# How can I move a signal in frequency so that it ends at the frequency zero?

I am trying to move a signal to 0 in the frequency domain but I am not sure how to do it.

• What do you mean by "move"?
– jojeck
Commented May 26, 2015 at 11:13
• I want the signal to be at 0. So I'd be "moving" them or shifting them from 0.25 and -0.25 to 0. Sorry if my explanation isn't that good. Commented May 26, 2015 at 11:16
• You want your signal to be a DC? What are you trying to achieve cause it sounds dodgy to me.
– jojeck
Commented May 26, 2015 at 11:18
• Well, the signal contains this picture (imgur.com/DnKpz5t), after filtering out the others, using a bandpass filter. . And I don't know what's going wrong. My teacher mentioned that I should try and see what happens when I move the signal to the zero frequency. Commented May 26, 2015 at 11:25
• I think he means moving the frequency of the bandpass filter. Why do you think this image is wrong? how is the input image? how do you expect the output image to be? Commented May 26, 2015 at 11:40

## 1 Answer

One way of moving a frequency response of signal is multiplying it with a sinusoid. Assume x(t) is your signal.Let the sinusoid be $$cos(w_0t)$$ If I multiply this to x(t) as $$y(t)=x(t)cos(w_0t)$$ Then Fourier transform of y(t) is $$Y(w)=0.5(X(w-w_0)+X(w+w_0))$$ This is because multiplying in time domain of two signals is equivalent to its convolution in frequency domain.Since fourier transform of the above sinusoid is$$C(w)=\frac{\delta(w-w_0)+\delta(w+w_0)}{2}$$Due to the sifting property of delta function we get the above Y(w) after multiplying.
Although here we get two signal the required one can be obtained by passing it through band pass filter