FW: [Discuss-gnuradio] Slashdot Q & A / Beginning of our FAQ

[Matthew Burnham]

Naively, I've just had a look to see what RS Components do in the way of
ADCs... and found that there's quite a few available with sample rates
in the region of 10MSPS for around UKP20 to UKP30 and some with 20MSPS+
for a little more (around UKP40) for single units. Would it not be
possible to come up with a cheap alternative to the Measurement
Computing board? I assume the minimum spec necessary with an I/F of
5.75MHz is 11.5MSPS?

Just a thought, which I'll look into further, but wondered if anyone had
considered this? I know it could get 'interesting' designing such
high-speed digital circuits... but I've got a Wavefinder Digital Radio
in front of me which squirts a considerable amount of data down the USB
port so it can't be THAT difficult. Has anyone else considered this
previously?

Perhaps as a starting point some lower-speed stuff could be done, if we
got a lower I/F than the cable TV tuner, with lower bandwidth, we'd need
a correspondingly lower ADC (maybe even something reasonably cheap
off-the-shelf?) which would allow those not able/willing to get the
Measurement Computing board to get into wide-band-ish stuff (for
examaple, be able to play two FM stations, provided they were much
closer to each other).

> The other path I'll call "wide band".  This is personally the
> area that I find most interesting because it is with wide 
> band that you are
> able to do things that you can't do with a conventional radio.   Chief
> among these is the ability to concurrently receive (or 
> transmit) multiple channels/stations/frequencies.  In the 
> examples directory of the GNU Radio code, you'll find an 
> example that receives and demodulates 2 FM broadcast stations 
> and puts one out the left channel and one out the right.  
> Matt Ettus, another GNU Radio developer, has built a demo 
> that receives 4 narrow band FM channels concurrently. These 
> demos run fine on a 1800+ Athlon, or 1.7 GHz P4.
> 
> For the wide band stuff our "standard configuration" is a TV
> tuner module designed for cable modems that tunes from 50MHz 
> to 890MHz with an IF of 5.75 MHz.  The module is a Microtune 
> 4937 DI5.  We connect the output of the tuner directly to a 
> 20M sample/second 12-bit A/D converter.  The converter we're 
> using is the Measurement Computing PCI DAS4020/12.  It'll do 
> 4 channels at 10M sample/sec or 2 channels at 20M 
> sample/second.  From the hobbyist's point of view, it's not 
> cheap, about $1300, but it is the cheapest, fastest off the 
> shelf solution that we found.
> 
> With our "standard configuration" we ought to be able to
> handle IS-136.  GSM would be possible if our RF front end 
> would cover the 
> 1.9 GHz range.  Vanu, Inc has a GSM receiver running on a 
> 1GHz pentium laptop, so we know it's possible.