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Change center frequency of I, Q values

Started by Raphael31 August 9, 2006
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

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 >
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, > > Raphael
Just 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