In a Phase Shift Overlay track circuit, the transmitter and receiver work together to provide reliable train detection. Each device has a specific role in sending, receiving, and interpreting the track circuit signal.
The transmitter generates a continuous low-voltage AC signal at a designated frequency. This signal is fed into the track at one end of the block and travels through the rails toward the receiver at the opposite end.
The receiver is tuned to detect the proper frequency and the phase angle of the received signal. It compares the received signal against a reference signal to determine whether the phase has changed.
When a train enters the block, the wheels and axles shunt the rails. This changes the impedance of the track circuit and causes a phase shift in the signal.
If the phase shift reaches the designed threshold, the receiver indicates that the track is occupied. This allows the PSO system to detect train presence using changes in phase rather than relying only on signal strength.
The transmitter must provide a stable frequency, and the receiver must be properly calibrated. If either device is unstable, poorly adjusted, or not maintained, false indications or unreliable operation may occur.