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Agilent
Performance Spectrum Analyzer Series
Swept and FFT Analysis

Application Note




Spectrum analysis measurements The Agilent Technologies perform- The PSA series cannot eliminate the
often involve trade-offs between ance spectrum analyzer (PSA) trade-offs described above, as some
accuracy, speed and dynamic range. series (model E4440A) provide of them are due to the inherent limi-
In most cases, emphasis on one of major improvements in each of tations of measurement physics.
these parameters adversely impacts these three measurement factors of However the innovations in digital
the other two. For example, the wide speed, dynamic range and accuracy. filtering (a digital implementation of
resolution bandwidths (RBW) used They also offer major increases a traditional swept IF section) and
to achieve fast measurement speeds in ease-of-use and measurement fast Fourier transform (FFT) analysis
result in higher noise levels and reliability. These improvements are of the PSA series provide the user
potentially reduce dynamic range. primarily the result of an innovative with a dramatic improvement in the
Increasing the sweep rate improves hardware and firmware architecture, trade-offs required.
measurement speeds for a given based on an analog foundation of
RBW but reduces accuracy due to exceptional performance. In addition, This product note will describe the
inadequate settling time for the the autocoupling implemented in the use of FFT and swept analysis in
intermediate frequency (IF) filters. PSA's firmware provides an easier the PSA series, along with the impli-
Alternatively, dynamic range may be and more reliable path to high cations for measurement speed,
emphasized at the expense of meas- quality measurements. accuracy, and dynamic range. It will
urement speed because of the need summarize the autocoupling func-
to use narrow (slow sweeping) RBW tions and demonstrate how to tune
filters and, in some cases, averaging. them to optimize one or more of the
major measurement parameters
described here.
Table of Contents Comparing the
Amplitude Accuracy of

Comparing the Amplitude Accuracy of Swept and FFT Techniques 2 The PSA series analyzers use both
swept and FFT analysis techniques
Comparing the Dynamic Range of Swept and FFT Techniques 3
for spectrum measurements.
Amplitude flatness errors are
ADC Bandwidth in Swept and FFT Techniques 3
present in both techniques, although
Effect of Autoranging on Swept and FFT Techniques 4 the eventual (after correction) accu-
racy of the swept technique is higher.
Effect of Signal Statistics on FFT and Swept Measurements 4 Figure 1 illustrates this point.

Speed of Swept Techniques Using a Digital Filter 5 In a swept spectrum analyzer the
amplitude accuracy of the RBW fil-
Faster Sweep Rates and Compensation of Sweep Rate Effects 6 ters (termed "RBW switching uncer-
tainty") can be quite high because
they are all centered at the same
Speed of FFT Techniques versus Swept 7
frequency. In the case of FFTs, how-
ever, the flatness of the IF preceding
Continuum of Performance Between FFT and Swept Analysis 8 the analog to digital (A/D) process is
Dynamic Range of the Multiple-FFT Measurement Type 8 a factor in determining accuracy. A
Measurement Speed of the Multiple-FFT Measurement Type 8 broad flat response is needed or else
errors result due to the non-flatness
Automatic Selection of Sweep Type 9 of the IF. This non-flatness is meas-
ured and results are compensated
accordingly, but the process is
Manual Selection of Sweep Type 9 imperfect and flatness errors as
large as