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Re: [Discuss-gnuradio] USRP data rates with a suggestion

From: Eng. Firas
Subject: Re: [Discuss-gnuradio] USRP data rates with a suggestion
Date: Fri, 10 Aug 2007 00:54:50 -0700 (PDT)


I have an important suggestion:

The USRP ADC resolution is 12 Bits. These 12 Bits are converted to 32 bits
(16 bits I and 16 Bits Q) after being processed by the FPGA DDC. However, if
we reduced the output bit width of the DDC to 12Bit I and 12 Bit Q (or
simply rotate right the resultant 16 bits I/Q by 4), then we packed and
transfered the resultant 24 bits  into a 3 bytes only (instead of 4 in the
case of 16bit complex), then we can have an instantaneous bandwidth of
10.67MHz (32M/3) with 12 bits I/Q which is in my opinion will be optimum in
the sense of ADC resolution and the obtained instantenouse bandwidth. 


Lisa Creer wrote:
> On 8/9/07, Johnathan Corgan <address@hidden> wrote:
>> It is possible to retain only the upper eight bits of each sample and
>> pack these into two bytes such that the USB can carry 16 Msps complex;
>> this gives you more passband bandwidth at the expense of dynamic range.
> Now that I know it can be done, how do I retain the upper eight bits of
> each
> sample?
> Is there code already written to do this? I'd like to maximize the
> passband
> bandwidth.
> Advanced applications of the USRP involve re-synthesizing the FPGA
>> firmware and performing DSP operations directly on the samples coming
>> from the ADCs before handing off the data to the USB.  This
>> theoretically gives you access to the entire 64 MHz receive bandwidth;
>> however, there isn't a great deal of space on the FPGA for custom code,
>> and the expertise/development time/debugging/pain involved is much
>> higher than just using GNU Radio on the host.
> I'll spare myself the pain and be content with a 16 MHz passband
> bandwidth.
> You have been most helpful. Thank you.
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