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MIXER AMPLIFIER AND TRANSMIT FSK/NSK FILTER SWITCHING
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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Section VII. STAGE ANALYSIS, RECEIVE SIGNAL PATHS
TM 11-5805-387-34-2
With no ground applied to resistor A3A2R24, no dividing action takes place and +20 volts dc supply voltage is
applied to the cathode of switching diode A3A2CR3 through resistor A3A2R16. Consequently, the diode is reverse
biased, which blocks the signal from being applied through series-dropping resistor A3A2R23 and pin 4 of connector
A3J1 to nsk filter FL1. With the modem in the 85-Hz diversity mode of operation, the reverse action takes place, and the
signal is applied to nsk filter FL1 and blocked from fsk filter FL2. The outputs of the nsk and fsk filters are applied to tty
amplifier A3A2Q6 (para 1-51).
1-50. TRANSMIT VOICE CIRCUITS
(fig. FO-10)
Transmitter module A3 contains microphone bias, attenuator and keying circuits used in the 50-ohm microphone
voice path, and a 600-ohm voice amplifier used In the 600-ohm.volce circuit. Figure FO-11 shows the external switch
paths used to route signals through the voice circuits.
Transmitter module A3 50-ohm microphone circuits function only during the 85-Hz plus voice modes of operation
of the modem. Resistor A3A2R38 provides dc bias voltage for microphone operation, and audio from the microphone is
routed through pin 12 of connector A3J1 and coupling capacitor A3A2C20 to an attenuator circuit composed of resistors
A3A2R39, A3A2R53, and potentiometer A3A2R40.  A portion of the signal that appears across 50 MIKE LEVEL
potentiometer A3A2R40 is passed through coupling capacitor A3A2C21 to the anode of keying switch diode A3A2CR8. If
the keying diode is forward biased, the audio signal is fed through isolating resistor A3A2R46, coupling capacitor
A3A2C23, and pin 10 of connector A3J1 to transmit voice filter FL3. Keying switch diode A3A2C8 prevents microphone
audio from being applied to transmit voice filter F L3 unless a ground is supplied on pin 11 of A3J1 (keyline). Resistors
A3A2R41 and A3A2R42 form a voltage-divider network which biases the anode of diode A3A2CR8 at approximately +
18.5 volts dc. The cathode of the diode Is connected to a voltage divider composed of resistors A3A2R44, A3A2R43,
and A3A2R45. With no ground applied on the keyline, +20 volts dc is applied to the cathode of diode A3A2CR8 through
resistor A3A2R45, reverse biasing the diode. When a ground Is applied to A3A2R44, the voltage-divider action of
resistors A3A2R44, A3A2R43, and A3A2R45 develop approximately + 8.5 volts dc on the cathode of the diode, which
forward bias It and allows audio to pass. Capacitor A3A2C22 prevents audio from appearing on the keyline.
The 600-ohm voice circuit (fig. FO-11 ) operates only in the voice or 85-Hz plus voice modes of operation of the
modem. The circuit consists of conventional common-emitter amplifier A3A2Q8, supplied by an audio input signal from
pin 18 of connector A3J1. The amplified signal is routed to pin 21 of A3J1 and switch S4G. In the voice mode of
operation, the audio is applied through the switch to transmit output amplifier A3A2Q7. In the 85-Hz plus mode of
operation, the amplified audio is returned to Pin 23 of A3J1, coupled through isolating resistor A3A2R60 and fed to
transmit voice amplifier FL3 through A3J1-20.
1-39

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