ResBW Coupling (ResBW)

ResBW Coupling determines how resolution bandwidth (ResBW) changes when you change the frequency span. The selections are as follows:

Auto coupling: This coupling chooses the narrowest ResBW value that is consistent with the Span, ResBW mode, Window Type, and number of frequency points. (Auto ResBW coupling is only available in Manual Frequency Points mode.) This ResBW that provides the best compromise between measurement speed and frequency resolution when you change the frequency span.
Offset coupling: This coupling attempts to maintain the same ResBW-to-span ratio when the frequency span is changed (see Offset coupling below for details).
Fixed coupling: This coupling does not change ResBW when the frequency span is changed. Fixed coupling is automatically selected when the value of Main Time Length is changed or when time gating is turned on. This is necessary to maintain the selected value of Main Time Length or Gate Length.
When Time Gating is enabled, only Fixed coupling is available. This is because the ResBW is directly dependent on the length of the time record used to compute spectral traces and Time Gating effectively fixes the time length (and thus ResBW).

The value of resolution bandwidth depends on the selected Window Type and the Main or Gate Time Length. The effect of selecting Auto or Offset coupling is to cause resolution bandwidth to depend on frequency span as well.

Offset coupling

The current resolution bandwidth can easily be overridden by manually entering a setting of your own. When ResBW is manually specified, the VSA will attempt to maintain the ResBW/Span ratio when the Span is changed. This is called offset coupling.

For example, a change to a narrower resolution bandwidth than the default (Auto coupling), will cause the VSA to switch to Offset coupling and maintain a narrower-than-normal resolution bandwidth for subsequent spans. Offset coupling causes the frequency spectrum to appear the same when the span is changed.

When ResBW Mode is arbitrary, the ResBW/Span ratio is exactly maintained. When ResBW Mode is 1-3-10, the ratio cannot be exactly maintained because ResBW is rounded up to the next 1-3-10 value.