Measurements
R&S
®
FSW
136
User Manual 1173.9411.02 ─ 19
5.3.1 About Channel Power Measurements
Measuring channel power and adjacent channel power is one of the most important
tasks for a signal analyzer with the necessary test routines in the field of digital trans-
mission. While, theoretically, channel power could be measured at highest accuracy
with a power meter, its low selectivity means that it is not suitable for measuring adja-
cent channel power as an absolute value or relative to the transmit channel power. The
power in the adjacent channels can only be measured with a selective power meter.
A signal 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 sine wave signal irrespective of the selected detector. This calibration cannot be
applied for non-sinusoidal signals. Assuming that the digitally modulated signal has a
Gaussian amplitude distribution, the signal power within the selected resolution band-
width can be obtained using correction factors. These correction factors are normally
used by the signal analyzer's internal power measurement routines in order to deter-
mine the signal power from IF envelope measurements. These factors apply if and only
if the assumption of a Gaussian amplitude distribution is correct.
Apart from this common method, the R&S
FSW also has a true power detector, i.e. an
RMS detector. It displays the power of the test signal within the selected resolution
bandwidth correctly, irrespective of the amplitude distribution, without additional correc-
tion factors being required.
The R&S
FSW allows you to perform ACLR measurements on input containing multi-
ple signals for different communication standards. A measurement standard is provi-
ded that allows you to define multiple discontiguous transmit channels at specified fre-
quencies, independent from the selected center frequency. The ACLR measurement
determines the power levels of the individual transmit, adjacent, and CACLR channels,
as well as the total power for each sub block of transmit channels.
A detailed measurement example is provided in
chapter 5.3.7, "Measurement Exam-
5.3.2 Channel Power Results
For channel or adjacent-channel power measurements, the individual channels are
indicated by different colored bars in the diagram. The height of each bar corresponds
to the measured power of that channel. In addition, the name of the channel ("Adj",
"Alt1", "TX1", etc. or a user-defined name) is indicated above the bar (separated by a
line which has no further meaning). For "Fast ACLR" measurements, which are per-
formed in the time domain, the power versus time is shown for each channel.
Channel Power and Adjacent-Channel Power (ACLR) Measurement
