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MIXER CIRCUIT
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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TRANSMIT OUTPUT CIRCUITRY
TM 11-5805-387-34-2
Mixer Circuit in 85-Hz Diversity Mode of Operation. With the MODE SELECTOR switch set at 85 HZ DIVERSITY,
crystal A3A2Y1 controls the frequency of mixer oscillator A3A2Q3, Q4 to produce an output of 23,800 Hz. In the 85-Hz
diversity mode, the divide-by-10 driver A3A2Q5 and divideby-10 integrated circuit A3A2Z1 are energized. (Divide-by-10
bypass switch A3A2CR10, which bypasses the divide-by-10 circuitry, is backbiased in 85-Hz diversity.)The 2,380-Hz
output of divide-by-10 integrated circuit A3A2Z1 is the mixer injection frequency applied to mixer A3A2T1, CR1, CR2. In
the mixer, the mixer injection frequency is combined with the output of isolation amplifier A3A2Q1. As explained in
paragraph 1-22, the output of isolation amplifier A3A2Q1 is 2,805-Hz nsk (2,762.5 Hz for a mark, 2,847.5 Hz for a
space). When subtractively combined with the 2,380-Hz mixer injection frequency in mixer A3A2T1, CR1, CR2, a mark
tone becomes 382.5 Hz and a space becomes 467.5 Hz. The mark and space tones are centered 42.5 Hz (85-Hz
frequency shift) around 425 Hz. The 425-Hz tone does not exist, but is used as a convenient means of expressing the
frequencies of tones. The 425-Hz nsk thus produced is amplified by mixer amplifier A3A2Q2 and is applied to switches
A3A2CR3 and CR4. With MODE SELECTOR switch S4 set at 85 HZ DIVERSITY, switch A3A2CR3 is biased on, so the
output of mixer amplifier A3A2Q2 is applied to the 425-Hz nsk portion of FL1. Filter FL1 filters out mixing harmonics,
and applies the filtered 425-Hz nsk tones to the transmit output circuitry. The 2,380 Hz produced by divide-by-10
integrated circuit A3A2Z1 is also applied to the receiver module for mixer injection.
Mixer Circuit in 85-Hz, 85-Hz + Voice, and Voice Modes of Operation. Mixing occurs only in 850Hz and 85-Hz
diversity modes. In all other tty modes (85-Hz and 85-Hz voice), mixer A3A2T1, CR1, CR2 is bypassed, and isolation
amplifier A3A2Q1 applies the 2,805-Hz nsk tones to the 2,805-Hz nsk portion of FL1. Filter FL1 filters out mixing
harmonics, and applies the filtered 2,805-Hz nsk tones to the transmit output circuitry.
1-24. TRANSMIT VOICE SIGNAL PATH
(fig FO-4)
The transmit voice signal path consists of a 50-ohm (nominal) impedance signal path and a 600-ohm (nominal) signal
path.
50-Ohm Voice Signal Path. The 50-ohm voice signal path processes voice audio produced by a carbon handset or
microphone. The signal path consists of 50-ohm mike filter A6L6, C6, C13; 50-ohm mike bias supply A3A2R38; a
keyline, and 50-ohm MIKE LEVEL control A3A2R40; 50-ohm mike keying switch A3A2CR8; transmit voice filter FL3;
voice filter amplifier A3A2Q9; and 50-ohm mike output filter A6L6, C6, C11. The 50-ohm voice signal path is switched
by the MODE SELECTOR switch. Operation in the 85-Hz plus voice modes is described below.
85-Hz plus voice mode operation of 50-ohm voice signal path. In the 85-Hz plus voice mode, the modem
supplies dc bias for the 50-ohm mike, switches, filters, and keys; controls the level of the 50-ohm mike signal; and
mixes the signal with tty tones produced and processed in the tty signal path. Voice signals generated in the 50-
ohm mike are applied through MICROPHONE input jack J3 to input filter A6L3, C3, C13. Resistor A3A2R40
controls the level of the applied signal and insures proper balance between voice signals that originates from a 50-
ohm source and tty tones. When the 50-ohm mike is switched on, the keyline is grounded through pin F of
MICROPHONE J3 (or SPEAKER + REMOTE J4, or AUXILIARY J5). This forward biases 50-ohm mike keying
switch A3A2CR8, applying the signal through isolation resistor A3A1 R46 to transmit voice filter FL3. Filter FL3
establishes a voice bandwidth of 200-to 2,260 Hz, preventing interference with the 2,805-Hz nsk tones when voice
is combined with tty tones in the transmit output section. The output of FL3 is applied to the transmit output
section.
1-15

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