zerophase
Zero-phase response of digital filter
Syntax
[Hr,w] = zerophase(b,a)
[Hr,w] = zerophase(sos)
[Hr,w] = zerophase(d)
[Hr,w] = zerophase(...,nfft)
[Hr,w] = zerophase(...,nfft,'whole')
[Hr,w] = zerophase(...,w)
[Hr,f] = zerophase(...,f,fs)
[Hr,w,phi] = zerophase(...)
zerophase(...)
Description
[Hr,w] = zerophase(b,a) returns
the zero-phase response Hr, and the frequency vector w (in
radians/sample) at which Hr is computed, given
a filter defined by numerator b and denominator a.
For FIR filters where a=1, you can omit the value a from
the command. The zero-phase response is evaluated at 512 equally
spaced points on the upper half of the unit circle.
The zero-phase response, Hr(ω), is related to the frequency response, H(ejω), by
where φ(ω) is the continuous phase.
Note
The zero-phase response is always real, but it is not the equivalent of the magnitude response. The former can be negative while the latter cannot be negative.
[Hr,w] = zerophase(sos) returns the zero-phase
response for the second order sections matrix, sos. sos is
a K-by-6
matrix, where the number of sections, K,
must be greater than or equal to 2. If the number of sections is less
than 2, zerophase considers the input to be the
numerator vector, b. 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)].
[Hr,w] = zerophase(d) returns the zero-phase
response for the digital filter, d. Use designfilt to generate d based
on frequency-response specifications.
[Hr,w] = zerophase(...,nfft) returns
the zero-phase response Hr and frequency vector w (radians/sample),
using nfft frequency points on the upper half of
the unit circle. For best results, set nfft to
a value greater than the filter order.
[Hr,w] = zerophase(...,nfft,'whole') returns
the zero-phase response Hr and frequency vector w (radians/sample),
using nfft frequency points around the whole unit
circle.
[Hr,w] = zerophase(...,w) returns
the zero-phase response Hr and frequency vector w (radians/sample)
at frequencies in vector w. The vector w must
have at least two elements.
[Hr,f] = zerophase(...,f,fs) returns
the zero-phase response Hr and frequency vector f (Hz),
using the sampling frequency fs (in Hz), to determine
the frequency vector f (in Hz) at which Hr is
computed. The vector f must have at least two elements.
[Hr,w,phi] = zerophase(...) returns
the zero-phase response Hr, frequency vector w (rad/sample),
and the continuous phase component, phi. (Note
that this quantity is not equivalent to the phase response of the
filter when the zero-phase response is negative.)
zerophase(...) plots the zero-phase response versus
frequency. If you input the filter coefficients or second order sections matrix, the
current figure window is used. If you input a digitalFilter, the step response is displayed in FVTool.
Note
If the input to zerophase is single precision,
the zero-phase response is calculated using single-precision arithmetic.
The output, Hr, is single precision.
Examples
Zero-Phase Response of FIR filter
Use designfilt to design a 54th-order FIR filter with a normalized cutoff frequency of 0.3π rad/sample, a passband ripple of 0.7 dB, and a stopband attenuation of 42 dB. Use the method of constrained least squares. Display the zero-phase response.
Nf = 54; Fc = 0.3; Ap = 0.7; As = 42; d = designfilt('lowpassfir','FilterOrder',Nf,'CutoffFrequency',Fc, ... 'PassbandRipple',Ap,'StopbandAttenuation',As,'DesignMethod','cls'); zerophase(d)
Design the same filter using fircls1, which uses linear units to measure the ripple and attenuation. Display the zero-phase response.
pAp = 10^(Ap/40); Apl = (pAp-1)/(pAp+1); pAs = 10^(As/20); Asl = 1/pAs; b = fircls1(Nf,Fc,Apl,Asl); zerophase(b)
Zero-Phase Response of Elliptic Filter
Design a 10th-order elliptic lowpass IIR filter with a normalized passband frequency of 0.4π rad/sample, a passband ripple of 0.5 dB, and a stopband attenuation of 20 dB. Display the zero-phase response of the filter on 512 frequency points around the whole unit circle.
d = designfilt('lowpassiir','FilterOrder',10,'PassbandFrequency',0.4, ... 'PassbandRipple',0.5,'StopbandAttenuation',20,'DesignMethod','ellip'); zerophase(d,512,'whole')
Create the same filter using ellip. Plot its zero-phase response.
[b,a] = ellip(10,0.5,20,0.4);
zerophase(b,a,512,'whole')
See Also
designfilt | digitalFilter | freqs | freqz | FVTool | grpdelay | invfreqz | phasedelay | phasez