Signal & System Parameters - Bandwidth Selection

[Erida]

Hi,
Still trying to figure out things with signals and systems... It doesn't go as well as I hoped so please help me...
I was checking out a computer program which allows you to specify the
parameters of any kind of signal going through any kind of system and
then provides you with the output.
I used a narrow bandpass filter in order to extract a single harmonic
from a pulse train with really narrow pulses. What I don't get is how
one should decide what bandwidth to use for the bandpass filter. Is the
fact that the pulses are very narrow or wide of any significance?

Thank you!


Erida

 

[berkeman]

Hi,
Still trying to figure out things with signals and systems... It doesn't go as well as I hoped so please help me...
I was checking out a computer program which allows you to specify the
parameters of any kind of signal going through any kind of system and
then provides you with the output.
I used a narrow bandpass filter in order to extract a single harmonic
from a pulse train with really narrow pulses. What I don't get is how
one should decide what bandwidth to use for the bandpass filter. Is the
fact that the pulses are very narrow or wide of any significance?
Thank you!
Erida

Will your program let you look at the frequency spectra of the input and output signals? That would be the most instructive thing for you at this point. The frequency spectra of a pulse train varies according to how wide the pulses are. If it's exactly a 50% duty cycle square wave, you will see only odd harmonics (fundamental, 3rd harm, 5th, harm, etc.), with the harmonics' amplitude falling off as 1/f. If the pulse train is not 50% duty cycle, then you will get some even harmonic energy, and the amplitudes will fall off with something other than a 1/f characteristic. Note also that there is less energy overall in the fundamental as you lower the duty cycle below 50%.

If you can do a Fourier transform or whatever on the waveforms in your software package, play with the duty cycle of the input waveform as you watch the results in the frequency domain. Then put a narrow BPF around the fundamental frequency, and watch the time domain and frequency domain results.