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

[Andy Green]

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On Tuesday 27 May 2003 17:48, Joseph DiVerdi wrote:

Hi Joseph -

> I have been working on using GNU-radio for some fairly specific
> scientific applications and although I cannot speak to the
> "completely generic" uses can offer some thoughts.
>
> IMHO, offering two channels of simultaneously captured input and of
> simultaneously updated output are rather important. This presages

Right - in fact for the intended use of the board the data is also 
balanced and in quadrature.  So two parallel channels are clearly 
needed.

> My particular application favors 200kSample/second (complex!)
> continuously (although I'm currently getting by with a dual
> channel, 24kSample/second sound card). This is by no means

Its easy enough to cut down the sample rate, the difficult choice is 
in deciding which ADC/DAC to use so that its cheap enough for 
'slower' uses like yours but fast enough to capture data from the 
most desirable uses.

What is the minimum resolution your application can live with?

> representative of the rest of the world and I suspect that you will
> learn from the group feedback that there are several natural
> popularly required bandwidths (and hence sampling rates) which will
> guide the design.

10.7MHz IF seems to have been discussed a lot.  This is unfortunately 
a lot of data, in quadrature at 12 bit resolution it is 
32.1MBytes/sec.  We can look an USB2 Isochronous mode which can 
probably cope with this throughput, although I understand that PCI 
bugs start to surface in some chipsets if you push your luck with the 
throughput.  Its a lot of data for the CPU to process in a meaningful 
way in realtime.  Has anyone else looked at attempting this at that 
datarate?

> Based on experience I always favor differential signal input and
> output, especially when operating at baseband. The quality of the
> data is more often assured over single-sided signals. There exist
> plenty of modestly priced, instrumentation amplifiers which would
> provide a high impedance front-end to the input. Providing space
> for a soldered or push-pin terminating resistor will afford your
> users with the option of their favored value.
...
> Additionally, the differential connections obviate the need for
> transformers and offer down-to-DC input and output.

Just the kind of advice I am looking for, thank you.

- -Andy
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