AF and RF spectrum analysis using SB16

by Jouni Verronen, OH8RO

I have made a small study on using a 16 bit Sound Blaster or a compatible soundcard in measurement applications.
One outcome is a spectrum analyzer program, SB_FFTC, which can be of use for other tinkerers.

Soundcards are common in PCs and there are also many ham applications available. I have had an SB16 card in a PC for years and occasionally used it with some software of others. To program SB looked difficult and information was scarce and sometimes confusing. However, SB16 is cheap and available and 16 bits to 20 kHz. Then I found a couple of free programs, one which could be used to run high speed DMA sampling and another to make FFT conversion. That allowed me to get the program running in a reasonable time.

While collecting programming material and getting to know SB16 better I began to feel, that a soundcard could be used as a key component for many other than strictly AF purposes. RF measurements can be made using a simple down converter in front of an SB16, as I will show.

The following texts with screendumps tell more:

Description of software
features, system requirements, basic AF spectrum measurement

More on AF measurements
dynamic range, distortion, noise and the kind

On RF measurements
a down converter description, examples of measurements

Download SB_FFTC package (405k)

A ham application

Actually, for some time I have had an idea of a narrowband monitor to my multi mode RX which would show +/- 7 kHz, all that come through the first 70 MHz IF filter. As to hardware it would add a parallel broad 455 kHz IF stage after the second mixer, a third mixer with 444 kHz injection and a post amplifier. The output spectrum from 4 kHz to 18 kHz could then be measured with an SB16. Even a bit older pentium would be fast enough to run the FFT.

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