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Re: [Discuss-gnuradio] Re: what is the rate(sampling) of the impulse res

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Re: what is the rate(sampling) of the impulse response using usrp_sounder.py ?
Date: Sat, 6 Dec 2008 12:26:07 -0800

On Sat, Dec 6, 2008 at 12:09 PM, Bruhtesfa Ebrahim <address@hidden> wrote:

> but, does the USB support this much sampling rate of 32 MHZ?


>  Also on simple gnu radio user manual, about gnuradio sounder it says :
> "The sounder uses a custom FPGA bitstream that is able to generate and
> receive a sounder waveform across a full 32 MHz wide swath of RF
> spectrum; the waveform generation and impulse response processing occur
> in logic in the USRP FPGA and not in the host PC. This avoids the USB
> throughput bottleneck entirely."
> This implies that the processed impulse response that goes to the USB is
> of lower sampling rate than 32MHZ. or no? please correct me if i am
> wrong.

The custom FPGA code performs the correlation in the USRP.  The
algorithm is a sequential correlator that processes an entire PN code
sequence per impulse reponse lag time.  So the data rate coming out of
the USRP is 64 Msps (ADC rate) divided by 8190 samples per PN sequence
(default degree of 12), or about 7.8 ksps.  Since each record is 4095
samples long, this outputs a full impulse response record about twice
a second.

The impulse response record still represents the original 32 Mchip/sec
correlation, so in your received data, each sample represents a lag of
about 31.25 ns.


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