R14, R15 biases the anodes of the switching diodes to approximately + 16.5 volts dc. The
cathodes of the diodes are connected to voltage dividers that are controlled by front panel MODE SELECTOR switch
S4G. Resistors A3A2R16 and A3A2R24 form the voltage divider for switching diode A3A2CR3; resistors A3A2R17 and
A3A2R26 form the divider for switching diode A3A2CR4.
Assume that the MODE SELECTOR switch is set at 850 Hz. Ground is supplied from contacts 6 and 1 of switch
S4G to resistor A3A2R26. The voltage-divider action of A3A2R26 and A3A2R17 biases the cathode of switching diode
A3A2CR4 to approximately ± 2.5 volts dc. Thus, the diode is forward biased and the signal is applied through the diode,
coupling capacitor A3A2C24, 2,000Hz fsk level adjustment potentiometer A3A2R52, and pin 6 of connector A3J1 to
terminal 1 of fsk filter FL2.