How is Trace Smoothing implemented within the Keysight 8753 and 8720 Series Network Analyzers?
Frequently Asked Questions (FAQs)
Summary
Trace smoothing processes the analyzers formatted data so that each displayed point on a trace represents the moving average of a number of trace points around the displayed point. The number of points included in the average is set by the smoothing aperture. Smoothing is performed using formatted data, that is, the data has been converted to the current display format (Log Mag, Phase, etc). Phase data is unwrapped before averaging such that the ±180° transitions are not affected. Polar or Smith Chart formats use a vector average.Question
How is Trace Smoothing implemented within the Keysight 8753 and 8720 Series Network Analyzers?
Answer
Trace smoothing processes the analyzers formatted data so that each displayed point on a trace represents the moving average of a number of trace points around the displayed point. The number of points included in the average is set by the smoothing aperture. Smoothing is performed using formatted data, that is, the data has been converted to the current display format (Log Mag, Phase, etc). Phase data is unwrapped before averaging such that the ±180° transitions are not affected. Polar or Smith Chart formats use a vector average.
The analyzer allows the user to directly set the smoothing aperture as a percentage of a full sweep. After entering the aperture percentage the display indicates the aperture width in frequency. The frequency aperture is always an even numbered multiple of the analyzers current frequency step size. The analyzers frequency step size can be calculated by:
(Stop Freq – Start Freq) / (Trace Points - 1)
The number of points included in the smoothing calculation is:
(Freq Aperture / Step Size) + 1
For example:
Assume the analyzer stimulus is configured with a start frequency of 400 MHz, a stop frequency of 700 MHz, and 101 points per sweep. The frequency step size is (700 MHz - 400 MHz) / (101 - 1) = 3 MHz.
If the smoothing aperture is set to 10% then the equivalent frequency aperture is 30 MHz. The 30 MHz frequency aperture defines the difference in frequency between the highest frequency point and the lowest frequency point used in the smoothing calculation as 30 MHz.
The number of points used in the averaging calculation is:
(30 MHz / 3 MHz) + 1 = 11. Therefore eleven points are used to calculate each final, smoothed data point. The 11 points consist of 5 points before the current point, the current point itself, and 5 points after the current point.
The trace endpoints are handled by extrapolation. As the smoothing aperture moves beyond the end of the trace the data from the last point is duplicated to fill up the required number of data points for averaging