The filter was now tested using a square wave input at the center frequency of the filter.
It can be seen from the graph that when the filter is excited with a square wave at the center frequency, the components of the square wave not at the centre frequency are filtered out. As expected, the final output from the filter is a sine wave at the frequency f_0.
The Matlab Code from 11a can be seen here.
The filter was now tested using a square wave input at a frequency of f_0/3.
In this case, the graph shows that the fundumental frequency which is outside the passband of the filter is attenuated and filtered out. The 3rd harmonic however, passes through the filter unattenuated as it's frequency falls right onto the center frequency of the bandpass filter.
The Matlab Code from 11b can be seen here.