Software derived test equipment: Oscilloscope, Signal generator and more.
By: Steve Adams G0KVZ
This was originally presented to the Vange Amateur Radio Society on the 18th November 2010.
After basic hand tools, a soldering iron, and a multimeter,
two of the most commonly needed (and versatile) pieces of
equipment that one will need are an oscilloscope,
and a signal generator.
There is no substitute for these complex devices, and they are quite expensive. But with today’s computers becoming increasingly fast, it is now possible to install software that enables a standard P.C. to emulate their basic functions.
It must be noted that these do not boast great specifications, but despite the limitations of reduced functionality / bandwidth, these programmes offer the user low-cost (free) alternatives that can perform very well for audio troubleshooting.
Four such programmes are featured below, and will provide the home constructor or radio amateur with basic equipment:
SweepGen turns a PC into an Audio Oscillator and Sweep Generator which can be used for testing audio or educational purposes. In conjunction with audio test instruments, you can make frequency response plots. SweepGen uses the sound card in your PC to produce sine waves that are mathematically correct almost to CD quality, indeed it's more likely that the quality of your PC sound card will be the limiting factor rather than the code in SweepGen.
The display is bright when the program believes it has a self-consistent reading. If the volume is too low (or fluctuating too much), the signal is rich in harmonics, or contaminated by extraneous sounds or electrical interference, then the display remains dim. If the signal is too loud then the display goes red.
This program uses the output from the 'Recording' mixer panel as its input.
Audio is sampled at 44.1kHz, with 16-bit resolution, mono. The FFT analyses 4096 samples at a time, yielding a transform with a resolution of 10.77Hz. Version 2.2 features a peak level meter, and allows various vertical and horizontal scales to be chosen. For best results at least a Pentium II class processor is recommended.
[The now-obsolete version 2.2 from Jan 2003 is still available at specan22.exe just in case the new version has any new issues.]
The new version 2.8 now has a menu option to open the relevant volume mixer (especially useful in Windows XP). It is also now aware of -and can make use of- multiple soundcards (in XP) -- this same addition is a huge bonus under the new sound model of Vista/Windows7 where all the input devices appear like individual soundcards at the programming level.
N.B. As the computer soundcard is not designed for use at high voltages great care should be exercised to ensure that the computer is not damaged as a result of using this software. this article (and the presentation) should not be seen as recommending this software, and is not intended as an endorsement. Any loss or damage that is suffered as a result of using this software is entirely at your own risk.