A thread over on the softrock user group list spurred the curiosity…..: http://groups.yahoo.com/group/softrock40/message/68324
G4ZFQ has RightMark test data for a high end Xonar D2X card, as well as several others. An internet search found other RightMark tests of several other Xonar cards, all of whose test data show curve trends remarkably similar to those of the D2X, albeit with somewhat worse IMD, spurious, and noise figures.
The curiosity is the test data shows a roll off on the frequencies above 50khz. The nature of the loopback test is an issue, but it also seems likely that using a sound card as the source may be having an effect on the test results at the higher edges of the sound card frequency response. But signal generators as input to the tests shows the same general trend. SDR at wider bandwidths pushes at these edges of a ‘sound’ card’s ability….So perhaps the SDR software is compensating for the expected performance drop-off at frequencies above audible levels?
The Test- (Pertinent Excerpt from list post):
Having not yet thought of a better way to do a meaningful real-world test on the sound card with what is available in the KazShack, I fired up the 80m softrock on the xonar DX.
Transmitting a cw signal(a string of dashes at about 18wpm) at 5w into a dummy load on separate radio, noting the SNR readings obtained by CW skimmer from the SoftRock center frequency(353395x) to its upper limit. With the xonar DX set to 192khz scan rate, the actual upper limit on the readings was 3629.60. SoftRock connected to normal antenna system, a NE facing K9AY with W7IUV pre-amp. In summary, a sound card test using the SoftRock system as input source.
After CW skimmer collected a bit of data, the SNR readings above 3600 improved to 37-39.
So the worst case for CW skimmer(as currently configured) using a Xonar DX is being 6db less sensitive at the upper edge of the 192khz bandwidth than it is at the center. That is actually a lot better than I expected for an audio device pressed into service outside normal audio ranges (and I already liked the Xonar DX).
My curiosity is now nagging me to run the same tests on all of the other in-shack cards more methodically at their maximum scan rates(mostly 96khz), and to find a lower level outside signal source. I’ll try to recruit a fellow in the near field who will better be able to generate a low level test signal. It would be useful to see what happens at the band edges when the best copy close to the center of the SoftRock’s scan range starts out at 20dB, 10dB, or 6dB SNR.
But with the WX here improving, all of that might not happen for several months.
Engineer the Possible…