Agilent Spectrum Analysis Basics - Agilent Technologies

Since the output of a spectrum analyzer is an X-Y trace on a display, let’s see
what information we get from it. The display is mapped on a grid (graticule)
with ten major horizontal divisions and generally ten major vertical divisions.
The horizontal axis is linearly calibrated in frequency that increases from
left to right. Setting the frequency is a two-step process. First we adjust the
frequency at the centerline of the graticule with the center frequency control.
Then we adjust the frequency range (span) across the full ten divisions with
the Frequency Span control. These controls are independent, so if we change
the center frequency, we do not alter the frequency span. Alternatively, we
can set the start and stop frequencies instead of setting center frequency and
span. In either case, we can determine the absolute frequency of any signal
displayed and the relative frequency difference between any two signals.
The vertical axis is calibrated in amplitude. We have the choice of a linear
scale calibrated in volts or a logarithmic scale calibrated in dB. The log scale
is used far more often than the linear scale because it has a much wider
usable range. The log scale allows signals as far apart in amplitude as 70 to
100 dB (voltage ratios of 3200 to 100,000 and power ratios of 10,000,000 to
10,000,000,000) to be displayed simultaneously. On the other hand, the linear
scale is usable for signals differing by no more than 20 to 30 dB (voltage ratios
of 10 to 32). In either case, we give the top line of the graticule, the reference
level, an absolute value through calibration techniques 1 and use the scaling
per division to assign values to other locations on the graticule. Therefore,
we can measure either the absolute value of a signal or the relative amplitude
difference between any two signals.
Scale calibration, both frequency and amplitude, is shown by annotation
written onto the display. Figure 2-2 shows the display of a typical analyzer.
Now, let’s turn our attention back to Figure 2-1.
Figure 2-2. Typical spectrum analyzer display with control settings
1. See Chapter 4, “Amplitude and Frequency
Accuracy.”
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