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RECEIVE DC LOOP
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
Next
VOICE AND TTY MONITOR SIGNAL PATH
TM 11-5805-387-34-2
1-34. DIVERSITY SWITCHING
(fig. FO-5)
In 85-Hz diversity, diversity switching circuitry samples tty channel 1 and tty channel 2, and squelches the weaker
channel, if the difference in level is greater than 3 decibels (db). Diversity switching circuitry consists of diversity control
adjusters A4A2R5 and A4A2R13; detectors A4A2CR3, CR4 and A4A2CR7, CR8; diversity channel control differentiating
amplifier A4A2Z2; threshold diodes A4A2VR4 and A4A2VR5; channel 1 switch A4A2Q3; and channel 2 switch A4A2Q4.
In 85-Hz diversity, the input to the modem consists of 425-Hz nsk and 2,805-Hz nsk. The 2,805-Hz nsk is applied
unchanged to tty channel 2, and the 425-Hz nsk applied to tty channel 1 is mixed up to 2,805-Hz nsk. The channel 1
(2,805-Hz nsk) is sampled through tty channel 1 diversity control adjust A4A2R5, and detected by detector A4A2CR3,
CR4. The dc output of detector A4A2CR3, CR4 is applied to diversity channel control differentiating amplifier A4A272.
Amplifier A4A272 compares the channel 1 sample with the channel 2 sample. Assume the level of channel 1 (2,805-Hz
nsk) is higher than that of channel 2. Amplifier A4A272 will switch to a high state for the channel 1 sample, and to a low
state for channel 2. Thus, channel 2 threshold zener diode A4A2VR5 is biased into nonconduction and threshold zener
A4A2VR4 conducts. When A4A2VR4 conducts, channel 2 switch A4A2Q4 is biased in a low state. With A4A2Q4 in a
low state, weaker channel 2 is grounded, squelching the channel 2 output. If channel 1 were the weaker channel, the
switching process would have been reversed and channel 1 would have been grounded through A4A2Q3. Threshold
zener diodes A4A2VR4 and A4A2VR5 establish a 3-db differential. When tty channel 2 is grounded, a voltage sample
taken from limiter amplifier A4A2Q5 energizes relay A2A1K1 in the scope module, switching the scope input to channel 1
frequency discriminator A4A1T1, Q7, Q8.  In the deenergized position, the scope output of fsk/nsk frequency
discriminator A4A2T2, Q9, Q10 is sampled.
1-35. TTY SQUELCH (MARK HOLD) SWITCHING
(fig. FO-5)
The tty squelch switching circuitry consists of tty squelch adjust A4A2R1; tty channel 1 diversity control adjust A4A2R5;
tty channel 2 diversity control adjust A4A2R13; detectors A4A2CR1, CR2, A4A2CR3, CR4, and A4A2CR7, CR8;
auctioneer circuit A4A2CR5, CR6; tty squelch differentiating amplifier A4A2Z1; threshold zener A4A2VR1; tty squelch
switch A4A2Q1; AUTO MARK HOLD switch S6; and dc-coupled bistable multivibrator A4A1Q5, Q6. Unfiltered signal
plus noise Is applied to tty squelch adjust A4A2R1. Tty squelch detector A4A2CR1, CR2 rectifies the signal plus noise,
and applies a dc voltage to tty squelch differentiating amplifier A4A2Z1 which Is proportional to the signal-plus-noise
level. Meanwhile, at diversity squelching (para 1-34), a dc voltage that is proportional to signal levels of filtered tty tones
in tty channels 1 and 2 has been derived through their respective diversity control adjusts and detectors. In all modes but
85-Hz diversity, signal is present in only one tty channel. In 85-Hz diversity, signal is present in both channels. In either
case, auctioneer circuit A4A2CR5, CR6 selects the higher output of the diversity control detectors for tty channel 1 or 2
and applies it as the second input to tty squelch differentiating amplifier A4A2Z1. Amplifier A4A2Z1 compares the level
of the dc derived from tty channel 1 or 2 signal (with wideband noise filtered out) to the level of dc derived from unfiltered
signal plus wideband noise. When the signal-to-noise ratio of signals applied to the modem falls below a preset level (as
measured in A4A2Z1), threshold zener A4A2VR 1 conducts, biasing tty squelch switch A4A2Q1 in a low state. Tty
squelch switch A4A2Q1 is connected direct to input D of dc-coupled bistable multivibrator A4A1Q5, Q6; D is effectively
grounded. If input D is lower than input C, a normal mark (that is, a positive voltage) will appear at ac-coupled bistable
multivibrator A4A2Q7, Q8 output G (para 1-29). Tty squelch switch A4A2Q1 can be disabled (locked in a high state) by
setting AUTO MARK HOLD switch S6 at OFF. Thus, with AUTO MARK HOLD switch at ON, and when the tty signal-to-
noise ratio falls below a preset level, tty signals will be squelched, locking receive tty equipment in a mark condition.
This action prevents copying unintelligible information, or the excitation of tty equipment by random noise when
transmission from the sending station pauses momentarily.
1-22

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