3 Spectrum Analyzer Mode
3.2 Swept SA Measurement
Details of each averaging type appear below:
Auto
When Auto is selected, the instrument chooses the optimum type of averaging for
the current instrument measurement settings. When one of the average types is
selected manually, the instrument uses that type regardless of other instrument
settings, and shows Man on the Average Type toggle.
Here are the auto-select rules for Average Type:
Auto selects VoltageAveraging if the Detector for any active trace is EMI Average or
QPD or RMS Average; otherwise, it selects Power (RMS) Averaging if a Marker
Function (Marker Noise, Band/Intvl Power) is on, or Detector is set to Man and
Average; otherwise, if Amplitude, Scale Type is set to Lin it selects Voltage
Averaging; otherwise, if the EMC Standard is set to CISPR, it selects Voltage;
otherwise Auto selects Log-Power Average.
Note that these rules are only applied to active traces. Traces which are not
updating do not impact the auto-selection of Average Type. When you select log-
power averaging, the measurement results are the average of the signal level in
logarithmic units (decibels). When you select power average (RMS), all measured
results are converted into power units before averaging and filtering operations, and
converted back to decibels for displaying. Be aware that there can be significant
differences between the average of the log of power and the log of the average
power.
Log-Pwr Avg (Video)
Selects the logarithmic (decibel) scale for all filtering and averaging processes. This
scale is sometimes called “Video” because it is the most common display and
analysis scale for the video signal within a spectrum instrument. This scale is
excellent for finding CW signals near noise, but its response to noise-like signals is
2.506 dB lower than the average power of those noise signals. This is compensated
for in the Marker Noise function.
The equation for trace averaging on the log-pwr scale is shown below, where K is
the number of averages accumulated. (In continuous sweep mode, once K has
reached the Average/Hold Number, K stays at that value, providing a continuous
running average.)
New avg = ((K–1)Old avg + New data)/K
This equation assumes all values in decibel scale.
384 Spectrum Analyzer Mode User's &Programmer's Reference
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