Understanding Real-valued signals / Negative Freq.

[haxorcize]

Hi,

I can't seem to understand two basic ideas:
1. I read many books and they related to both complex-valued and real-valued signals, saying the Imaginary part is non-realizable. I don't understand what that really means.
As far as I understood, the use of imaginary numbers is only so we pack a signal with the I and Q together. However, in the books they refer to Baseband complex, and Passband real value.

2. How come a spectrum can have negative frequencies? Why do we consider them in baseband, but not in passband?
What does it mean a negative frequency anyway?

[charan langton]

Please think of Imaginary signal and real signal as two independent signals. They are both "real".

A complex signal is made of two independent signals, a I signal and a Q signal. We can manipulate each individually. But we do not transmit complex signals, we transmit real signals, this is what is called passband. It is a real signal which contains within it (added) both I and Q signals.

We usually only have the signal split into I and Q when it is inside hardware at the source, or what is generally called baseband, although I am fuzzing the ideas a bit. Baseband actualy doen not mean that the signal is complex. It just means it is at its source frequency, or usually a low frequency. However, to turn this signal into a passband signal, it needs to be up-converted to a carrier frequency. This process is done by converting the baseband signal into two projections, a I and a Q signal which are multiplied by cosine and sine and then added together to create a passband signal centered at a carrier frequency.

So passband and baseband do not directly have anything to do with real and complex. Those two terms just indicate a source signal and a modulated signal. However, passband is generally a real signal.

So these are just some thoughts to help you understand.

Charan Langton

[haxorcize]

Thank you!
Your explanation helped a lot!