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[Discuss-gnuradio] USB2 <-> fast ADC & DAC

From: Andy Green
Subject: [Discuss-gnuradio] USB2 <-> fast ADC & DAC
Date: Tue, 27 May 2003 15:58:50 +0100
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Hi folks -

My friend Milosch and I are interested to make a generic USB2 <-> fast 
ADC and DAC board, based around a Cypress FX2 chip, a FIFO and a fast 
ADC and/or DAC.  We are both experienced engineers and expect no 
difficulty with the digital side.

This is intended to be a completely generic USB-powered baseband 
module with all processing done back at the PC in realtime.  Milosch 
did some tests and found it was possible to transfer 20MBytes/sec 
like this with latencies of under 2mS over USB2.  It is this transfer 
rate which is the limiting factor, not the conversion rates on the 
ADC or DACs.

We have a use for this design already, but we are interested to see if 
with some guidence from yourselves it can be made to be of use to the 
gnu radio project too, since it will be cheap, small and selfpowered.  
Some areas that we would welcome opinions on:

 - I assumed that a baseband-only device like this is in fact useful 
for this project.  However I see existing designs are interested in 
outputting modulated RF at ~30MHz via an FPGA or DSP.  Is a 
baseband-only device even useful?

 - What resolution and speed ADC is needed to be considered useful?  
We can transfer up to ~10Msps at 16 bits simplex, or ~5Msps at 16 
bits duplex (ie, simultaneous tx and rx).  The choice of ADC and DAC 
specs has obvious impact on the cost.

 - What kind of analogue IO is most useful?  1Vp-p 50ohm input and 
output?  Transformer coupled?  Filtered?

 - I saw on the list AGC mentioned, is this desirable on this kind of 
module to have programmable attenuation or amplification?

 - Has this kind of Native Signal Processing ((c)Intel 199x) at these 
kind of datarates been tried before?  Considering that on a modern PC 
SSE2 or MMX is available, what are the opinions about being able to 
have the CPU process data in realtime for common modulation schemes?

Any advice is welcome :-)

- -Andy
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