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8 Hints for
Better Spectrum Analysis
Application Note 1286-1
The Spectrum Analyzer
The spectrum analyzer, like an When digital technology first became To get better spectrum analyzer
oscilloscope, is a basic tool used for viable, it was used to digitize the measurements the input signal must
observing signals. Where the oscil- video signal, as shown in Figure 2. be undistorted, the spectrum ana-
loscope provides a window into the As digital technology has advanced lyzer settings must be wisely set for
time domain, the spectrum analyzer over the years, the spectrum analyzer application-specific measurements,
provides a window into the frequency has evolved to incorporate digital and the measurement procedure
domain, as depicted in Figure 1. signal processing (DSP), after the final optimized to take best advantage of
IF filter as shown by the dotted box, the specifications. More details on
Figure 2 depicts a simplified block to be able to measure signal formats these steps will be addressed in the
diagram of a swept-tuned super- that are becoming increasingly hints.
heterodyne spectrum analyzer. complex. DSP is performed to pro-
Superheterodyne means to mix or vide improved dynamic range, faster
to translate in a frequency above sweep speed and better accuracy.
audio frequencies. In the analyzer,
a signal at the input travels through
an attenuator to limit the amplitude
of the signal at the mixer, and then
through a low-pass input filter to
eliminate undesirable frequencies.
Past the input filter, the signal gets
mixed with a signal generated by the
local oscillator (LO) whose frequency Time domain Frequency domain
is controlled by a sweep generator. measurements measurements
As the frequency of the LO changes,
Figure 1. Measurement domain
the signals at the output of the mixer,
(which include the two original
signals, their sums and differences
and their harmonics), get filtered by Digital signal processor
the resolution bandwidth filter (IF Preselector Resolution
Input or input bandwidth Log Envelope
filter), and amplified or compressed attenuator filter Mixer filter IF gain amp detector
in the logarithmic scale. A detec-
tor then rectifies the signal passing
through the IF filter, producing a
DC voltage that drives the vertical Video
filter
portion of the display. As the sweep
generator sweeps through its Local
oscillator
frequency range, a trace is drawn
A/D
across the screen. This trace shows
Sweep
the spectral content of the input generator
signal within the selected frequency Display
range.
Figure 2. Block diagram of a superheterodyne spectrum analyzer
2
Hint 1. Selecting the Best Resolution Bandwidth (RBW)
The resolution bandwidth (RBW) However, the narrowest RBW setting
setting must be considered when is not always ideal. For modulated
concerned with separating spectral signals, it is important to set the RBW
components, setting an appropriate wide enough to include the sidebands
noise floor and demodulating a of the signal. Neglecting to do so
signal. will make the measurement very
inaccurate.
When making demanding spectrum
measurements, spectrum analyzers Also, a serious drawback of narrow
must be accurate, fast and have RBW settings is in sweep speed. A
high dynamic range. In most cases, wider RBW setting allows a faster
emphasis on one of these parameters sweep across a given span compared
adversely impacts the others. Often- to a narrower RBW setting. Figures
times, these tradeoffs involve the 4 and 5 compare the sweep times Figure 5. Sweep time of 84.73 s for 3 kHz RBW
RBW setting. between a 10 kHz and 3 kHz RBW
when measuring a 200 MHz span It is important to know the funda-
One advantage of using a narrow using sample detector. mental tradeoffs that are involved
RBW is seen when making measure- in RBW selection, for cases where
ments of low-level signals. When the user knows which measurement
using a narrow RBW, the displayed parameter is most important to
average noise level (DANL) of the optimize. But in cases where mea-
spectrum analyzer is lowered, increas- surement parameter tradeoffs
ing the dynamic range and improving cannot be avoided, the modern
the sensitivity of the spectrum ana- spectrum analyzer provides ways to
lyzer. In Figure 3, a