Hi All, I use a samplig card with DDC(Digital Down Conversion with quadrature mixer and filter to eliminate frequency at 2*fc) that provides me directly I, Q values of the signal. As the demodulation is performed offline, with the recorded point, the values are not centered around the carrier of interest. I need to track the center frequency of the carrier during the demodulation to compensate important variation of doppler. I have already the Costas Loop module programmed, but I don't how to make the conversion of the I and Q to another frequency without creating new 2fc composants, that I will need to filter later. How can I center the I, Q by calcultation around the desired center frequency, by calculation (I can't change the LO of the DDC) ? Thanks for your help, Raphael
Change center frequency of I, Q values
Started by ●August 9, 2006
Reply by ●August 9, 20062006-08-09
You multiply your I,Q values by e^jwt where w is the delta angular frequency by which you want to shift the current spectrum. w can be either positive or negative depending on the direction of shift. Cheers Bhaskar "Raphael31" <raphaelsanchez31@gmail.com> wrote in message news:1155159206.586420.132730@75g2000cwc.googlegroups.com...> Hi All, > I use a samplig card with DDC(Digital Down Conversion with quadrature > mixer and filter to eliminate frequency at 2*fc) that provides me > directly I, Q values of the signal. > > As the demodulation is performed offline, with the recorded point, the > values are not centered around the carrier of interest. I need to track > the center frequency of the carrier during the demodulation to > compensate important variation of doppler. > > I have already the Costas Loop module programmed, but I don't how to > make the conversion of the I and Q to another frequency without > creating new 2fc composants, that I will need to filter later. > > How can I center the I, Q by calcultation around the desired center > frequency, by calculation (I can't change the LO of the DDC) ? > > Thanks for your help, > > Raphael >
Reply by ●August 9, 20062006-08-09
Raphael31 wrote:> Hi All, > I use a samplig card with DDC(Digital Down Conversion with quadrature > mixer and filter to eliminate frequency at 2*fc) that provides me > directly I, Q values of the signal. > > As the demodulation is performed offline, with the recorded point, the > values are not centered around the carrier of interest. I need to track > the center frequency of the carrier during the demodulation to > compensate important variation of doppler. > > I have already the Costas Loop module programmed, but I don't how to > make the conversion of the I and Q to another frequency without > creating new 2fc composants, that I will need to filter later. > > How can I center the I, Q by calcultation around the desired center > frequency, by calculation (I can't change the LO of the DDC) ? > > Thanks for your help, > > RaphaelJust complex-multiply the DDC output signal by exp(j*2*pi*f*n/Fs), where f is the frequency that you want to shift by in Hz and n is the sample index. This won't create any unwanted components since it is a complex mix. John