The Spectrum Application (RF Measurements)
R&S
®
FPL1000
465
User Manual 1178.3370.02 ─ 03
With logarithmic averaging, the dB values of the display voltage are averaged or sub-
tracted from each other with trace mathematical functions.
With linear averaging, the level values in dB are converted into linear voltages or pow-
ers before averaging. Voltage or power values are averaged or offset against each
other and reconverted into level values.
For stationary signals, the two methods yield the same result.
Logarithmic averaging is recommended if sinewave signals are to be clearly visible
against noise since with this type of averaging noise suppression is improved while the
sinewave signals remain unchanged.
For noise or pseudo-noise signals, the positive peak amplitudes are decreased in loga-
rithmic averaging due to the characteristic involved. The negative peak values are
increased relative to the average value. If the distorted amplitude distribution is aver-
aged, a value is obtained that is smaller than the actual average value. The difference
is -2.5
dB.
This low average value is usually corrected in noise power measurements by a 2.5
dB
factor. Therefore the R&S
FPL1000 offers the selection of linear averaging. The trace
data is linearized before averaging, then averaged and logarithmized again for display
on the screen. The average value is always displayed correctly irrespective of the sig-
nal characteristic.
Trace Smoothing
A Video Bandwidth Filter (VBW) is a hardware-based method of smoothing the trace
(see also
Chapter 7.6.1.2, "Smoothing the Trace Using the Video Bandwidth"
on page 391). However, other sweep and bandwidth settings may be coupled to the
VBW, and for some signals, a VBW may not be freely selectable to obtain the required
smoothing effect. Therefore, a software-based trace smoothing function is also availa-
ble.
(Software-based)
smoothing
is a way to visually remove anomalies in the trace that
may distort the results. The smoothing process is based on a moving average over the
complete measurement range. The number of samples included in the averaging proc-
ess (the
aperture
size) is variable and is a percentage of all samples that the trace con-
sists of.
Figure 7-50: Sample size included in trace smoothing
Effects of smoothing on post-processing functions
Note that in Spectrum mode, all functions performed after the sweep, such as limit
checks, markers, or channel power measurements, are based on the smoothed trace
data. Thus, the results differ from results based on the original trace.
Trace Configuration
