Bandwidth returns inf with discrete time transfer function
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I am trying to find the 3dB frequency of discrete time transfer function using bandwidth but it returns inf. Here is the code:
alpha1=0.9;
ts = 0.1;
z = tf('z',ts);
H = ((1-alpha1)/2)*(1-z)/(1-alpha1*z);
fb = bandwidth(H,-3);
Am I doing anything conceptually wrong?
4 个评论
Mathieu NOE
2021-9-30
Hi
read the doc
fb = bandwidth(sys) returns the bandwidth of the SISO dynamic system model sys. The bandwidth is the first frequency where the gain drops below 70.79% (-3 dB) of its DC value.
seems that this function cannot be used if you hae a high pass filter behaviour in your tf; because then the dc gain is 0 (-Inf in dB)
you have to write your own function for bandwith that overcomes this limitation
I recognize it's strange that this case is not dealed correctly in the native matlab function (shame on you TMW !)
all the best
Shivanshu Sahoo
2021-9-30
Mathieu NOE
2021-9-30
see the results of low pass vs high filters
bandwith works ok only for the low pass case
% high pass digital filter
alpha1=0.9;
ts = 0.1;
z = tf('z',ts);
H = ((1-alpha1)/2)*(1-z)/(1-alpha1*z);
figure(1),bode(H)
fb = bandwidth(H,-3);
% low pass analog filter
sys = tf(1,[1 1]);
fb = bandwidth(sys)
figure(2),bode(sys)
% high pass analog filter
sys = tf([1 0],[1 1]);
fb = bandwidth(sys)
figure(3),bode(sys)
Shivanshu Sahoo
2021-9-30
采纳的回答
The -3 dB frequency is straightforward to calculate —
alpha1=0.9;
ts = 0.1;
z = tf('z',ts);
H = ((1-alpha1)/2)*(1-z)/(1-alpha1*z);
fb = bandwidth(H,-3)
[mag,phase,wout] = bode(H);
mag = squeeze(mag);
wdB3 = interp1(mag2db(mag/max(mag)), wout, -3)
magdB3 = interp1(wout, mag2db(mag), wdB3)
phase = squeeze(phase);
figure
semilogx(wout, mag2db(mag))
hold on
plot(wdB3, magdB3, '+r', 'MarkerSize',10)
hold off
grid
xlim([min(wout) max(wout)])
.
4 个评论
Shivanshu Sahoo
2021-9-30
Star Strider
2021-9-30
It is the frequency corresponding to the -3 dB power.
.
Shivanshu Sahoo
2021-9-30
Star Strider
2021-9-30
As always, my pleasure!
.
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