How to Read a Spectrum Analyzer
- 1). Enter the frequency range, or span, that will be displayed on the spectrum analyzer's X-axis. This is often a necessary first step when you are trying to display a signal, because the frequency range affects the way the spectrum analyzer scans the input signal; if the frequency range is incorrect, your signal might be missed. You could start by setting the range to a factor of 10 above and below your signal of interest. For example, if you want to analyze a 1 MHz signal, choose a span of 100 kHz to 10 MHz.
- 2). Set the frequency axis to be displayed on a log scale. This ensures the frequency response will resemble typical Bode plots, with which many engineers and hobbyists are familiar. On a log scale, each interval on the axis corresponds to a factor-of-ten increase in the frequency. Each factor-of-ten interval is called a decade.
- 3). Set the Y-axis to display in units of dB, or decibel. This logarithmic unit is the standard for representing values in the frequency domain. Your spectrum analyzer may use dBm as the default unit; dBm is referenced to milliwatts instead of watts, meaning 0 dBm = one milliwatt.
- 4). Look at the line graph on your spectrum analyzer's display and match up a frequency on the X-axis with a power level on the Y-axis. Most spectrum analyzers have a cursor function: you move the cursor to a specific point on the line and the display reports the frequency and power at that point. You can also look at the display in a general way and compare the overall configuration of the graph to standard spectrum configurations, such as the characteristic sideband arrangements of an amplitude-modulated (AM) or frequency-modulated (FM) radio signal.
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