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TRANSMITTER MODULE A3, 2,805-HZ NSK GENERATION
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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TRANSMIT VOICE SIGNAL PATH
TM 11-5805-387-34-2
Nsk operation. Shaper amplifier A3A1Q4 and divide-by-8 circuit A3A1Q5 through Q10 operates as described
in fsk operation. The divide-by-8 output is applied direct to sine wave pass filter A3A1C1, C2, L1 as before, with
C1 switched in to modify the pass band width for nsk operation. However, the divide-by-8 output (A) is also
divided by 10 (A3A1Q11 through Q18), resulting in a divide-by-80 output (B). The divide-by-80 output is also
applied to sine wave pass filter A3A1C1, C2, L1 where it is combined with the divide-by-8 output (C). The
resulting waveform is applied to sine wave pass filter A3A2C1, C2, L1, which acts to round off the applied
square wave into a sine wave (D, dotted line). At this point, a mark tone is 2,762.5 Hz and a space tone is
2,847.5 Hz, the frequencies of the 2,805-Hz nsk mark and space tones. The output of the sine wave pass filter
is fed to isolation amplifier A3A2Q1, which stabilizes load and source impedances. The 2,805-Hz nsk produced
is used for 85 Hz, 85 Hz + voice, and the higher diversity channel. The production of the lower diversity
channel (425-Hz nsk) is discussed in paragraph 1-23.
1-23. MIXER CIRCUIT
(fig FO-4)
The mixer circuit mixes certain mark and space tones with BFO mixer injection frequencies to establish the center
frequency for the tones of the 850-Hz mode and the lower band of 85 Hz diversity operation (2,000-Hz fsk and 425-Hz
nsk, respectively). The center frequency of the 2,805-Hz nsk mark and space tones (85-Hz modes) has been established
(para 1-22),and consequently bypasses the mixer circuit. The mixer circuit consists of mixer oscillator A3A2Q3, Q4;
divide-by-10 integrated circuit A3A2Z1 and its driver amplifier A3A2Q5; mixer A3A2T1, CR1, CR2; divide-by-10 bypass
switch diode A3A2CR10, and mixer filters FL1 and FL2. The mixer circuit operates only in the 850-HZ and 85-HZ
DIVERSITY settings of the MODE SELECTOR switch.
Mixer Circuit in 850-Hz Mode Operation. With MODE SELECTOR switch S4 set at 850 HZ, crystal A3A2Y2 controls
the frequency of mixer injection oscillator A2A3Q3, Q4 to produce an output of 26,050 Hz. In the 850-Hz mode, the
divide-by-10 integrated circuit is not energized, and diode CR10 is forward biased.  The 26,050-Hz mixer injection
frequency is applied to mixer A3A2T1, CR1, CR2 where it is combined with the output of isolation amplifier A3A2Q1.
The output of isolation amplifier A3A2Q1 is 27,625 Hz for a mark and 28,475 Hz for a space. When subtractively
combined with the mixer injection frequency in mixer A3A2T1, CR1, CR2, a mark tone becomes 1575 Hz, and a space
becomes 2425 Hz. The mark and space tones are centered 425 (850-Hz frequency shift) around 2,000 Hz. Also the
2,000-Hz tone actually does not exist, but is used as a convenient method of expressing the frequencies of mark and
space tones. The 2,000-Hz fsk tones are amplified by mixer amplifier A3A2Q2. The output of mixer amplifier A3A2Q2 is
applied to switches A3A2CR3 and CR4. In the 850-Hz mode, switch A3A2CR4 is biased on, so the output of mixer
amplifier A3A2Q2 is applied to 2,000-Hz nsk filter FL2. Filter FL2 filters out mixing harmonics, and applies the filtered
2,000-Hz fsk tones to the transmit output circuitry.
1-14

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