Question
I measure phase noise with the R&S FSEA. I get different results with different SPAN. If the span is 100 kHz, the result is -119dBc/Hz as expected. With a span of 116 kHz, the result will be -110 dBc/Hz which is not as expected. Why?
SSB Measurement Result has Changed after a Change of Span
Answer
When you change the span, you also change the Main PLL Bandwidth, which yields different measurement results.
A span change from 100 kHz to 116 kHz will change the Main PLL BW from High to Medium.
You can set the Main PLL manually in the right-hand submenu of the COUPLING menu. This allows you to optimize the phase noise for different applications.
For more details, especially optimum settings depending on span and RBW, please refer also to the operating manual (coupled settings).