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?

On Fri, Oct 17, 2008 at 12:39 PM, Pablo M. [email protected]

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