With a wide bandwidth, more noise is included in the measurement. With a narrow band-
width, less noise enters the resolution bandwidth filter, and the measurement is more accu-
rate. If the resolution bandwidth is narrower, the noise floor will drop on the display of the
spectrum analyzer. This is because the IF filter of the analyzer has been made narrower in
bandwidth, which lets in less noise. As the measured noise level drops, smaller signals that
were previously obscured by the noise can now be measured.
Effect of Video Bandwidth
Spectrum analyzers typically use another type of filtering after the detector called video fil-
tering. This filter also affects the noise on the display but in a different manner than the res-
olution bandwidth. In video filtering, the average level of the noise remains the same but
the variation in the noise is reduced. Hence, 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 com-
presses into a thinner trace, while the position of the trace remains the same. Thus, chang-
ing the video bandwidth (VBW) does not improve sensitivity; however, it does improve
discernability and repeatability when making low-level measurements.
As a general rule of thumb, most field spectrum analyzer measurements are made at a video
bandwidth that is a factor of 10 to 100 less than the resolution bandwidth. Thus, for a reso-
lution bandwidth of 30 kHz, the typical video bandwidth setting options are either 3 kHz or
300 Hz.
Sweep Limitations
With some spectrum analyzers, the user has control over sweep time (the elapsed time of
each sweep, sometimes called scan time). An analyzer cannot be swept arbitrarily fast while
maintaining its specified accuracy, but will have a sweep rate limitation depending on the
resolution bandwidth, video bandwidth, and frequency range selected. The sweep rate is not
usually chosen by the user but is determined by the frequency range swept divided by the
sweep time. The limitation on sweep rate comes from the settling or response time of the
resolution and video bandwidth filters. If an analyzer is swept too quickly, the filters do not
have time to respond, and the measurement is inaccurate. Under such conditions, the ana-
lyzer display tends to have a “smeared” look to it, with the spectral lines being wider than
normal and shifted to the right.
Fortunately, the Anritsu Handheld Spectrum Analyzer has mechanisms designed into it that
relieve the user from having to calculate the sweep rate. When changing the RBW and
VBW, the sweep rate will change accordingly. The sweep rate will be faster for a wide
RBW or VBW and slower for a narrow RBW or VBW. The sweep rate can also be changed
manually, by pressing the
MEAS/DISP
key and selecting the
Min Sweep Time
soft key.
Enter a sweep time from 0.05 ms to 2000 seconds. If the minimum sweep time entered by
the user is less than the value calculated by the Site Master to assure accurate results, the
calculated value will prevail. Regardless of the minimum sweep time setting, the Site Mas-
ter will never sweep faster than the RBW and VBW settings will allow.
Attenuator Functions
Attenuation adjusts the hand held spectrum analyzer input attenuator. In
Auto
mode, as the
reference level is increased, the attenuation is increased. In
Manual
mode, the input attenua-
tion can be adjusted using the keypad or the Up/Down arrow key. The attenuator range is 0
to 51 dB, in 1 dB steps. When
Dynamic
attenuation is selected, the instrument automatically
sets the input attenuation and preamplifier to achieve the best possible dynamic range.
5-2
Chapter 5 Spectrum Analyzer Measurements
