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Operating Concepts
Noise Reduction Techniques
Smoothing Smoothing (similar to video filtering) averages the formatted active channel data over a
portion of the displayed trace. Smoothing computes each displayed data point based on one
sweep only, using a moving average of several adjacent data points for the current sweep.
The smoothing aperture is a percent of the swept stimulus span, up to a maximum of 20%.
Rather than lowering the noise floor, smoothing finds the mid-value of the data. Use it to
reduce relatively small peak-to-peak noise values on broadband measured data. Use a
sufficiently high number of display points to avoid misleading results. Do not use
smoothing for measurements of high resonance devices or other devices with wide trace
variations, as it will introduce errors into the measurement.
Smoothing is used with Cartesian and polar display formats. It is also the primary way to
control the group delay aperture, given a fixed frequency span. Refer to “Group Delay
Principles” on page7 -29. In polar display format, large phase shifts over the smoothing
aperture will cause shifts in amplitude, since a vector average is being computed. The
effect of smoothing on a log magnitude format trace is illustrated in Fi gure 7-19.
Figure 7-19 Effect of Smoothing on a Trace
IF Bandwidth Reduction IF bandwidth reduction lowers the noise floor by digitally reducing the receiver input
bandwidth. It works in all ratio and non-ratio modes. It has an advantage over averaging
as it reliably filters out unwanted responses such as spurs, odd harmonics, higher
frequency spectral noise, and line-related noise. Sweep-to-sweep averaging, however, is
better at filtering out very low frequency noise. A tenfold reduction in IF bandwidth lowers
the measurement noise floor by about 10 dB. Bandwidths less than 300 Hz provide better
harmonic rejection than higher bandwidths.