Common Analysis and Display Functions
R&S
®
FSW
457
User Manual 1173.9411.02 ─ 19
After the selected number of sweeps the average trace is saved in the trace mem-
ory. Until this number of sweeps is reached, a preliminary average is displayed.
When the averaging length defined by the "Sweep Count" is attained, averaging is
continued in continuous sweep mode or for "Continue Single Sweep" according to
the following formula:
N
MeasValue
Trace
N
Trace
old
*
)
1
(
where N is the sweep count
7.3.1.4
How Trace Data is Averaged - the Averaging Mode
When the trace is averaged over several sweeps (Trace mode: "Average"), different
methods are available to determine the trace average.
With logarithmic averaging, the dB values of the display voltage are averaged or sub-
stracted from each other with trace mathematical functions.
With linear averaging, the level values in dB are converted into linear voltages or pow-
ers prior to 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 and 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
FSW offers the selection of linear averaging. The trace data
is linearized prior to averaging, then averaged and logarithmized again for display on
the screen. The average value is always displayed correctly irrespective of the signal
characteristic.
7.3.1.5
Combining Several Trace Results - Trace Math Evaluation
If you have several traces with different modes, for example an average trace and a
maximum trace, it may be of interest to compare the results of both traces. In this
example, you could analyze the maximum difference between the average and maxi-
mum values. To analyze the span of result values, you could subtract the minimum
trace from the maximum trace. For such tasks, mathematical functions on trace results
are provided.
Trace Configuration
