3-6 Setting Bandwidth Parameters Spectrum Analyzer Measurements
3-18 PN: 10580-00447 Rev. H Spectrum Analyzer MG
3-6 Setting Bandwidth Parameters
Bandwidth parameters are set using the “BANDWIDTH Menu” on page 3-20.
Resolution Bandwidth
Resolution Bandwidth (RBW) determines frequency selectivity. The spectrum analyzer traces the shape of the
RBW filter as it tunes past a signal. The choice of resolution bandwidth depends on several factors. Filters take
time to settle. The output of the filter will take some time to settle to the correct value so that it can be
measured. The narrower the filter bandwidth (resolution bandwidth), the longer the settling time needs to be,
and therefore, the slower the sweep speed.
The choice of resolution bandwidth will depend upon the signal being measured. If two closely-spaced signals
are to be measured individually, then a narrow bandwidth is required. If a wider bandwidth is used, then the
energy of both signals will be included in the measurement. Thus, the wider bandwidth does not have the
ability to look at frequencies selectively, but instead simultaneously measures all signals falling within the
resolution bandwidth. Therefore, a broadband measurement would include all signals and noise within the
measurement bandwidth into a single measurement.
On the other hand, a narrow-band measurement will separate the frequency components, resulting in a
measurement that includes separate peaks for each signal. There are advantages to each. The ultimate
decision will depend upon the type of measurement required.
There is always some amount of noise present in a measurement. Noise is often broadband in nature; that is, it
exists at a broad range of frequencies. If the noise is included in the measurement, the measured value could be
in error (too large) depending upon the noise level. With a wide bandwidth, more noise is included in the
measurement. With a narrow bandwidth, less noise enters the resolution bandwidth filter, and the
measurement is more accurate. If the resolution bandwidth is narrower, the noise floor will drop on the
spectrum analyzer display. As the measured noise level drops, smaller signals that were previously obscured
by the noise might now be measurable. Zero span is used for noise and noise-like measurements that are
usually wider than the RBW. The RBW is ideally set to be as wide as the bandwidth of the signal you are
measuring.
Video Bandwidth
Spectrum analyzers typically use another type of filtering after the detector that is called video filtering. This
filter also affects the noise on the display, but in a different manner than the resolution bandwidth. In video
filtering, the average level of the noise remains the same, but the variation in the noise is reduced. Therefore,
the effect of video filtering is a “smoothing” of the signal noise. The resultant effect on the analyzer’s display is
that the noise floor compresses into a thinner trace, while the average position of the trace remains the same.
Changing the video bandwidth (VBW) does not improve sensitivity, but it does improve discernibility and
repeatability when making low-level measurements. As a general rule, most field spectrum analyzer
measurements are made at a video bandwidth that is a factor of 10 to 100 less than the resolution bandwidth.
Using this ratio, with a resolution bandwidth of 30 kHz, typically, the video bandwidth is set between 300 Hz
and 3 kHz, although it can be set anywhere from 1 Hz to 10 MHz.
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