Electrical – Can a filter change the frequency of the input signal

band passcutoff frequencyfilter

I am given a square wave with Vmin = 0V and Vmax=5V , T(period)=8ms and duty cycle 25%. I am asked to find the properties(low cutoff , high cutoff , bandwidth) of a band pass filter that will make the signal a sine with T=4ms. My answer would be that the Δf = 1/Δt where Δt=4ms . So the Δf=250Hz and therefore the actual value of lowcutoff and highcutoff doesn't matter as long as their difference is 250Hz. It was pointed to me that this is wrong , I don't understand why. What would be the right answer to this problem?

Best Answer

You can express the pulse train as a sum of sine waves. The second harmonic is at 250Hz. Your bandpass filter needs to be centered at 250Hz with a bandwidth sufficiently narrow to block the higher harmonics and the fundamental.

Here's a crude LTSpice simulation. Rough simulation