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Change cutoff frequency for lowpass analog filter

`lp2lp`

transforms an analog lowpass filter prototype with a cutoff
angular frequency of 1 rad/s into a lowpass filter with any specified cutoff angular
frequency. The transformation is one step in the digital filter design process for the
`butter`

, `cheby1`

, `cheby2`

, and `ellip`

functions.

`lp2lp`

is a highly accurate
state-space formulation of the classic analog filter frequency transformation. If a lowpass
filter has cutoff angular frequency ω_{0}, the standard
*s*-domain transformation is

$$s=p/{\omega}_{0}$$

The state-space version of this transformation is

$$At={\omega}_{0}\cdot A$$

$$Bt={\omega}_{0}\cdot B$$

$$Ct=C$$

$$Dt=D$$

The `lp2lp`

function can perform the transformation on two different
linear system representations: transfer function form and state-space form. See `lp2bp`

for a
derivation of the bandpass version of this transformation.