Alberto: Here's the post that I do not agree with:
"There is also something to be said about using IQ demodulation to get your sideband rejection ... " As I said previously, I-Q formats are being used in the SDR packages I know in order to get image rejection. Let's say your Softrock is tuned to 1840 kHz and a modulated carrier is present at 1850 kHz. We'll want to remove the image, as you and others successfully do. In the process, we aren't removing any sideband from the desired carrier. If a sideband is to be removed, you're doing so with your FIR afterwards. Now for the Costas part: you may of course multiply the 1850 kHz carrier with an in-phase and orthogonal carrier. By further phase shifting the orthogonal product by 90 degrees and then summing or differencing the two products, we will reject one sideband or the other. That's what I call true "sideband rejection" as opposed to "image rejection" or "sideband filtering". Clearer? Chuck >From: "i2phd" <[EMAIL PROTECTED]> >Reply-To: [email protected] >To: [email protected] >Subject: [soft_radio] Re: Softrock as receiver back end >Date: Wed, 31 Jan 2007 22:15:54 -0000 > >--- In [email protected], "CHARLES HUTTON" <[EMAIL PROTECTED]> wrote: > > > > Have I missed something? I don't see any relationship between I-Q > > and sideband suppression. I do see that Rocky - WinRadio - etc. are > > using the I-Q data to achieve image suppression. True sideband > > suppression would involve using a Costas Loop, which I don't see in > > any of the packages. > > > > Chuck > > Chuck, > > there is a bit of confusion on what we are talking about. The >previous three or four messages were related to the crisper quality >the absence of crystal filters, typical of a DC receiver, can give to >the audio. > >The issue of image rejection is a separate one. I don't fully >understand you when you say that "true" sideband suppression can be >done only by using the Costas loop... can you please explain in what a >"true" image suppression done with the Costas loop differs from the >image suppression done by a software program that extracts the >information from a phasor described by its two rotating components I >and Q ? > >73 Alberto I2PHD > >
