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Cost-Effective Fast Data Acquisition, and the LT5506

I am working on a radio project in my spare time. The proposed architecture will use an LT5506 to bring a signal down to baseband, through some low-pass filters, then into a DSP chip for all the 'real' processing. I'd like to prototype this thing in pieces before I put it all onto one tiny board. So I have some questions for the group:

LT5506, 40-500MHz I/Q demodulator with built-in VGA

Have you ever used it?

Do you know if there's an eval board available (Linear doesn't appear to have anything, but I may not be navigating their site correctly).

Do you know of any alternatives? Something that had built-in low-pass filters would be tres cool.

Data Acquisition:

I'd like to buy (or build, if I have to) an eval board for the LT5506 and acquire chunks of data to feed to a PC. For this I'd need a data acquisition device that could sample two channels at no less than 10kHz;

50kHz would be nicer; 16 bits would be nice but I could live with 12 (for 8 bits or less I can use my 'scope). I've found one that's $400 or so for analog in/USB out -- is that the best I can do? Any suggestions will be appreciated.

Low-Pass Filters:

My thinking right now is that I'll just come out of the LT5506 into passive RC filters, oversample the snot out of the signals, and do a quickie filter-and-decimate before I do the complex signal processing. Anyone know of a really small low-pass filter I could use that would have better performance? The LT5506, naturally, has balanced outputs and its specifications go to hell if you don't use them as such, so any solution will have to accept a balanced input.

Thanks all.

--
Tim Wescott
Wescott Design Services
Voice: 503-631-7815
Cell:  503-349-8432
http://www.wescottdesign.com
Reply to
Tim Wescott
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The data acq part: just get a stereo USB sound port. 16 bits at

22.05ks/s should be easy. I have one that's 24 bits, and it wasn't anything like $400. You can get 16 bits stereo to 48k or 96k s/s for much less than $100 these days, even isolated. Nice thing about delta-sigma converters is that the antialias filtering is relatively easy. In a design I did some fifteen years ago, I used a five-pole LC filter implemented with little smt chokes and C0G caps that worked just fine* and allowed a wide range of output data rates without changing the filter, but typically you don't need anything like that sort of rolloff if you're sticking to a narrow range of clock frequencies.

Call your local Linear Technology rep for current eval board availability.

Cheers, Tom

  • I was expecting to be able to see some distortion from those little chokes, but distortion from the AAF was well below the 16-bit ADCs' distortion.
Reply to
K7ITM

Thanks. Someone else pointed out the soundcard part off list (d'oh). Linear Tech doesn't do eval boards for that part, I was hoping maybe someone knew of someone who does. I think I'm just going to do a quick-turn eval board myself -- it's one of those itty bitty packages with the grounding pad in the center, so it pretty much has to be reflowed to be useful.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/
Reply to
Tim Wescott

One way I've considered doing those heat pads on homebrew boards is to drill a large hole there, say to just clear a 10AWG wire, tin the heat pad, solder the part down, and finally slip a very short (e.g. 0.09") length of tinned #10 into the hole, heating it to reflow to the part, and solder around the the wire to the ground plane. Should work, but haven't tried it (yet). Such a short, fat wire is pretty low thermal resistance. You can also add some heat sinking on the back to pull heat away from the wire stub. Toughest part might be cutting the wire cleanly perpendicular to its axis. A reasonably clean cut followed by a bit of filing would do it, and I can imagine better ways if I were making several.

Cheers, Tom

Reply to
K7ITM

Sounds like you're thinking single-sided board. I was just going to pop for a quick turn house that delivers two-sided, plated-through holes. The price is not too bad, all things considered.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/
Reply to
Tim Wescott

Well, I generally do double-sided boards, but the bottom side on a board like that would be (mostly) ground plane/heat sink. The solid copper plug will conduct away a lot more heat than one (or a few) plated holes.

I do agree that the proto shops offer an economical and valuable service, but still do boards myself when they are simple and I want them fast (like an hour or less from the completed layout). Analog layouts generally don't need many vias, and I just fill those with

30AWG wire.

Good luck with your project. I have something somewhat similar going on myself at the moment.

Cheers, Tom

Reply to
K7ITM

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