When high-frequency signals are carried on transmission lines of any significant length, care must be taken that the transmission medium is matched to its terminations. The source and load impedances should equal the characteristic impedance of the transmission line, as this minimizes signal reflections. The presence of impedance discontinuities or mismatches will degrade the amplitude and phase accuracy, as well as the temporal fidelity, of measurements made with a digitizer. The example below shows one of the most common mismatch errors encountered in such measurements.
Selectable termination impedances are provided at the digitizer inputs to accommodate the most popular coaxial cable characteristic impedances: 50 Ω and 75 Ω. The diagram below illustrates what happens when a coaxial cable of the wrong characteristic impedance (75 Ω) is used with 50 Ω source and load impedances.
Figure 2. Cable with characteristic impedance of 75 Ω connected to load and source impedances of 50 Ω
Figure 3. Distortion resulting from a cable's impedance mismatched with the impedance of a source and load
The pulse encounters impedance mismatches at each end of the cable, whereupon it is partially reflected. The reflected pulse traverses the cable back and forth numerous times, diminishing at each end by the reflection coefficient, .
vr = reflected voltage
vi = incident voltage
zt = termination impedance
z0 = characteristic impedance
The resulting voltage waveform recorded by the digitizer is distorted by the asymptotic decay of the reflected pulse, as shown above in Figure 3, exaggerated for visual effect. Impedance discontinuities of smaller magnitude and/or duration have correspondingly smaller effects. Also displayed is the waveform that results when a cable of matched impedance (50 Ω) is used.