What is a Spectrum Analyzer


The spectrum analyzer is a specialized type of a sound level meter. Consequently, it allows the engineer to examine the amplitude versus the frequency spectrum of a sound. They use multiple sets of filter networks and vertical colums 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 20hz to 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 for equalizations centers as well.




The frequencies used are as follows :

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




The “Spectrum analyzer” measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. For the AD-FMCOMMS3-EBZ that is 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 how to use the fast lock feature of the AD9361, with a small contrived example. It is multi-threaded, so it does actually 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. The technique used is to combine sweeping the LO and using FFT analysis on the captured signals for consideration of 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, for consideration of narrow spans, the FFT can  increase sweep time without distorting the displayed spectrum.


Resources & source texts