How to correctly apply dsp.FIRFilter

The code is simple:

N = 40;

fs = 20 * 10 ^ 9;

freq = [0, 1 * 10 ^ 9, 2 * 10 ^ 9, 3 * 10 ^ 9, 5 * 10 ^ 9, 7.5 * 10 ^ 9, 10 * 10 ^ 9]

mag = [1, 1, 1, 0.3, 0.1, 0.005, 0.001]

d = fdesign.arbmag('N,F,A', N, freq, mag, fs);

W = [1, 1, 1, 1, 1, 1, 1];

fir = design(d, 'equiripple', 'weights', W, 'SystemObject', true);

sents = ones(1, 300) * amplitude;

rcvds = fir(sents);

% plot

figure(1)

stem(sents)

hold on

stem(rcvds)

legend('sents', 'received')

By viewing freqz(fir), seems the filter is correctly designed, it's low pass:

But in the code I input a DC signal, the result does not seem to be correct, the output is almost 0.

Is the way I perform the filter wrong?

Seems I got correct result when apply the filter by function filter:

fir = fir.Numerator

rcvds = filter(fir, 1, sents)

figure(2)

stem(sents)

hold on

stem(rcvds)

legend('sents', 'received')

[Kera Cai]

I did not really get that. Static or time-varying FIR filter - MATLAB (mathworks.com), this page says to apply a dsp.FIRFilter object fir: y = fir(x) to get the output.

Answer 1

[Paul]

The input to fir() should be a column vector.

N = 40;
fs = 20 * 10 ^ 9;
freq = [0, 1 * 10 ^ 9, 2 * 10 ^ 9, 3 * 10 ^ 9, 5 * 10 ^ 9, 7.5 * 10 ^ 9, 10 * 10 ^ 9];
mag  = [1, 1,          1,          0.3,        0.1,        0.005,        0.001];
d = fdesign.arbmag('N,F,A', N, freq, mag, fs);
W = [1, 1, 1, 1, 1, 1, 1];
fir = design(d, 'equiripple', 'weights', W, 'SystemObject', true);
sents = ones(1, 300);% * amplitude;
rcvds = fir(sents.');
% plot
figure(1)
stem(sents)
hold on
stem(rcvds)
legend('sents', 'received')

[Kera Cai]

Thank you!