TRANSMIT SIGNAL PATH SIMPLIFIED BLOCK DIAGRAM

TM 11-5805-387-34-2

1-15. TRANSMIT SIGNAL PATH SIMPLIFIED BLOCK DIAGRAM

The transmit signal path consists of two channels; a tty channel, which converts mark and space dc pulses into mark-

space (tty) tones, and a voice channel, which processes voice signals for transmission alone or in combination with tty

tones.

Transmit Tty Channel. The transmit tty channel consists of a send dc loop, a send sensing circuit, mark-space tone

generation circuitry, a mixer circuit, mixer tty filter, and transmit output circuitry. The send dc loop provides a current

supply, or routes an externally supplied current to send tty equipment that is connected in series with the send dc loop.

When the send tty equipment is keyed, loop current flows for a mark pulse and does not flow for a space pulse, resulting

in a time synchronized dc pulse input to the send sensing circuit. The send sensing circuit isolates the send dc loop from

the rest of the modem and produces a pulse of the proper shape and magnitude to trigger the mark-space tone

generation circuitry. When the send sensing circuit applies a positive pulse to the mark-space tone generation circuitry, a

mark tone is produced; when no pulse is applied to the tone generation circuitry, a space tone is produced. The tones

produced by the mark-space tone generation circuitry are not of the frequency at which they will be transmitted; the tones

are frequency divided, and in 850 Hz, and 85-Hz diversity are subtractively mixed in a mixer circuit to establish the

center frequency. After leaving the mixer circuit, the tty mark and space tones are at the proper frequency for

transmission. The tones are filtered to remove mixing harmonics and are applied to the transmit output circuitry where

they are amplified, and applied through the modem RCVR TRANS AUDIO jack to the associated radio transmitter or

landline.

Transmit Voice Channel. The transmit voice channel has a 50-ohm microphone (mike) input, and a 600-ohm input

for mike or line. The voice channel is operative only in the 85 Hz + voice mode and the voice mode.

In the 85 Hz + voice mode, the 600-ohm voice input is amplified by the 600-ohm voice amplifier and applied to

a transmit voice filter. The 50-ohm mike input is applied direct to the transmit voice filter. The transmit voice

filter limits the bandwidth of the voice signal to enable the upper portion of the radio transmitter sidebands to

be used for tty tones. The output of the transmit voice filter is applied to the transmit output circuitry, where it

is combined with tty tones generated in the transmit tty channel to produce the composite tty and voice

transmission.

In the voice mode, no tty information is transmitted, so the transmit voice filter is bypassed. The 600-ohm

voice input is amplified by the 600-ohm voice amplifier and is applied direct to the transmit output circuitry.

The transmit output circuitry further amplifies the 600-ohm voice signal, which is then applied through the

modem RCVR TRANS AUDIO jack to the associated radio transmitter or landline. In the voice mode, the 50-

ohm mike input is routed through the modem for switching purposes. The switched 50-ohm mike input is

applied through the modem RCVR TRANS AUDIO jack to the associated radio transmitter or landline.

1-16. RECEIVE SIGNAL PATH SIMPLIFIED BLOCK DIAGRAM

(fig. FO-3)

The receive signal path consists of two main channels: a tty channel which converts mark and space tones into mark

and space pulses for operation of tty machines, and a voice channel which separates voice signals from tty tones and

amplifies the voice for loudspeaker or headset.

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