Proposed P.C. based spectrum analysers

[fotios]

Originally posted by jackinnj
Here's some fat in the fire -- I don't think you can adequately test an amplifier with a PC sound card as most of the off-the-shelf devices are configured -- the bandwidth is too limited -- you really need to be able to see the power bandwidth -- which means that you need a device capable of acurately measuring power and distortion out to 100 - 150 kHz. For examining speakers, preamps to the devices already mentioned I would add the True-Audio analyzer -- although the freeware version is limited to octaves only.

You can use the sound card for frequencies above 20kHz but you need a local oscillator, IF and some filtering -- these functions are much easier to do with DDS chips, then the analogue stuff I worked with in my youth. The ham radio world abounds with spectrum analyzers which work from DC to giga-Hertz. In fact there was another article in the most recent issue of QEX http://www.mydarc.de/DG8SAQ/VNWA/index.shtml in which Dr. Baier - a physicist in Ulm -- uses the parallel port to direct the DDS chips and the sound card to acquire the data.

I was at first intrigued with the USB devices from Measurement Computing which is now owned by National Instrument -- because of their low cost and easy access to the USB (bus). If you want to invest a little time, solder and ingenuity you can analyze just about anything with them -- it's a matter of not getting hung up on their transmission rates, just using these to move data packets to Softwire or Measurement Studio.

Stay tuned ...

Dear jackinnj thanks for your interest and your comprehension about what means a measurement for me. If you throw a look in my threads, you can easily distinguish that i use allways osciloscope curves to prove that i say. Especially square wave curves. Unfortunatelly most people can not understand these are much more usefull from a curve of THD+noise but we are compeled to present and this. This item - of THD+noise - i believe has examined primarily from the high quality components offered today from different companies. I am waiting for something good from you before go in construction of custom maded notch filters!!!
Regards
Fotios

[unclejed613]

i have built a voltage divider to use the soundcard to measure higher voltages than a soundcard can normally handle. as far as the bandwidth issue, i still have a hitachi scope and function generator to fall back on for things outside the bandwidth of the PC. i will also probably need a better soundcard for THD20 measurements. but for basic in-band measurements such as THD1 and 20-20k freq response, phase, etc... measurements, the PC is perfect. the only thing i could want in addition is some way of viewing the distortion residual signal so i could use the pc for setting output bias and troubleshooting nonlinearities (pc audio software writers please take note......)....

i have a copy of spectrum lab by DL4HF, which has configurable signal processing, and i might try to configure a "diff amp" section to perform the required subtraction and yield a distortion residual. if i'm successful i will upload the config file for the configurable dsp, as well as instructions for using it.