TM 11-7440-232-12/NAVELEX 0967-LP-275-5010
CHAPTER 4
ORGANIZATIONAL MAINTENANCE
PRINCIPLES OF OPERATION
Section I. BLOCK DIAGRAM DESCRIPTION
4-1. GENERAL
The Digital Data Modem MD-701(*)/UY is a modulator-demodulator used for transmitting and receiving serialized
digital data at speeds of 600, 1200, and 2400 bps over a standard 3kHz voice channel. It has a transmit branch, which
consists of two modules and a RF oscillator, and a receive branch, which consists of five modules and the same RF
oscillator. A power supply is common to both branches. The MD-701A/UY and MD701B/UY have a data carrier alarm
module which provides a fail-safe alarm output if the modem malfunctions.
Transmitter
Accepts serial-by-bit binary data at specified bps rate.
Produces synchronous frequency modulated output.
Matching transformer couples transmitter output to vf path.
Supplies square wave output to transmitter clock at specified bps rate.
Provides test pattern generator for checking modem operation.
Transmitter Clock
Receives stable 4.8 MHz sine wave from RF oscillator.
Provides 2400 Hz square wave timing signal to transmitter.
Produces secondary timing source for synchronizing related data equipment.
Receiver Line Filter
Matching transformer couples input signal to low pass filter.
Low pass filter amplifies signal and applies it to band pass filter.
Band pass filter initially shapes duobinary signal.
Adjustable Equalizer
Equalizes any delay and amplitude distortion that occurs in receive signal path.
Limiter Discriminator
Receives binary or duobinary encoded frequency modulated signals.
Converts signals into analog form.
Controls a path between the output transmitter line filter and the limiter
discriminator when REC MODE switch is placed in BACK-TO-BACK mode for
checking modem operation.
Provides local testing of some receive circuits.
Decoder Error Detector
Regenerates signals from limiter discriminator to a serial-by-bit NRZ form.
Monitors the received data pattern and provides alarm signal when errors are
detected.
Monitors the received carrier signal level and provides alarm signal if level
decreases below a preset level.
Provides an error visual indication by means of an ERRORS indicator lamp.
4-1
