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Agilent 8560E/8590E Spectrum
Comparing Power Measurements
on Digitally Modulated Signals
Product Note

Measurement Software Personalities The instrument first calculates the instantaneous
Measuring the power of digital CDMA signals is not power values of the trace elements by antilogging
as straightforward as the procedure employed when the dBm values. These values are summed across
measuring their analog counterparts. While there the channel bandwidth and the average over the
are several ways to perform these in-channel and number of trace elements is calculated. The instan-
out-of-channel measurements, a spectrum analyzer taneous power at every trace element is measured
and software-implemented power detection makes using a selected filter that has a resolution band-
the measurement correctly, accurately, and quickly. width from 0.5 to 3.0% of the channel bandwidth.
The Agilent Technologies 8590 E-series and 8560
E-series spectrum analyzers can perform these meas- In order to obtain the true spectral density (mean
urements alone, but when the 8590 E-series is com- power normalized at 1 Hz), the power average is
plemented by measurement software personalities divided by the equivalent noise bandwidth. The
such as the Agilent 85725C for CDMA, the process total power in the bandwidth is then obtained by
is easier and more reliable results are obtained. multiplying the result by the channel bandwidth
However, there are differences between measure- and then converting it back to dBm for display.
ments made by the instruments alone and with the The instrument also displays the power spectral
personalities that are important to understand. density (dBm/Hz).

In particular, the measurement personalities allow It is important to remember that video averaging
total linear power to be averaged over multiple is not the same as averaging several measurements.
sweeps in order to achieve more repeatable results, In fact, video averaging should not be used when
while the spectrum analyzers alone require the measuring noise-like signals, such as digitally
average to be calculated manually. modulated signals, because it will introduce errors
into the measurement. This is because video aver-
The Measurement aging (or video filtering) averages the logarithmic
Whether adjacent-channel power is being meas- values of the trace elements. For noise this error
ured by the spectrum analyzer alone or with the is