Setting up a Phase Shift Overlay, or PSO, track circuit is more detailed than setting up a traditional amplitude-based AC track circuit because the system must establish and monitor the phase relationship of the signal.
The first step is defining the track block. PSO systems may not always require insulated joints for the PSO signal itself, but the electrical limits of the block must still be clearly established.
A specialized AC transmitter is installed at one end of the block. The transmitter sends a continuous AC signal at a specific frequency and phase through the rails.
A phase-sensitive receiver is installed at the opposite end of the block. The receiver compares the received signal against a reference signal and detects phase changes that indicate occupancy or a fault condition.
Tuned circuits or impedance matching units may be installed at the block boundaries. These devices help define the electrical length of the track circuit and reduce signal reflection or signal loss.
The system must be carefully calibrated. This includes setting transmitter output, adjusting tuning equipment, and setting the receiver phase detection threshold.
After setup, testing verifies that the receiver detects phase shifts caused by train shunts at different locations in the block. Broken rail detection should also be tested by confirming that major impedance changes produce the proper receiver response.