Forum: GNU Radio About OFDM modulation

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Pablo M. (Guest)
on 2008-10-17 20:40
(Received via mailing list)
Hi!
I am working with an OFDM modulator for digital radio. I use a
4096-point
IFFT implemented in a FPGA, and because this is my sampling frequency
4.096MHz. At the final of my system i need to obtain an analog signal.
The
problem is that my DAC works with an 80MHz clock. I can interpolate my
signal (originaly at Fs=4.096MHz) to 81.196MHz but not to 80MHz. Then my
question are:

What problems might I have in the case of using the DAC at a frequency
of
80MHz?
Brian P. (Guest)
on 2008-10-17 20:51
(Received via mailing list)
On Fri, Oct 17, 2008 at 12:39 PM, Pablo M. 
<removed_email_address@domain.invalid>
wrote:
> Hi!
> I am working with an OFDM modulator for digital radio. I use a 4096-point
> IFFT implemented in a FPGA, and because this is my sampling frequency
> 4.096MHz. At the final of my system i need to obtain an analog signal. The
> problem is that my DAC works with an 80MHz clock. I can interpolate my
> signal (originaly at Fs=4.096MHz) to 81.196MHz but not to 80MHz. Then my
> question are:

Which DAC?  Are we talking about a USRP, USRP2, other?

> What problems might I have in the case of using the DAC at a frequency of
> 80MHz?

Are you supplying the sample clock of 80MHz for samples which are
supposed to be at 81.196MHz?  Is 80MHz the maximum sample rate for the
DAC?  There seem to be a lot of questions left unanswered.

Let me propose a solution - use a resampling filter to get to a time
base which makes sense, then do the proper upconverstion on your
signal.

In your case, you have a 4.096Msps signal which you want to get to
80MHz.  Maybe you want to have an integer upconversion of 10 or 5?  In
that case, you want 4.096Msps to really be 8Msps or 16Msps.  To get
there, you re-sample your 4.096Msps signal by a rate 125/64 or 125/32
(respectively).  Follow this by your up by 10 or 5, and you have an
80Msps signal running into an 80Msps DAC.

A quick Google search for resampling filter comes up with this FAQ:

    http://dspguru.org/info/faqs/multrate/resamp.htm

Hope this helps!

Brian
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