Hi I think you will find that with the two DDS's at 90 degrees to each other you have sin(wt) and cosine(wt). That lets you do a multiply on each and ultimately get both the I and he Q signals you need.
Another way to look at it: If you have 0 and 90 degrees out of the two DDS's, that gives you 0, 90, 180, and 270 degrees by simple buffering or inversion. If you need something else it would have to be 45 degrees based ( 0,45,90, 135,180, .225, 270, 315). . Since it's the 90 degree differences that are "useful" there's no need for 45 degrees. I have seen places where they talk about +/- 45 degree phase shifts, but then they never use the zero degree phasor. I think that's a very confusing way to explain what's going on ... Bob On Jan 24, 2010, at 10:12 AM, Stephan Schaa wrote: > > Hello Bob! > > Yes, thats the reason why the the RXs with only one frequency generator > (like SDR 1000, or firefly) run at 4x RX frequency. They use the higher > frequency to generate the two signals with 90 degrees shift at 2x RX > frequency for I/Q generation. > > So in order to receive for example 30 MHz you would need to run the two DDS > Clocks at 60 MHz if you want to receive the complete signal without losses in > a system with two clock generatores with a 90 degrees shift. > > As the DDS Clock gets only up to 37 MHz I would think that > a) the DDS frequency is doubled somehow somewhere in the LD-1 or > b) the mixer works in "undersampling-mode" with a single balanced mixer. > (This would mean that there should be some loss in information, but I don't > know if that is a problem in digital reception...) > > Stephan > > > > > > Hi > > > The "normal" approach uses twice the frequency so it can generate the > quadrature LO by division. If you run two DDS's you don't have to get the > quadrature signals that way. You can have them each generate one of the two > required phases of the LO directly. I suspect that's why the radio will run > up to the top end of the DDS frequency range. > > Of course if the Nyquist frequency is 37 and you are at 36.9 you will have a > spur at 37.1 to deal with. It'll take a lot more than a third order filter to > do that. > > Bob > > > On Jan 24, 2010, at 5:48 AM, Stephan Schaa wrote: > >> > > >
