# Sampling and spectrum replication

1. Sep 15, 2009

### likephysics

Why is the frequency spectrum of a sampled signal replicated?
We have a Low pass filtered signal sampled well above nyquist rate. I can't understand why the spectrum is replicated.

2. Sep 15, 2009

### vk6kro

I haven't done this, but I understand they assume that each waveform is the same as the next one and so they take one sample off successive waves but progressing across the wave in each case.
So, if they look at the incoming waves and look a little later at each wave, they build up a low frequency version of the waveform that follows the shape of the waveform reasonably well.

This is how sampling oscilloscopes work.

3. Sep 16, 2009

### mustafa_2009

can you explain at which frequency you got this well sampled signal, and what is your source may be it si also sampled as the same frequency that is why you get the same result.

4. Sep 16, 2009

### likephysics

mustafa, let me rephrase -
Why does sampling produce multiple spectrum at 1/T, where T is the sampling rate.
If you look at the sampled data spectrum is should look like this -
http://en.wikipedia.org/wiki/File:ReconstructFilter.png
Why are there spectrum at -fs and +fs?
I think this is bcoz the FT of an impulse train is also an impulse train spaced 1/T apart in the Freq domain.