Understanding How Resolution and Aliasing Impact Time-Domain Measurements

Using time-domain analysis, you can determine the location of discontinuities or impedance mismatches. How closely you can pinpoint the location of the discontinuities is limited by the following main factors:

• Resolution—Resolution is controlled by the span of your measurement. The broader the frequency span of your measurement, or bandwidth, the more information presented during the time-domain analysis. This additional information increases the precision with which you can locate points.
  
  

  Note  If you have a DUT with limited bandwidth and the device passes the DC component, you will see a gradual roll-off of DUT performance at higher frequencies. If the limited-bandwidth DUT does not pass the DC component, you see the resolution degrade.

  Resolution is also influenced by the processing method, window selection options, and the relative amplitude of the processed signals.
• Aliasing—In the frequency domain, measurements are made at a finite number of points, not continuously. The transformation of this data then causes aliasing in the time domain because the transform is a circular function that repeats itself outside of its inherent range of 1 divided by the step size. The more data points available, the smaller the step size and the longer the time frame before the data repeats. For example, with a 20 GHz frequency sweep and 401 data points, the alias-free range is 10 ns.