RECEIVE SIGNAL PATH SIMPLIFIED BLOCK DIAGRAM

TM 11-5805-387-34-2

Receive Tty Channel. The receive tty channel consists of an isolation amplifier, an input filter, a mixer, diversity

switching circuitry, two frequency discriminators, mark-space pulse generation circuitry, a receive sensing circuit, a loop

current switch dc supply. Tty tones from the associated radio receiver or landline are applied to the tty channel isolation

amplifier, which prevents interaction between tty and voice channels and provides a constant input impedance. At the

output of the input filter, the receive tty channel is split into two channels, designated tty channel and tty channel 2. (In

the 850-Hz mode, tty channel 1 begins before the input filter; wideband noise and mixing harmonics are filtered after the

mixer.)

Tty chann el 1. Tty channel 1 functions in 850-Hz and 85-Hz diversity modes to process 2,000Hz fsk and 425-

Hz nsk tty tones, respectively. The 2,000-Hz fsk or 425-Hz nsk tones are fed to a mixer, which recenters them

to 2,805-Hz nsk. The centered tty tones are fed to the nsk fsk frequency discriminator, which puts out a dc

voltage of one polarity for a mark tone input, and a dc voltage of the opposite polarity for a space tone input.

Investigation of tty channel 2 will be made before continuing through the tty receiver path.

Tty channel 2. Tty channel 2 functions in all 85-Hz modes (including diversity) to process received nsk tty

tones centered about 2,805 Hz. The 2,805-Hz nsk is fed directly to the nsk only frequency discriminator, which

puts out a dc voltage of one polarity for a mark tone input, and the opposite polarity for a space tone input.

Tty receive path, reunited. The outputs of tty channels 1 and 2 are joined at the input of the mark-space

pulse generation circuitry. Combination of the output voltages of tty channels 1 and 2 does not occur. In all

modes but 85-Hz diversity, only one tty channel is operative; in 85-Hz diversity, diversity switching grounds the

weaker channel. The mark-space pulse generation circuitry shapes the output of the frequency discriminator

into a neat square wave for driving the receive sensing circuitry. The receive sensing circuitry isolates the

receive dc loop from the rest of the modem and produces a pulse of proper shape and magnitude to operate

the loop current switch. The loop current switch controls current supplied by the receive dc supply. The loop

current switch is biased on for a mark tone input to the modem, and off for a space tone input. Thus, receive

dc loop current flows for a mark tone input, and does not flow for a space tone input, keying the receive tty

equipment that is connected in series with the rest of the receive dc loop.

Receive Voice Channel. The receive voice channel consists of an isolation amplifier, receive voice filter FL5, and a

receive audio amplifier. Voice, or 2,805-Hz nsk combined with voice is applied to the modem voice input isolation

amplifier from the associated radio receiver or landlines. The voice input isolation amplifier, in conjunction with the tty

channel isolation amplifier, prevents interaction between tty and voice channels. In 85-Hz plus voice mode, the output of

the voice channel isolation amplifier is applied to receive voice filter FL5, which filters out 2,805-Hz nsk tty tones. The

filtered signal, now consisting of voice only, is amplified by the receive audio amplifier and applied to a monitor speaker

or headset. In all other modes, the unfiltered output of the voice channel isolation amplifier supplied directly to the

receive audio amplifier permits monitoring of voice circuits, or tty tones.

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