Home » Radio » Software Defined Radio » Sound Card Oscilloscope for SDR

Sound Card Oscilloscope for SDR

sound card oscilloscope

You can use a sound card oscilloscope for many things. My favorite use is to study noise and interference signatures on the shortwave radio spectrum.

When you are examining signals, noise or interference, you will find an oscilloscope and spectrum analyzer quite useful. In the picture above, you can see me examining impulse noise generated by a switch mode power supply at a neighbor’s house. In the past, I have relied on SpectrumView audio spectrum analyzer from WD6CNF.

Recently, I came across the Sound Card Oscilloscope from Christian Zeitnitz in Europe, shown above right. It’s a fine tool that provides both oscilloscope and spectrum analyzer in one package. Here’s how it works.

If you want to analyze noise or interference in the time or frequency domain, just export AM mode audio to a sound card oscilloscope. I typically use 7-10 kHz bandwidth. Most SDR software lets you easily pipe sound to a virtual audio cable. Zeitnitz’s sound card oscilloscope defaults to 16 bit sampling at 44,100 kHz. So, just make sure that your exported audio sampling rate matches up.

To study these signals, you need really good control over amplitude and timing. A good sound card oscilloscope lets you adjust level and time base. Also, a good trigger circuit is required to get a stationary display on the screen. Shown above is a stable 80 millisecond scan showing interfering impulse train starting up every 8.3 milliseconds, which corresponds to 120 Hz power system noise.

This software will also display peak frequency and peak pulse amplitude. Selecting the frequency domain, you can see the same signature in a spectrum analyzer. If you have never done this before, I suggest you give it a try.

Sound Card Oscilloscope and Noise Blankers

Noise blankers can do a good job on simple ignition noise and powerline spark gap interference. Not so much on RFI from switch mode power supplies, which is much more complex. A sound card oscilloscope lets you make NB threshold and other adjustments and watch the effect.

One thing you should keep in mind when playing with a noise blanker is to turn off the AGC and just use a fixed amount of gain in your SDR. Otherwise the automatic gain will just amplify the remaining noise artifacts between blanking pulses. With AGC off, you can properly evaluate the performance of your noise reduction tools in terms of SNR improvement.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.