R&S FSL Measurements on Modulated Signals
1300.2519.12 2.25 E-11
Measurements on Modulated Signals
For measurements on AM and FM signals refer to the Quick Start Guide, chapter 5, "Basic
Measurements Examples".
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 measuring adjacent 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 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 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 bandwidth can be obtained using correction factors. These correction
factors are normally used by the spectrum analyzer's internal power measurement routines in order to
determine 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 FSL 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. The absolute
measurement uncertainty of the R&S FSL is < 1.5 dB and a relative measurement uncertainty of < 0.5
dB (each with a confidence level of 95%).
There are two possible methods for measuring channel and adjacent channel power with a spectrum
analyzer:
• IBW method (I
ntegration Bandwidth Method)
The spectrum analyzer 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 section "Method of calculating the channel power".
• Using a channel filter
For a detailed description, refer to the following section.
Measurement using a channel filter
In this case, the spectrum analyzer makes zero span measurements using an IF filter 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 special receivers optimized for a particular transmission method. It is available in R&S
FSQ, FSU, FSP, FSL and ESL series.
The R&S FSL has test routines for simple channel and adjacent channel power measurements. These
routines give quick results without any complex or tedious setting procedures.
To Next Page
Loading...
