Friday, May 23, 2008

Sound Card Testing

Various references on SDR receiver usage with PC sound cards emphasize the importance of a good sound card design for good receiver performance. A message thread at the DRM Software Radio Forum site provides test results for a number of sound cards and motherboard-based sound chips.

After building the simple sound card tester described in the June 2007 issue of Elektor Electronics and on certain web sites, I proceeded to test the sound card functions in my two PCs.

The older 600 MHz Pentium 3 PC has a Soundblaster card installed. It produced the following display on the SDRdio software from I2PHD with the test signal turned on.

This compares favorably with the screen shots of other sound cards reported in the forum referenced above. Another screen shot with the test signal off shows the background noise level of the card.

My newer, faster PC has only the motherboard-based sound chips, and its test results were not so good. Below is a screen shot with the test signal on. In addition to the two test signal spikes, there are many spurs that represent aliasing and other undesired products that would muddle the performance of a connected SDR receiver.

Similar views were obtained using the Winrad software from I2PHD. It is clear that this PC will require a separate sound card for satisfactory SDR performance.

-- John