Propagation Rate
Speed value indicates how fast an electrical pulse travels through a conductor within a specific dielectric environment. Technicians use tdr phase velocity to calibrate time domain reflectometry equipment so that time measurements can be converted into physical distances. The velocity is always lower than the speed of light in a vacuum because the surrounding material slows down the electromagnetic wave.
Accurate settings are required to locate the exact position of an impedance mismatch or a break in the line.
Material Calibration
Dielectric constant of the substrate determines the tdr phase velocity for any given transmission line structure. A typical FR-4 material might allow a signal to travel at roughly half the speed of light. Before testing a batch of boards, the operator must enter the correct velocity factor into the instrument.
This factor is expressed as a fraction or a percentage of light speed. If the value is incorrect, a defect that appears to be at the five inch mark might actually be four inches away, leading to failed repairs.
Trace Measurement
Modern high speed designs rely on matched trace lengths to manage signal timing. By measuring the flight time of a pulse and applying the known tdr phase velocity, the instrument calculates the electrical length of the net. This non destructive test verifies that the fabrication process met the design requirements.
