isstable

Determine whether filter is stable

Syntax

flag = isstable(b,a)

flag = isstable(sos)

flag = isstable(d)

Description

flag = isstable(b,a) returns a logical output equal to 1 if the filter specified by numerator coefficients b and denominator coefficients a is stable.

flag = isstable(sos) returns 1 if the filter specified by second order sections matrix sos is stable.

flag = isstable(d) returns 1 if the digital filter d is stable. Use designfilt to generate d based on frequency-response specifications.

Examples

Filter Stability

Design a sixth-order Butterworth highpass IIR filter using second order sections. Specify a normalized 3-dB frequency of 0.7. Determine if the filter is stable.

[z,p,k] = butter(6,0.7,"high");
SOS = zp2sos(z,p,k);    
flag = isstable(SOS)        

flag = logical
   1

zplane(z,p)

Redesign the filter using designfilt and check it for stability.

d = designfilt("highpassiir",DesignMethod="butter",FilterOrder=6, ...
               HalfPowerFrequency=0.7);
dflg = isstable(d)

dflg = logical
   1

zplane(d)

Create a filter and determine its stability at double and single precision.

b = [1 -0.5];
a = [1 -0.999999999];
act_flag1 = isstable(b,a)

act_flag1 = logical
   1

act_flag2 = isstable(single(b),single(a))

act_flag2 = logical
   0

Input Arguments

b, a — Filter coefficients

vector

Filter coefficients, specified as vectors.

Data Types: single | double

sos — Second order sections

k-by-6 matrix

Second order sections, specified as a k-by-6 matrix where the number of sections k must be greater than or equal to 2. Each row of sos corresponds to the coefficients of a second order (biquad) filter. The ith row of the matrix corresponds to [bi(1) bi(2) bi(3) ai(1) ai(2) ai(3)].

Data Types: double

d — Digital filter

digitalFilter object

Digital filter, specified as a digitalFilter object.

Output Arguments

flag — Logical output

1 (true) | 0 (false)

Logical output, returned as 1 or 0. If the poles lie on or outside the circle, the function returns 0. If the poles are inside the circle, the function returns 1.

Data Types: logical

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• If the first input to isstable is a variable-sized matrix during code generation, then it must not reduce to a vector at runtime.

Version History

Introduced in R2013a