Time domain reflectometer for measuring impedance discontinuities on a powered transmission line

A time domain reflectometer for measuring impedance discontinuities on a powered transmission line system, the system including a conductor to which at least one transceiver is attached, the transceiver having safe operating limits defined by an upper voltage limit and a lower voltage limit and the transceiver being disabled when the average voltage level on the conductor falls below a collision detection threshold which lies between the upper and lower voltage limits, and wherein the tap impedance of the transceiver on the conductor is distorted if the upper voltage limit is exceeded, said reflectometer comprising:

• source means for applying a negative voltage test waveform to the conductor which test waveform generates a return reflected waveform when an impedance discontinuity is encountered along the conductor;
• bias means for selectively applying a DC bias voltage to the conductor of a value more negative than the collision detection threshold in order to disable the transceiver and thus prevent transmissions from the transceiver from interfering with the reflected waveform and to provide additional headroom for a positive relfected waveform without exceeding the upper voltage limit;
• and
• means for displaying the reflected waveform to permit measurement of the impedance discontinuity along the conductor.