# Change center frequency of I, Q values

Started by 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

```