AVERAGING Menu—Both menus show that averaging has been toggled on.
1. IFBW is set to 1 kHz and Trace Smoothing set to OFF (at left).
2. IFBW is set to 30 kHz, Trace Smoothing set to ON with Smoothing applied to 20% of the sweep (at right).
Averaging
The Averaging button toggles the function OFF and ON.
Avg. Factor
The Avg. Factor (Averaging Factor) represents the number of measurements performed at each frequency point in the case of per-point averaging, and represents the number of sweeps averaged (in a running average sense) for per-sweep averaging.
Averaging Type
The Averaging Type button toggles between per-point and per-sweep averaging.
• Per-Point Averaging
Per-point averaging acquires additional samples at each frequency (or power) point and performs the averaging process at that time. In this sense, it is quite similar to an IFBW reduction (adding 10 per point averages is equivalent to a 10x reduction in IFBW). Since the time between sample acquisitions is small in this case, per point averaging works best at removing high-rate noise.
• Per-Sweep Averaging
Per-sweep averaging averages a given frequency (or power) point’s behavior on subsequent sweeps. This can be a very long time constant between samples (depending on sweep speed) so this type of averaging does best with low-rate noise.
Per-sweep averaging is performed on a rolling basis. That is, if 10 per sweep averages are selected, the most recent sweeps are used to compute the result.
Since per-sweep averaging has a long time constant, setup changes or DUT changes can appear to have an odd effect. Powering down an active DUT, for example, may lead to an S21 display to slowly drift away since it takes some time for the gain change to work its way through the sweep count. If a setup or DUT change is made, it may be desirable to reset the averaging count.
IFBW
The Intermediate Frequency Bandwidth (IFBW) is allowed in the range of 1 Hz to 1 MHz. At lower IFBWs, additional per point averaging will have little effect.
At very low frequencies, where the IFBW may be on the order of the system frequency, there could be measurement issues. By default, the IFBW will be limited at system frequencies below 3 MHz although this can be overridden under the System menu.
Trace Smoothing
Trace smoothing is toggled OFF and ON by this button. Trace smoothing performs a weighted averaging around each frequency point using a window size set by the percentage of smoothing. Since this process combines data at different frequency points, it should be used with care since it can remove valid frequency response information.
Averaging and Smoothing Conclusions
In time domain, averaging and IFBW apply to the basic frequency domain data. Smoothing applies to the time domain data. Everything on this menu is channel-based.
Strictly speaking, the above averaging comments apply to linear/log/CW frequency sweeps and power sweeps. When in segmented sweep mode, the behavior is slightly different. One can assign IFBW and per-point averaging on a segment-by-segment basis (so that, for example, one could use a wide IFBW and little averaging in a low insertion loss part of the DUT’s response and a narrow IFBW and more averaging in a very high insertion loss part of the response where one wanted to maximize dynamic range). Sweep-by-sweep averaging, by definition, applies to the sweep rather than a segment so it can be applied only to the aggregate. Unlike in linear frequency sweep, one can then simultaneously apply both point-point averaging (from the segmented sweep table now) and sweep-by-sweep averaging (still from the averaging menu). If sweep-by-sweep averaging is applied in segmented sweep and one goes back to linear sweep, the averaging setting will carry but any use of point-by-point averaging in segmented sweep will not carry back to linear sweep.
Smoothing is also applied on a sweep basis and cannot be dictated by segment when in segmented sweep.
