# Side Lobes in the Magnitude Response of a Low Pass FIR Filter

I have the impulse response (from the filter coefficients) of an FIR filter obtained from MATLAB's "interp" function using the command: [y, b] = interp(x, 5); % where b contains the interpolation filter coefficients. When I plot the magnitude response of this filter using "freqz" in MATLAB, I get the following What do those side lobes between 0.5 and 0.7 (x pi rad/sample) and 0.9 and 1 (x pi rad/sample) mean for my FIR filter? Why are they there?

• because your design put them here. We don't know how you've designed this, but that's the reason. – Marcus Müller Jul 18 at 21:12
• The filter coeffs I used were the coeffs returned by the MATLAB function "interp". This function is supposed to design a low pass filter to filter the interpolated signal but I don't understand why it has these extra side lobes apart from the main pass band. – BlueSweater Jul 18 at 22:19
• that would be critical information that should be in your question. Also, how exactly you called that function. – Marcus Müller Jul 19 at 8:51
• Thanks for the input! I edited my question. – BlueSweater Jul 19 at 18:23