Talk:Downconverter

Does the author of this page realize that it is physically impossible to remove negative frequency components of the signal? Any real physical quantity will have its negative frequency spectral content equal to the complex conjugate of the positive frequency spectral content. The power spectrum is even (mirrored) about DC, while the phase spectrum is odd (sign reversed).

Downconverting works by mixing the incomming RF with the local oscillator (LO), thus shifting both the positive and negative frequency content both up and down. Thus there would now be four bands (two of positive frequency, and two conjugate bands), unless the LO was centered in the initial RF band. The former case results in content around an interim frequency (IF), along with the mirror band at a much higher frequency. The latter case results in a DC centered band, along with the mirror band centered much higher in frequency. In either case (IF centered or DC centered), a low pass filter will remove the unwanted spectra.

DV


 * It is possible to reject the negative frequency component using a Hilbert transform.


 * Note also that if the LO is complex, the frequency content moves in one direction only (toward -ve infinity). Oli Filth(talk&#124;contribs) 19:52, 14 September 2009 (UTC)