--- In [email protected], "CHARLES HUTTON" <[EMAIL PROTECTED]> wrote: > > 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?
Well Chuck, it is clearer now that you have a false idea on how a software package implementing an SDR works... the general method (there are others) to suppress the image in a software program is not that of brute-force filtering out what is not wanted... what is mostly used is a Hilbert transformer that further shifts of another 90 degrees the Q component, then you add or subtract I and the re-shifted Q to select the upper or the lower sideband. This method is conceptually the exact equivalent of the Costas loop... 73 Alberto I2PHD
