If the following scenario applies to you:

  • you are using the Span setting of 25MHz or down to 0.1MHz, and 
  • your input signal is the same as the device's center frequency, then

continue with the explanation below.  If not, see if this article on signal input and attenuation applies to you before open a support case.  If you do open a case, please send us also an S240v5 screen capture of the signal measured and settings (from the menu bar: View > Take Screenshot...) along with the input level used.

When the Span is 25MHz or down to 0.1kHz, S240v5 applies the decimation setting to the device, which results in {I,Q} data being sent back.  The software, thus, starts applying an IQ imbalance and DC offset correction to the data (see the R55x0 Programmer's Guide page 17 and 18 for further explanation).  This correction can cause an issue when the input signal aligned with the center frequency of the box.  It's a known issue with the S240v5 software (not the performance of the device). This issue could be minimized by either:

  • reduce the RBW to a small value (0.1kHz or smaller as the Span decreases), or
  • don't align the input signal and the box to the same frequency (10kHz difference is sufficient).

Why this issue only applies to this particular Span range, please read  the "coupled mode" used in S240v5 User Guide, page 14, to understand what happened when changing the Span.  For Span above 25MHz, only SH mode is used (which returns I-only data, not {I,Q}).