isstable
Determine whether filter is stable
Syntax
flag = isstable(b,a)
flag = isstable(sos)
flag = isstable(d)
Description
flag = isstable(b,a)flag, equal to true if
the filter specified by numerator coefficients, b,
and denominator coefficients, a, is a stable
filter. If the poles lie on or outside the circle, isstable returns false.
If the poles are inside the circle, isstable returns true.
flag = isstable(sos)true if
the filter specified by second order sections matrix, sos,
is stable. sos is 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 sos matrix corresponds to [bi(1)
bi(2) bi(3) ai(1) ai(2) ai(3)].
flag = isstable(d)true 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
See Also
designfilt | digitalFilter | isallpass | islinphase | ismaxphase | isminphase | zplane