Instruments such as the Analog Arts ST985 (www.analogarts.com), based on the TDR and wave transmission concept, characterize the length, impedance, and other characteristics of a cable.
When a signal travels through a cable, it is undisturbed until it encounters an impedance change, and if it encounters an open circuit, all of the wave reflects back to the source.
The characteristic impedance of the cable and the load impedance, determine what happens to the signal at the load. A higher load impedance reflects a part of the signal back to the source. There is no reflection when the load impedance is smaller than the impedance of the cable. However, smaller loads cause signal loss. An equal load impedance prevents signal reflection while delivering maximum amount of power to the load.
An impedance mismatch can happen whenever the signal medium changes. For example, when the source is connected to the cable or at the load. These mismatches cause reflection and signal loss.
In general, the source output impedance, cable characteristic impedance, and load impedance must be made equal for optimal performance. Impedance matching maximizes the power transfer to the load and eliminates signal reflection. A perfect impedance matching transfers half of the source power to the load.
When the signal wavelength becomes a significant portion of the cable length, without a proper termination, reflection distorts the signal. The reflected wave changes the amplitude of the signal throughout the cable. At the source, it acts like a load, which if not properly addressed, can damage the signal generator. At certain frequencies, the reflected wave interferes constructively with the signal. The resultant signal is referred to as a standing wave.
A TDR consists of a pulse generator, a monitoring instrument device, and a DSP. Generally, in these instruments, the generator outputs a fast transition pulse. This pulse travels to the end of the cable and reflects back. But first it is divided by the voltage divider formed due to the source resistance and the cable impedance. The division factor is then calculated based on which the impedance of the cable is found.
The divided signal travels through the cable at a fraction of the speed of light in vacuum. This fraction is referred to as the velocity factor of the cable. For a coax it is about point 67. The time it takes for the signal to reach the end of the cable and reflect to the source, is used to calculate the length of the cable.
Impedance matching maximizes the power transfer to the load and prevents reflection.
Damaging a cable, by twisting, bending, or kinking, or placing improper signal connectors in the signal path cause an impedance mismatch resulting in reflection, and consequently less than ideal signal handling.