Re: [Discuss-gnuradio] Sampling rate of USRP and data rate through USB

[Ryan Seal]

Lin Ji wrote:
> Hi,
>   There is one thing that I can not figure out:
>   The ADC on the USRP has a sampling rate at 64Msps, that is, for a 
> data at 1Mbps, for every incoming bit there will be 64 samples. 
> Suppose every sample is 16bits, the bit rate is 1024Mbps,that is a 
> huge amount of data flow. Now if a user want to same these samples to 
> file through USB, even though the USB 2.0 interface has a high 
> throughput at 480Mbps, it's not enough. If there is a buffer(like a 
> FIFO or something) that is handling the ADC to USB tranfer, sooner or 
> later this buffer will get full, how is this situation been handled?
>   Thanks.
> /Lin
>   
Data is decimated using a CIC filter implemented in the FPGA. Data is 
sampled on the front-end at 64MSPS and then the signal is filtered and 
decimated (sample rate is reduced by dropping a number of samples) to 
point that can be handled by the USB 2.0 bus (32 MBytes/Sec). If you 
have a complex 16-bit data stream, you have 16-bit In-phase, 16-bit 
Quadrature; therefore, you have 4 bytes per complex sample. It is stated 
that only 8 MSPS (complex samples) can be sent across the USB bus and 
this corresponds to 8 MSPS * 4 bytes = 32 MBytes/Sec. In other words, 
you have to decimate you front-end 64 MSPS signal by a minimum of 8and 
this gives you 8 MHz of bandwidth; of that, it is stated that only 6 MHz 
or so is usable due to wrapping/aliasing from the filter roll-off.

32 MBPS becomes 32 MBPS * 8 bits/byte = 256Mbps ; which is little more 
than 50% of the theoretical max.

--Ryan

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