While I have little experience looking at phase noise I have a lot with using commercial sound cards in acoustic analysis. I use a system from this guy : http://libinst.com/ called Praxis. The designer had to resolve the same issues re: calibration of the chain. He uses an external box that generates a square wave of known amplitude and frequency. The spectrum is known so it's easy to check the system calibration. I would think its possible to inject the square wave into the freq trim pin of the reference oscillator for cal purposes. If the varactor diode isn't filtered you could get response to 100 KHz.
To deal with the dynamic range issue you can equalize the preamp. If you know the eq you can correct for it in the fft. The same problem exists in a magnetic phono cartridge (remember those?) and the preamp has a 40 dB difference in gain between 20 Hz and 20 KHz. This measurement is a little different since the compensation is different, less gain at low frequencies and more at high frequencies. Some of the semi-pro soundcards sample at 192 KHz. You need to check the response, since to get better s/n numbers some mfr's roll of the top early and are way down at 90 KHz. Low frequency response is a different issue. The mfr's don't publish schematics. Look suggests that DC response is possible. The absolute DC accuracy won't be good since it's not an issue for soundcards. I have a lot of experience working with this card: http://www.esi-audio.com/products/julia/ and can furnish measurements of it. I also have resources for modifying the analog section to some degree. It also has single ended and differential inputs to help reduce common mode noise. Its well supported under Linux, important, since both the Windows and the Mac sound system will muck around with your bits. -Demian _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
