The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> complex filter
https://designers-guide.org/forum/YaBB.pl?num=1210062381

Message started by aaron_do on May 6^th, 2008, 1:26am

Title: complex filter
Post by aaron_do on May 6^th, 2008, 1:26am

Hi all,

second question of the day is about the complex filter. From my understanding, all real signals (sine, cosine...) can be split into positive and negative frequencies. A complex filter uses the combination of I and Q signals to reject the positive or negative half of the spectrum.

So my question is, what comes out at the other end? If the output has no negative half, then it must be a complex signal. So what would we see on the oscilloscope?

As a sub-question, is it possible to realize a signal with only a positive or only a negative frequency?

thanks,
Aaron

Title: Re: complex filter
Post by pancho_hideboo on May 6^th, 2008, 1:51am

aaron_do wrote on May 6^th, 2008, 1:26am:

So my question is, what comes out at the other end?

If you observe only Isig or Qsig, this signal has both negative and positive frequency components.
If Isig+j*Qsig has only positive frequency spectrum, Qsig will be Hilbert transformation of Isig.

aaron_do wrote on May 6^th, 2008, 1:26am:

As a sub-question, is it possible to realize a signal with only a positive or only a negative frequency?

Yes, it is possible as far as you treat signal as complex signal Isig+j*Qsig.