How do I use the "prewarp" method with the C2D command in Control System Toolbox 5.2.1 (R13SP1)?

I am using the "prewarp" method as the third input to the C2D command. However, I receive the following error:

 ??? Error using ==> zpk/c2d

 The critical frequency must be specified when using prewarp method.

What is the critical frequency and how do I specify it?

Answer 1

[MathWorks Support Team]

This bug has been fixed for Release 14 (R14). For previous releases, please read below for any possible workarounds:

There is a difference in the information available from the MATLAB file help and from the documentation on the C2D function in Control System Toolbox 5.2.1 (R13SP1). If you wish to view the help for C2D, which includes a brief description of the critical frequency parameter and how to specify it, type the following at the MATLAB command prompt:

help c2d

The correct syntax is:

 SYSD = C2D(SYSC,Ts,'prewarp',Wc)

where Wc is the critical frequency. The critical frequency is the frequency where it is desired that the phase and gain in the digital spectrum exactly match the (original) analog spectrum. We have verified that documentation on the additional parameter for C2D with the "prewarp" method is missing from the Control System Toolbox.