Spectrum Measurements
R&S
®
ESR
248
User Manual 1175.7068.02 ─ 12
The frequency span must at least cover the channels to be measured plus a mea-
surement margin of approx. 10 %.
If the frequency span is large in comparison to the channel bandwidth (or the adja-
cent-channel bandwidths) being examined, only a few points on the trace are avail-
able per channel. This reduces the accuracy of the waveform calculation for the
channel filter used, which has a negative effect on the measurement accuracy. It is
therefore strongly recommended that the formulas mentioned be taken into consid-
eration when selecting the frequency span.
For channel power measurements the
softkey sets the frequency
span as follows:
"(No. of transmission channels – 1) x transmission channel s 2 x transmis-
sion channel ban measurement margin"
For adjacent-channel power measurements, the
softkey sets the
frequency span as a function of the number of transmission channels, the trans-
mission channel spacing, the adjacent-channel spacing, and the bandwidth of one
of adjacent-channels ADJ, ALT1 or ALT2, whichever is furthest away from the
transmission channels:
"(No. of transmission channels – 1) x transmission channel s 2 x (adja-
cent-channel s adjacent-channel bandwidth) + measurement margin"
The measurement margin is approx. 10 % of the value obtained by adding the
channel spacing and the channel bandwidth.
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Resolution bandwidth (RBW)
To ensure both, acceptable measurement speed and required selection (to sup-
press 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.
With the exception of the IS95 CDMA standards, the
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.
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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:
"VBW ≥ 3 x RBW"
The
softkey sets the video bandwidth (VBW) as a function of the
channel bandwidth (see formula above) and the smallest possible VBW with regard
to the available step size will be selected.
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Detector
Measurements
