Measuring Channel Power And Adjacent Channel Power - Rohde & Schwarz R&S FSP Series Operating Manual

R&S FSP
2.5.3

Measuring Channel Power and Adjacent Channel Power

Measuring channel power and adjacent channel power is one of the most important
tasks in the field of digital transmission for a Spectrum Analyzer with the necessary
test routines. While, theoretically, channel power could be measured at highest
accuracy with a power meter, its low selectivity means that it is not suitable for mea-
suring adjacent channel power as an absolute value or relative to the transmit chan-
nel power. The power in the adjacent channels can only be measured with a
selective power meter.
A Spectrum Analyzer cannot be classified as a true power meter, because it displays
the IF envelope voltage. However, it is calibrated such as to correctly display the
power of a pure sinewave signal irrespective of the selected detector. This calibra-
tion is not valid for non-sinusoidal signals. Assuming that the digitally modulated sig-
nal has a Gaussian amplitude distribution, the signal power within the selected
resolution bandwidth can be obtained using correction factors. These correction fac-
tors are normally used by the Spectrum Analyzer's internal power measurement rou-
tines in order to determine the signal power from IF envelope measurements. These
factors are valid if and only if the assumption of a Gaussian amplitude distribution is
correct.
Apart from this common method, the R&S FSP also has a true power detector, i.e.
an RMS detector. It correctly displays the power of the test signal within the selected
resolution bandwidth irrespective of the amplitude distribution, without additional
correction factors being required. With an absolute measurement uncertainty of <
0.5 dB and a relative measurement uncertainty of < 0.2 dB (each with a confidence
level of 95%), the R&S FSP comes close to being a true power meter.
There are two possible methods for measuring channel and adjacent channel power
with a Spectrum Analyzer:
The IBW method (Integration Bandwidth Method) in which the R&S FSP measures
with a resolution bandwidth that is less than the channel bandwidth and integrates
the level values of the trace versus the channel bandwidth. This method is described
in the section on noise measurements.
Measurement using a channel filter.
In this case, the R&S FSP makes measurements in the time domain using an IF fil-
ter that corresponds to the channel bandwidth. The power is measured at the output
of the IF filter. Until now, this method has not been used for Spectrum Analyzers,
because channel filters were not available and the resolution bandwidths, optimized
for the sweep, did not have a sufficient selectivity. The method was reserved for spe-
cial receivers optimized for a particular transmission method.
The R&S FSP has test routines for simple channel and adjacent channel power
measurements. These routines give quick results without any complex or tedious
setting procedures.
Operating Manual 1164.4556.12 - 06
Getting Started
Measurements on Modulated Signals
2.29