How to sweep a sinusoidal signal using the system transfer function?

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James Lockhart
James Lockhart 2020-1-4
评论： Star Strider 2020-1-10
I have a bandpass filter schematic. I also have the corresponding transfer function.
Is it possible to sweep a sinusoidal signal, say from 0.10 Hz to 100 Hz, using either the schematic or the transfer fucntion, or both?

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回答（1 个）

Star Strider
Star Strider 2020-1-4
I am not certain what you want.
A Bode plot of a filter with those frequency limits is straightforward, with the freqs function (using an example from the documentation):
a = [1 0.4 1];
b = [0.2 0.3 1];
f = logspace(-3, 1, 150); % Define As Hz
w = f*2*pi; % Convert To rad/sec
h = freqs(b,a,w);
mag = abs(h);
phase = angle(h);
phasedeg = phase*180/pi;
figure
subplot(2,1,1)
loglog(w,mag)
grid on
xlabel('Frequency (rad/s)')
ylabel('Magnitude')
subplot(2,1,2)
semilogx(w,phasedeg)
grid on
xlabel('Frequency (rad/s)')
ylabel('Phase (degrees)')
To convert the frequency axis to Hz:
xt = get(subplot(2,1,1), 'XTick');
set(subplot(2,1,1), 'XTick',xt/(2*pi), 'XTickLabel', xt, 'XLim',[0.1 100]/(2*pi)) % Convert Frequency Axis To Hz
set(subplot(2,1,2), 'XTick',xt/(2*pi), 'XTickLabel', xt, 'XLim',[0.1 100]/(2*pi)) % Convert Frequency Axis To Hz
Use freqs for continuous-time filters, and freqz for discrete filters.
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James Lockhart
James Lockhart 2020-1-10
Star Strider:
I really appreicate your help. Your are pretty close to what I am looking for as follows.
  1. I have the transfer feuction of a LC passive bandpass filter.
  2. Now, I need the response of the filter with input of a random signal, or noise, or sweeping a sinusoid signal or a chirp signal.
  3. This what I don't know how to do in Matlab.
Thankd a lot
Star Strider
Star Strider 2020-1-10
I already did the equivalent of that. I am not certain what you want.
I would just do this:
a = [0.07 0 20.2];
b = 0.2;
freqz(b, a, 2^12, 100)
That is the equivalent of this:
t = linspace(0.0, 1, 2E+2);
L = numel(t);
Ts = t(2)-t(1);
Fs = 1/Ts;
Fn = Fs/2;
in = zeros(size(t));
in(fix(L/2)) = 1;
out = filter(b, a, in);
FTin = fft(in)/L;
FTout = fft(out)/L;
FTtf = FTout./FTin;
ref = max(abs(FTtf));
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
figure
subplot(3,1,1)
plot(Fv, abs(FTin(Iv))*2)
grid
subplot(3,1,2)
plot(Fv, abs(FTout(Iv))*2)
grid
subplot(3,1,3)
plot(Fv, 20*log10(abs(FTtf(Iv))/ref))
grid
and produces the same result. The ‘in’ signal is a single pulse at t=L/2. The last subplot (transfer function) is in dB. The others are linear.

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