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DIVERSITY SWITCHING
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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Section VI. STAGE ANALYSIS, TRANSMIT SIGNAL PATHS
TM 11-5805-387-34-2
1-36. VOICE AND TTY MONITOR SIGNAL PATH
(fig. FO-5)
The receive voice and tty monitor signal path consists of a main signal path and audio squelch switching.
Main Signal Path. The signal path consists of voice channel isolation amplifier A7Q2, receive voice filter FL5, AUDIO
GAIN control R1, driver A1A1Q1, and push-pull amplifier A1A1Q2, Q3, T1. The same input which is applied to tty
channel isolation amplifier A7Q1 (para 1-26) is simultaneously applied to voice channel isolation amplifier A7Q2. In 85
Hz plus voice, 2,805-Hz nsk tty tones are filtered out by receive voice filter FL5; in all other modes, the filter is bypassed,
and tty tones or voice signals are applied to driver A1A1Q1 through AUDIO GAIN control R1. Driver A1A1Q1 amplifies
the input signal and drives push-pull amplifier A1A1Q2, Q3, T1. Amplifier A1A1Q2, Q3, T1 puts out a 2-watt audio signal
for loudspeaker, and a 10-milliwatt (mw) signal for headphones. Thus, voice transmissions can be monitored in voice
and 85 Hz plus voice modes; in all other modes, tty tones can be monitored.
Audio Squelch Switching. SQUELCH SENS control R9 samples the signals applied to AUDIO GAIN control R1, and
applies the sample to detector A1A1Q2, which puts out a dc voltage proportional to the strength of the sample signal.
The dc voltage is applied to phase reversal amplifier A1A1Q3. Amplifier A1A1Q3 produces an amplified dc voltage
inversely proportional to its input. When the level of the input sample falls to a predetermined level, the output of
amplifier A1A1Q3 rises to a level that causes threshold zener A1A1VR1 to conduct, biasing switch A1A1Q4 into
conduction. With switch A1A1Q4 in a low state, the input to driver A1A1Ql is grounded, squelching the output of the
voice any tty monitor signal path.
1-37. DETAILED BLOCK DIAGRAM ANALYSIS OF SCOPE MODULE
(fig. FO-7)
The scope module consists of a scope amplifier section, a scope channel control, a scope Intensity squelch, and a
scope power supply.
Scope Amplifier and Channel Switching. Scope amplifier and channel switching consists of horizontal amplifier
A2A1Q1, T1, vertical amplifier A2A1Q2, T2, threshold zener A2A1VR1, channel switch A2A1Q5, and relay A2A1K1. Tty
channel 1 frequency discriminator A4A1Q9, Q10 is tuned to 2,805 Hz. Because 2,805-Hz fsk (or nsk) is applied to the
frequency discriminator, the mark and space pulses are below and above resonance, respectively, producing phase
differences that result in a differential voltage output. The differential output depends on the input frequency. This
differential voltage is applied to scope module horizontal channel 1 and vertical channel 1. Similarly, a differential
voltage produced by nsk frequency discriminator A4A2T1, Q7, Q8 is applied to scope module horizontal and vertical
channels 2. When tty channel 2 is squelched because of the operating mode, or in 85-Hz diversity because of a stronger
signal on tty channel 1, limiter amplifier A4A2Q5 produces a strong positive output to the scope channel control When a
strong positive voltage is applied, threshold zener A2A1VR1 conducts, switching channel switch A2A1Q5 into a low state.
Current flows through A2A1Q5, energizing relay A2A1K1. When energized, relay A2A1K1 selects scope channel 1. If
receive tty channel 2 is being used, channel switch A2A1Q5 is biased off, and unenergized relay A2A1K1 applies scope
channel 2 to the horizontal and vertical amplifiers. The horizontal and vertical amplifiers raise the level of the input
signal and apply it to the horizontal and vertical deflection plates, respectively, of the scope CRT.
1-23

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