Resolution and Video Bandwidth

Power Spectrum measurements let you set both the Resolution Bandwidth (RBW Resolution Band Width (RBW or ResBW): specifies the minimum frequency bandwith that two separate frequency spectra can be resolved and viewed seperately. For FFT (digital) based VSA's the process is equivalent to passing a time-domain signal through a bank of bandpass filters, whose center frequencies correspond to the frequencies of the FFT bins. For a traditional swept-tuned (non-digital) spectrum analyzer, the resolution bandwidth is the bandwidth of the IF filter which determines the selectivity.) and the Video Bandwidth (VBW). You can manually set these parameters, or you can set the Span-to-ResBW ratio and/or VBW-to-ResBW ratio to automatically set these values. You can also set the maximum FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. size.

Resolution Bandwidth

The standard swept analyzer uses the last in a series of analog IF filters to determine resolution bandwidth.

In contrast, the 89600 VSA software with PXI hardware uses a digital IF and a DSP algorithm to determine resolution bandwidth (RBW). The minimum and maximum RBW for Power Spectrum measurements is determined by these parameters:

• Window Type

• Number of frequency points

• Span

The narrowest practical RBW with Power Spectrum measurements is constrained by the length of time you are willing to wait for a measurement to be performed. The widest RBW is limited by the frequency span. The maximum RBW decreases as you narrow the frequency span and increases as you increase the frequency span.

ResBW Parameters

The ResBW Tab (MeasSetup > ResBW) lets you set the following parameters:

• ResBW -- Sets the width of the resolution bandwidth (RBW) filter. RBW determines how close frequency components in the signal spectrum can be and still be displayed as distinct components on the screen. For example, a large RBW may only reveal one signal, when in fact if the RBW is decreased, another signal may also be present and will show up as an additional signal.

  • Auto checkbox -- When checked (default setting), automatically changes ResBW to match the ratio set by Span to ResBW Ratio.

• Span to -3dB ResBW Ratio -- Sets the ratio of the frequency span to the resolution bandwidth. This ratio is only used when the ResBW Auto checkbox is checked.

• VBW -- Lets you change the analyzer post-detection filter (VBW or “video bandwidth”). VBW is annotated at the bottom of some traces (e.g., Spectrum).

  • Auto Checkbox -- When checked (default setting), selects automatic coupling of VBW to RBW using the ratio set by VBW to ResBW Ratio. To decouple VBW from RBW, uncheck this checkbox.

• VBW (Actual) (Time Input Domain only) -- Displays the actual video bandwidth (VBW). Actual VBW may be different from the desired VBW is when gating is used and the gate length is longer than what is needed by the current RBW/VBW ratio (in which case the gate length is prioritized, resulting in a different actual VBW value instead of the desired VBW value). For Time Input Domain, the VBW value shown at the bottom of some traces (e.g., Spectrum) is the Actual VBW value (not the desired VBW value).

  • Auto Checkbox -- When checked (default setting), selects automatic coupling of VBW to RBW using the ratio set by VBW to ResBW Ratio. To decouple VBW from RBW, uncheck this checkbox.

• VBW to -3dB ResBW Ratio- Selects the ratio of the video bandwidth to the resolution bandwidth. This ratio is only used when the VBW Auto checkbox is checked.

  • Auto Checkbox -- When checked (default setting), maintains a VBW to ResBW Ratio of 1 (VBW equal to ResBW)

• ResBW Metric -- Lets you choose the way that the width of the RBW filter is specified for the power spectrum measurement. You can select from the following ways to specify the RBW filter width: Equivalent Noise, -3dB, or -6dB.

 

Max FFT Size

The Max FFT Size parameter lets you set the maximum FFT size to allow for power spectrum measurements. Larger sizes can use significantly more computer memory but can result in faster sweeps, especially at high Span to RBW ratios. This parameter is ignored for zero span measurements.