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ATT-20 Audio Track Transceiver
3.
FUNCTIONAL DESCRIPTION
3.1. General
The transmitter produces an output signal in the high audio frequency range and is sine
wave modulated to provide noise immunity. First, the receiver detects the modulation
information signal. It then amplifies and rectifies this signal to provide a DC output which
drives the relay coil. The functional description of the ATT-20 applied to the shunt track
circuit is covered in Section 3.2. Section 3.3 covers the series mode track circuit.
3.2. Shunt Mode Track Circuit
As illustrated in Figure 2-1, the transmitter output and receiver input are cabled directly
to the rails. The point at which the transmitter wires are connected defines one end of
the track circuit. The other end of the track circuit is defined by the receiver connections
to the track. The receiver detects the presence of a train by the loss of the audio
frequency signal which is shunted away by the train axle.
The shunt mode of operation is defined as a vital application.
3.3. Series Mode Track Circuit
As illustrated in Figure 2-2, the transmitter and receiver are wired in series via a jumper
located on the ATT-20 unit. This jumper connects the transmitter output terminal to the
receiver input terminal. The remaining transmitter output lead and the remaining
receiver lead are twisted as a pair with each is connected to its respective rail. The
presence of a train axle across the rails completes the series signal path, thus detecting
the presence of a train
The series mode of operation is a non-vital application.
3.4. Detailed Circuit Description
3.4.1. Transmitter
The transmitter circuit diagram is shown in Figure 3-1. Transistor Q1 and its associated
circuitry comprise the modulation oscillator which produces 390 Hz. A constant signal
level is maintained by Zener diode D1. Transistor Q2 buffers the modulation oscillator.
Transistor Q7 and its associated circuitry comprise the carrier frequency oscillator.
Selection of capacitor C3 determines the carrier frequency.
Transistors Q3, R8 and R13 mix the carrier and modulation signals. Resistor R14
provides gain control. Inductor L1, capacitor C5, and resistor R17 form a bandpass filter
which allows the modulated carrier to pass unaltered by low frequency or higher order
harmonic disturbances.
Copyright 2019
SM 6299 Rev. 3, March 2019
3-1
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