Spectrum Analyzer
Last Edited: Dec 24, 2023
What Is a Spectrum Analyzer
A spectrum analyzer is a specialized type of sound level meter. Consequently, it allows the engineer to examine the amplitude versus the frequency spectrum of a sound. These analyzers use multiple sets of filter networks, and vertical columns display the information arranged from low to high frequencies, left to right. In addition, these analyzers usually have third-octave spacings between the frequencies within a range between 20 Hz and 20 kHz.
Octave Bands
Occasionally, they have octave bands or doubling of the frequencies above each preceding frequency column. The third-octave bands are determined by using the cube root of 2. For instance, the next band above 1hz would be a cube root of 2, followed by the cube root of 2 squared, the cube root of 2 to the third power, the cube root of 2 to the fourth power, etc. Each succeeding octave is multiplied by a factor of 10. The ISO ( International Standards Organization ) preferred frequencies are also for equalization centers.
Frequencies
The frequencies used are as follow: ⅓ 8va bands 1, 1.25, 1.63, 2, 2.5, 3.15, 4, 5, 6.3, 8, 10, 12.5, 16.3, 20, 25, 31.5, 40, 50, 63, 80, 100, 125, 163, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1630, 2k, 2500, 3150, 4k, 5k, 6300, 8k, 10k, 12,5k 16k, 20k, etc; as necessary 8va bands 31.5, 63, 125, 250, 500, 1000, 2k, 4k, 8k, 16k, etc; as necessary.
AD-FMCOMMS3-EBZ & AD9361
The "Spectrum analyzer" measures the magnitude of an input signal versus frequency within the entire frequency range of the instrument for the AD-FMCOMMS3-EBZ, 70 MHz - 6000 MHz (the limits of the AD9361). This is not meant to be the world's best spectrum analyzer, but an example of using the fast lock feature of the AD9361, with a small contrived example. It is multi-threaded, so it does get adequate performance. Additionally, many portable spectrum analyzers also allow the user to select the "A," "B," or "C" weighting scale as well. They are often parts of the metering circuit on larger audio recording consoles.
Hybrid SuperHeterodyne - FFT
The overall method it uses is known as a hybrid superheterodyne-FFT. This technique combines sweeping the LO and using FFT analysis on the captured signal to consider wide and narrow spans. Moreover, this technique typically allows for faster sweep time. One benefit of digitizing the intermediate frequency is the ability to use digital filters, which have a range of advantages over analog filters, such as near-perfect shape factors and improved filter settling time. Also, considering narrow spans, the FFT can increase sweep time without distorting the displayed spectrum.
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