PHYSICAL SIGNIFICANCE OF NEGATIVE FREQUECY

[KAMAL M KARDA]

kamal karda

hi,
i am stuck with understanding the physical importance of negative frequency.
can someone please give me a brief idea about it.
thanx[/quote]

 

[echo47]

You are probably use to thinking about simple sinewaves. Positive and negative frequency sinewaves look the same.

Now consider a quadrature wave. That's a pair of signals, one a sinewave, the other a cosinewave. Plot them on an X-Y graph and you get a dot that revolves around the origin either clockwise our counter-clockwise depending on positive or negative frequency. A zero hertz signal would be a motionless dot who's position clearly tells you the signal's magnitude and phase.

Such magnitude and phase are extremely useful information. For example, you can take a narrow-bandwidth radio signal, mix it down to zero hertz, and do fancy signal processing. It's usually much easier to do signal processing around zero hertz than at some oddball non-zero frequency.

Maybe grab youself a book on digital receivers, and try writing small programs that simulate what's going on. MATLAB is great for playing with this kind of stuff.

By the way, the Nyquist sampling theorm now works differently. You recall, for a simple sinewave you must sample at least twice the signal's bandwidth, or else you will lose information. But for a quadrature wave, you can sample at one times the signal's bandwidth.

After you work with this stuff for a while, you will begin saying to yourself "oh that makes sense!" more and more often.