mkfilter 2nd order Bessel bug?

Hello, in the mkfilter() function of the Robust Control Toolbox, I am unable to create a 2nd order Bessel filter, I receive

>> bess = mkfilter(1,2,'bessel');

Undefined function 'ssdata' for input arguments of type 'double'.

.

3rd order works though:

>> bess = tf(mkfilter(1,3,'bessel'))

Transfer function:

      1240

-------------------------------

s^3 + 37.7 s^2 + 592.2 s + 1240

Is this a bug in mkfilter? Or is there some theoretical impossibility of a 2nd order Bessel filter?

Answer 1

[Zack Peters]

Hi,

This does look to be a possible bug in MKFILTER. Fortunately, the code for MKFILTER is accessible. If you got to lines 60 and 62 of mkfilter.m and change them from:

sys = nd2sys(1,b1);
sys = nd2sys(1,b2);

respectively to:

sys = tf(1,b1);
sys = tf(1,b2);

Then you may be able to get the desired output that you are looking for.

~Zack