Frequency Response: Inputs & Effects

[chlorine]

Hi everybody,

I had a simple question that I've never really thought about until I actually had to do it...

If you input a single one frequency signal, will you get a whole new different frequency response? How would it effect the system? Or would it make sense if you inputed like a white noise (all frequency) signal to get a good result of a system's frequency response? Thanks..

 

[berkeman]

It depends on the system. The simplest systems are Linear, Time-Invariant (LTI) systems. For LTI systems, no, when you put in a single frequency, the output can only be different in amplitude and phase, not a different frequency.

To get the full transfer function of an LTI system, you either need to sweep the input frequency and observe the gain and phase shift of the output response, or you need to put in an impulse, and infer the transfer function from the impulse response of the LTI system.

For non-linear systems, all bets are off.

 

[cepheid]

It's hard to interpret your question. If you send a single frequency (sinusoidal) signal into a linear system, the output will be a sinusoid whose amplitude and phase is determined by the frequency response. If you are trying to measure the frequency response, then I suppose you could send in sinusoids of various different frequencies and then plot the amplitude and phase of the resulting output to get a general idea. Or you could send in a single impulse (spike), and measure the output (the system's impulse response). The frequency response is then given by the Fourier transform of the impulse response.

Edit: they say great minds think alike berkeman, but I guess you beat me to it. :)

 

[berkeman]

Yeah, it's fun to work in pairs. I think the OP believes it now! Duplicate simultaneous answers.

I always wished there were semaphores or something in the reply mechanism... but where's the fun in that?

 

[chlorine]

HMM interesting. Thanks guys. You know, its hard to match the things you read in textbooks with everything in the real world. This gave me more insight.