Re: [Discuss-gnuradio] Alternative interface from PC to a USRP.

[Lamar Owen]

On Sunday 10 July 2005 06:20, Tim Ansell wrote:
> Forgive me if I'm wrong (I only kinda lurk, rather then participate),
> but wasn't the whole idea of Gigabit Ethernet to allow long cable runs?

Partially.  There are other advantages to GigE; but I'm kindof just mentioning 
alternatives.

> So that you can put the USRP close to your dish saving on expensive
> low-loss cables. (It also had the advantage of working over fibre optic
> which gave even longer cable runs and isolation.)

There are fiber cables for USB2 extension, but they are kindof expensive.  
GigE fiber converters are a few hundred dollars too.  SCSI over fiber is also 
available.

> I didn't think speed was really an issue.

The USRP is capable of sampling 256MS/s (64MS/s across four channels), so you 
need 256MB/s to not lose data, if you are interested in, say, doing a 
continuous spectrum capture across a 128MHz band.  We have an application for 
Radio Astronomy that involves interferometry where bandwidth is king; the 
researcher would love to get two polarizations times two frequencies times 
two antennas (8 channels) with 200MHz of bandwidth per channel; this is a lot 
of data, yes, but he claims that you can't have too much bandwidth doing what 
he is doing.  For that application, we have put in a grant proposal for an 
SRC MAPstation with 16 8-bit 200Ms/s ADC's.  The SRC MAPstation uses three 
high-speed (200MHz) Xilinx FPGA's to do massively parallel calculations; the 
interface to the PC is SRC's proprietary SNAP interconnect that plugs into a 
DDR-SDRAM socket on the PC motherboard.  They sell 'servers' with up to 128 
MAP processors and crossbar switching of the SNAP interconnect for hundreds 
of gigaflops of performance and up to 6GB/s I/O bandwidth for things like 
ADC's.  They are not cheap.

We were going to do algorithm development on a USRP prior to spending the 
hundreds of kilobucks for the MAPstation stuff.
-- 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772
(828)862-5554
www.pari.edu