R&S ESU Series Operating Manual page 214

R&S ESU
Resolution bandwidth (RBW)
To ensure both an acceptable measurement speed and the required selection (to
suppress spectral components outside the channel to be measured, especially of
the adjacent channels), the resolution bandwidth must not be selected too small or
too large. As a general approach, the resolution bandwidth is to be set to values
between 1% and 4% of the channel bandwidth.
A larger resolution bandwidth can be selected if the spectrum within the channel to
be measured and around it has a flat characteristic. In the standard setting, e.g. for
standard IS95A REV at an adjacent channel bandwidth of 30 kHz, a resolution
bandwidth of 30 kHz is used. This yields correct results since the spectrum in the
neighborhood of the adjacent channels normally has a constant level. For standard
NADC/IS136 this is not possible for example, since the spectrum of the transmit sig-
nal penetrates into the adjacent channels and a too large resolution bandwidth
causes a too low selection of the channel filter. The adjacent-channel power would
thus be measured too high.
With the exception of the IS95 CDMA standards, the ADJUST SETTINGS softkey
sets the resolution bandwidth (RBW) as a function of the channel bandwidth:
RBW ≤ 1/40 of channel bandwidth.
The maximum possible resolution bandwidth (with respect to the requirement RBW
≤ 1/40) resulting from the available RBW steps (1, 3) is selected.
Video bandwidth (VBW)
For a correct power measurement, the video signal must not be limited in band-
width. A restricted bandwidth of the logarithmic video signal would cause signal
averaging and thus result in a too low indication of the power (-2.51 dB at very low
video bandwidths). The video bandwidth should therefore be selected at least three
times the resolution bandwidth.
The ADJUST SETTINGS softkey sets the video bandwidth (VBW) as a function of
the channel bandwidth as follows:
VBW ≥ 3 × RBW.
The smallest possible VBW with regard to the available step size will be selected.
Detector
The ADJUST SETTINGS softkey selects the RMS detector.
The RMS detector is selected since it correctly indicates the power irrespective of
the characteristics of the signal to be measured. In principle, the sample detector
would be possible as well. Due to the limited number of trace pixels used to calcu-
late the power in the channel, the sample detector would yield less stable results.
Averaging, which is often performed to stabilize the measurement results, leads to a
too low level indication and should therefore be avoided. The reduction in the dis-
played power depends on the number of averages and the signal characteristics in
the channel to be measured.
Operating Manual 1302.6163.12 - 03
Instrument Functions
Analyzer Mode
4.183