hello
I am not sure you can retrieve the data (mag,phase) from fvtool object , but you can use freqz instead
now both code below and the fvtool plot seems to indicate different cut off frequencies vs what you asked for in designfilt (I rarelly use it so there might be an explanantion I am not aware off)
plot from fvtool :
try this
% Parameters
Fs = 44100; % Sampling frequency (Hz)
Fc1 = 15.75; % Lower cutoff frequency (Hz)
Fc2 = 47.25; % Upper cutoff frequency (Hz)
Q = 1; % Quality factor
% IIR Biquad bandpass filter design
d = designfilt('bandpassiir', ...
'FilterOrder', 2, ... % Second-order (biquad)
'HalfPowerFrequency1', Fc1, ...
'HalfPowerFrequency2', Fc2, ...
'SampleRate', Fs);
% Frequency response visualization
freq = logspace(0,3,1000);
h =freqz(d.Numerator,d.Denominator,freq,Fs);
mag_dB = 20*log10(abs(h));
[f_6db_low,f_6db_up] = find_zc(freq,mag_dB,-6) % find both -6 dB cut off frequencies
figure,
semilogx(freq,mag_dB)
hold on
plot(f_6db_low,-6,'r+');
plot(f_6db_up,-6,'b+');
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [ZxP,ZxN] = find_zc(x,y,threshold)
% put data in rows
x = x(:);
y = y(:);
% positive slope "zero" crossing detection, using linear interpolation
y = y - threshold;
zci = @(data) find(diff(sign(data))>0); %define function: returns indices of +ZCs
ix=zci(y); %find indices of + zero crossings of x
ZeroX = @(x0,y0,x1,y1) x0 - (y0.*(x0 - x1))./(y0 - y1); % Interpolated x value for Zero-Crossing
ZxP = ZeroX(x(ix),y(ix),x(ix+1),y(ix+1));
% negative slope "zero" crossing detection, using linear interpolation
zci = @(data) find(diff(sign(data))<0); %define function: returns indices of +ZCs
ix=zci(y); %find indices of + zero crossings of x
ZeroX = @(x0,y0,x1,y1) x0 - (y0.*(x0 - x1))./(y0 - y1); % Interpolated x value for Zero-Crossing
ZxN = ZeroX(x(ix),y(ix),x(ix+1),y(ix+1));
end
