# Bandwidth and Filter Type

In the channel Setup dialog box, you are able to specify a Bandwidth limit and the Filter Type used:

- Bandwidth — When the Filter Type is not Off, this field lets you reduce the bandwidth of the channel to be less than the full bandwidth provided by the oscilloscope hardware.

  If you select a channel bandwidth that is lower than Infiniium can accommodate, Infiniium will set the signal bandwidth control to the lowest possible value.

  Because filter bandwidths are typically the -3 dB roll-off point, the maximum bandwidth that can be specified is 105% of the oscilloscope's licensed bandwidth.

- Filter Type — Lets you select the response of the individual channel bandwidth limit filter:

  - Off — In this case, no bandwidth limiting filter is applied, and the Bandwidth field shows the full bandwidth provided by the oscilloscope hardware.

  - 4th order Bessel — Specifies a 4th order Bessel response for the bandwidth limit filter. This response has a more gradual roll-off.

    To achieve the 4th order Bessel response, the maximum bandwidth you can specify is about 2/3 of the maximum bandwidth you could specify with the Brick-Wall filter. Also with the Bessel filter, the brick-wall response takes over at what would be the brick-wall filter's maximum bandwidth. Therefore, you can have a combination of roll-off responses with this selection.

  - Butterworth — Specifies a 4th order Butterworth response for the bandwidth limit filter. This response has a roll-off that is between the Brick Wall and Bessel responses.

    To achieve the 4th order Butterworth response, the maximum bandwidth you can specify is about 80% of the maximum bandwidth you could specify with the Brick-Wall filter. Also with the Butterworth filter, the brick-wall response takes over at what would be the brick-wall filter's maximum bandwidth. Therefore, you can have a combination of roll-off responses with this selection.

  - Bandpass — This option is used with the Phase Noise analysis application. Because you do not have control over the slope of the transition from the pass band to the rejected bands, this option has limited usefulness as a general-purpose bandpass filter.

  - Brickwall — Specifies a Brick-Wall response for the bandwidth limit filter. This response has a sharp roll-off.