Practical Considerations; Masking - Agilent Technologies N4413A User Manual

Performing Time Domain Measurements

Practical Considerations

Practical Considerations
In general, the more accurately the frequency domain data can be measured, the more
accurate the time domain data will be. Using the step mode rather than the sweep mode
provides additional frequency stability of the source that can greatly improve the time domain
data. The following are several other important considerations.

1. Masking

Masking occurs when the effect of one discontinuity affects the behavior of another
discontinuity located further from the source. For example, the step response of a perfectly
matched 25Ω lossless transmission line is shown below.
Notice that the response starts at a reflection of 0, steps down to −0.33 (corresponding to 25Ω),
and then steps back up to a value slightly less than 0. If the time domain response truly
showed impedance versus distance, the response should return to zero. It does not because
some of the energy is lost due to the reflections at the discontinuities.
A lossy transmission line will also affect the response. Consider the lossy 100Ω lossy
transmission line below. Notice that the region of the DUT has a slope in the step response,
and again the end-point is not at zero reflection.
Finally, consider the example of a short circuit at the end of an ideal transmission line. The
figure below shows the response as clearly having a reflection coefficient of −1 in the proper
location. If there is a matched attenuator in the line, the short shows up as having a smaller
reflection coefficient.
94 Chapter 4