SDR Soundcard Tester

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The key to using a
soundcard successfully in digital signal processing or digital radio
applications lies principally in the characteristics of the soundcard
itself. This applies in particular to SDR
(software defi ned radio) programs that turn your PC into a top-class
AM/SSB/CW receiver, assuming your soundcard cooperates. If you want to
experiment with SDR and avoid a lot of
frustration, it is worth checking first whether the PC soundcard you plan
to use is suitable. There are three essential elements to success:

  • the soundcard must have a stereo line-level input;
  • the card must be equipped with an input anti-aliasing filter; and
  • the sample rate must be at least 48 kHz and the card must be able to cope with signals up to 24 kHz.

Many laptops have only a mono microphone input, sometimes also
rather limited in bandwidth. In this case it may be possible to use an
external USB soundcard. Most desktop PCs these
days have an internal integrated soundcard, although some of these do
not feature an anti-aliasing fi lter. Attempts to disable the integrated
soundcard and replace it with a better one often meet with failure;
again, an external USB soundcard is a possible solution.

SDR Soundcard Tester Circuit

SDR Soundcard Tester Circuit Diagram

To avoid guesswork, the best way to proceed is to test the soundcard
using this very small circuit. This will help to diagnose any problems
and will help determine whether the card is suitable for use with an SDR
program. Figure 1 shows a simple square-wave generator built around an
NE555 timer IC. At the output is a 15 kHz signal rich in higher
harmonics. Using this we can determine whether or not the soundcard can
process the harmonics at 30 kHz, 45 kHz and so on. An anti-aliasing
filter at the soundcard input should attenuate all signals above 24 kHz.
The frequency of the test generator is, within limits, dependent on its
supply voltage.

Using an adjustable power supply, a frequency range from 10 kHz to
20 kHz can therefore be covered. There are two RC networks at the output
of the test circuit, a high-pass filter and a low-pass filter, acting
as simple phase shifters. At the basic frequency of 15 kHz these provide
a total phase difference of 90 degrees, corresponding exactly to the
typical situation at the output of an SDR receiver circuit using an I-Q mixer: signals at the same frequency but differing in phase. To test the soundcard we need an SDR program running on the PC as well as the circuit of Figure 1. Suitable software includes SDradio (available for download from http://digilander.libero.it/i2phd/sdradio/).

When things are running correctly, the screen should display just
two signals: the wanted signal at 15 kHz and a weaker image at –15 kHz
(Figure 2). Suppression of the image may not be particularly good as the
test circuit does not have very high phase and amplitude accuracy. If,
however, the signals have the same level, there is a problem in the
processing of the two channels: it is probable that the soundcard only
has a monophonic input. If there is no anti-aliasing filter at the input
of the soundcard the spectrum will show a large number of extra lines
(Figure 3): it is easy to work out which harmonic corresponds to which
alias frequency.

The results obtained using an I-Q receiver were grim: frequencies
all the way out to 100 kHz were wrapped into the audible range,
resulting in bubbling, hissing and whistling. In theory it would be
possible to add an anti-aliasing filter to the output of the receiver to
allow use with soundcards that are not equipped with such a filter. In
practice, however, it is not easy to achieve the required sharp cutoff
and symmetry between the two channels. A typical soundcard has a low
pass filter set at 24 kHz which by 27 kHz is already attenuating the
signal by some 60 dB. This is only practical using digital fi lters; an
adjustable analogue circuit to achieve this performance would be so
complex that the simplicity benefits of SDR receiver technology would entirely evaporate.

Author: Burkhard Kainka – Copyright: Elektor Electronics 2007