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Re: [Discuss-gnuradio] Problem with USRP interpolation


From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Problem with USRP interpolation
Date: Tue, 01 Aug 2006 11:28:34 -0700
User-agent: Mozilla Thunderbird 1.0.8-1.1.fc4 (X11/20060501)

hanwen wrote:

Hi,
Recently, I'm trying to do CDMA commnication at 1.28Mchip/s with USRP and I've met a problem. The sample rate of DAC on USRP is 128M, so samples avialabe for each chip is 128M / 1.28M = 100. The interpolation rate of USRP must be [4, 512] and multiple of 4. So the only choice for me seems to be setting the software interpolation rate to 5 and the interpolation rate of USRP to 20 (multiple of 4). 5 * 20 = 100. But the software interp is still too high for the root_raise_cosin interp filter (interp=5, len(taps)=16), and a lot of "Uu" comes out. However, the software interp cannot be reduced further. I wonder if the sample rate of DAC can be change to mutiple of 2, not 4. In that case, I can set the software interp rate to 2 and interp rate of USRP to 50, for which the burden of interp filter can be reduced.

With the current FPGA build, the interpolation must be a multiple of 4. If you wanted to go to the effort, you could build your own version which would: - have the digital upconverter in the FPGA instead of using the one in the AD9862
- run the data to the AD9862 at 128 MHz instead of 64

Right now, I have to use 1.3333Mchip/s. The software interp rate of 2 works well. But 1.28Mchip/s is exactly what I want.

You might be better off using an external clock into the USRP which is a more convenient multiple.

Matt





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