Transmit Branch cont'd

TM 11-7440-232-12/NAVELEX 0967-LP-275-5010

(4) The FSK modulator is controlled by the clock signal. Input signals to this circuit are supplied by the data

amplifier for low speed operation, or by the encoder for high speed operation. Whenever the modulator input is a high

level, it produces a 1200 Hz output. If the input is low, it allows the positive-going edge of the clock signal to change, this

results in a 2400 Hz output. In the duobinary mode, the encoder output for a "0" bit causes the modulator output to

alternate between a half cycle of 1200 Hz and a full cycle of 2400 Hz. This results in an average line frequency of 1800

Hz. A "1" bit in the duobinary mode is represented by a line frequency of either 1200 or 2400 Hz.

(5) The amplifier following the FSK modulator amplifies the modulated signal to a level suitable for application

to the transmit line and then extends this signal to the low pass filter. The filter passes signal products up to about 4 KHz

so that the output resembles a sine wave. The coupling transformer between the filter and the line provides impedance

matching from the unbalanced modem output to balanced lines of 300, 600 or 900 ohms. The output signal from the low

pass filter also extends to the REC MODE switch in the limiter discriminator.

(6) When the REC MODE switch is operated to the BACK-TO-BACK position the transmitter output is applied

to the receiver input. This loop back arrangement enables the receiver output to be compared with a known data pattern

at the transmitter input.

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