Sound Card Stepped Sweep Response

It is possible to improve upon the simple swept response. Whenever the signal frequency falls exactly on a spectral line frequency (which means there are an exact integer number of cycles in 1024 samples), there are no leakage sidelobes at all, hence no amplitude errors. There is no need for a window function in this case, and in fact it would be detrimental.

The trick is then to use only these discrete frequencies and no others. Instead of a smooth, continuous sweep, the frequency must be stepped over the desired range with just the proper steps to fall only on the spectral lines.

To do this, the Sweep Start to End frequencies in the Sweep dialog must first be set with frequency entry mode set to Step Lines, and the Lines value set to 1.000. That will assure that the sweep starts and ends on exact spectral lines, and will also assure that there will be an integer number of spectral line frequencies between them.

Make sure Linear sweep mode is selected, then click the Stairs button. This will automatically compute the number of frequency steps, and also set the sweep to stepped mode.

Now Sweep Length no longer controls the length of the overall sweep, but instead the duration of each step. It must be at least 1024 samples to have enough for an FFT, plus some extra to allow time for the display processing before the next frequency step. For slow systems, this might mean a total of 4096 samples or more.

You will need to set Sweep Sync. This sets the update sync interval to the Sweep Length, which will allow a trace update on each frequency step. Trigger must be active, and set to Gen Sync mode.

Make sure you are in Spectrum mode (ALT+S) and go to the Spectrum Averager dialog (CTRL+A). Set Peak mode, and set the averager frames Request to 1 to enable an unlimited number of frames. Hit the Averager button (ALT+A) to start the response plot. Note that toggling this back off will abandon the current plot and return to instantaneous spectrum mode; use Pause if you want to freeze the display and optionally save the response.

You can tell if Sweep Length is too low because you will see a gap in the response if Daqarta misses a sync point. Check the Trace Update interval to see if it can be set lower. The lowest useful value here is probably around 10 msec, which is the default for most systems (55 msec for Win95 and 98). If you still have gaps in the response, increase the Sweep Length.

See also Frequency Response Measurement