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Section V. DETAILED BLOCK DIAGRAM ANALYSIS OF RECEIVE SIGNAL PATHS
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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RECEIVE MARK-SPACE PULSE GENERATION
TM 11-5805-387-34-2
1-29. RECEIVE TTY CHANNEL 2
(fig. FO-5)
Receive tty channel 2 consists of the 2,805-Hz nsk pass portion of FL4, amplifier A4A262, limiter amplifier A4A2Q5,
discriminator driver A4A2Q6, and nsk frequency discriminator A4A2T1, Q7, Q8.  Filtered 2,805-Hz nsk tones, with
wideband noise removed, are amplified by A4A2Q2 and applied to limiter amplifier A4A2Q5, which controls the level of
the signal applied to discriminator driver A4A2Q6. The limiting action insures proper function of frequency discriminator
A4A2Q7, Q8 by providing an input of constant amplitude at all times. The 2,805-Hz nsk signal consists entirely of a
2,762.5-Hz tone and a 2,847.5-Hz space tone. The frequency discriminator puts out a dc voltage of one polarity for a
2,762.5-Hz input and the opposite polarity for a 2,847.5-Hz input. The ellipses seen on the scope pattern are derived
from varying phases within the discriminator. The operation of the scope module is discussed in detail in paragraph 1-
1-30. RECEIVE TTY CHANNEL 1
(fig. FO-5)
Receive tty channel 1 consists of mixer A4A1CR1, CR2, T1, mixer injector amplifier A4A1Q1, relay K1, nsk mixer filter
FL6, fsk mixer filter FL7, amplifier A4A1Q2, limiter amplifier A4A1Q3, discriminator driver A4A1Q4, relay A4A1K1, and
fsk/nsk frequency discriminator A4A1T2, Q9, Q10. Receive tty channel 1 is used only in 85-Hz diversity and 850-Hz
modes.
Receive Tty Channel 1 in 85-Hz Diversity. Filtered 425-Hz nsk tones with wideband noise removed are applied to
mixer A4A1CR1, CR2, T1, where they are additively mixed with 2,380 Hz from mixer injection amplifier A4A1Q1. The
resulting 2,805-Hz nsk (2,380-Hz nsk + 425 nsk = 2,805-Hz nsk) is passed through the contacts of unenergized relay K1
into narrowband nsk mixer filter FL6, where mixing harmonics are removed. The 2,805-Hz nsk is then amplified by
amplifier A4A1Q2, limited by limiter amplifier A4A1Q3, and applied to discriminator driver A4A1Q4, which drives fsk/nsk
frequency discriminator A4A1T2, Q9, Q10 through the contacts of unenergized relay A4A1K1. Except for relay A4A1K1,
the operation of the latter half of the receive tty channel 2 is identical with that of tty channel 1.
Receive Tty Channel 1 in 850 Hz. Unfiltered 2,000-Hz fsk tones are applied to mixer A4A1CR 1, CR2, T1, where
they are subtractively mixed with a mixer injection frequency from bfo oscillator A1A3Q1. Using BFO control R2, the
mixer injection frequency may be varied from 3,805 to 5,805 Hz, to permit reception of fsk signals with center
frequencies ranging from 1,000 to 3,000 Hz. In all discussions, a 2,000-Hz center frequency is assumed. If the 2,000-Hz
fsk is subtractively mixed with an assumed nominal 4,805-Hz injection frequency, a 2,805-Hz fsk output results (4,805 Hz
2,000-Hz fsk = 2,805-Hz fsk). Because the mixing is subtractive, the frequencies of the mark and space tones will be
reversed at this point; the mark is the higher (3,230 Hz) and the space is the lower (2,380 Hz). Relay K1 is energized in
850 Hz; therefore, the 2,805-Hz fsk (reversed) is applied to fsk mixer filter FL7, which has a wider passband than FL6, to
permit passage of fsk tones.  Filter FL7 blocks mixing harmonics and wideband noise.  The filtered 2,805 Hz-fsk
(reversed) is amplified by amplifier A4A1Q2, limited by limiter amplifier A4A1Q3, and applied to discriminator driver
A4A1Q4. In the 850-Hz mode, energized relay A4A1K1 widens the passband of fsk/nsk frequency discriminator A4A1T2,
Q9, Q10 for fsk operation. Relay A4A1K1 also reverses the operation of the discriminator from nsk operation, so that the
higher tone input to the discriminator (3,230 Hz) will produce a mark pulse output, and the lower tone will produce a
space pulse output. The output polarity of the frequency discriminator as a function of input tty tone frequency is
reversed in the 850-Hz mode. This reversal corrects the tone reversal caused by subtractive mixing in mixer A4A1CR1,
CR2, T1.
1-19

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