On Mon, 2009-02-23 at 07:45 -0800, Eric Blossom wrote: > On Mon, Feb 23, 2009 at 07:43:08PM +1100, Nick Withers wrote: > > Just resending as this didn't seem to get through to the list... > > > > Presumably I must be subscribed to post? > > Yep. > > > On Tue, 2009-02-17 at 20:37 +1100, Nick Withers wrote: > > > Hi all, > > > > > > I'm new to GNURadio / the USRP, but am working on a project, as part of > > > my honours for a Bachelor of Engineering degree, to implement a > > > pulse-doppler RADAR wind profiler (some information at > > > http://nickwithers.com/usrp-wind-profiler). > > > > > > If anyone's interested, perhaps doing something similar, or with tips > > > they can offer off the top of their noggin I'd love to hear from you :-) > > Looks interesting. Good luck with it! > > > > I'm basing my work off Johnathan Corgan's radar-mono code (thanks > > > Johnathan!) and have a couple of questions I was hoping someone could > > > point me in the right direction with (dare say I'll be getting referred > > > straight to the FAQ, it's all a bit overwhelming!). > > > > > > I want, if possible, to bypass the DAC's interpolation, filtering etc. > > > (the chief reason being to avoid the 44 MHz upper-bound on signals I can > > > output from it (I'm using the Basic{T|R}X boards and aiming to work > > > around 70 MHz), > > OK. > > > see http://www.gnuradio.org/trac/wiki/UsrpFAQ/DUC > > > (incidentally, I've done a crap job of finding supporting information in > > > the AD9862 data sheet)). > > It's a complicated part. And there are a few bugs in the data sheet > (mostly mislabeled diagrams). I don't have mine in front of me, but > look for the section on configuring the DAC. There are many diagrams > showing possible configurations, depending on whether you want real or > complex output. We've got it configured for complex output so that we > can use the digital upconverter.
Which I want (have?) to use to, to get to 70 MHz. > They call it the coarse and fine > modulator. We use both features. You can have it interpolate less on > chip (we use 4), but then you've got to feed it data at a faster rate, > and the filtering will be worse. We drive the The two DACs with two > streams at 128 MS/s. (Without looking at the data sheet and/or the > code, I don't remember how we do that. I think it's interleaved, but > it may be pumped on opposite clock edges.) > > Can you say more about your requirements? E.g., what RF range are you > trying to access? Center and bandwidth of your desired signal? Is it > centered at 70MHz or is 70MHz the top? Will you be using an external > upconverter? (I'm clueless about what RF range is used for wind > measurement.) For my part I want to transmit and receive at 70 MHz (externally it'll be mixed up/down to/from 1280 MHz). I've got a 5 MHz to play with, with should be more than enough, I believe. I've been told I need to do coherent detection to maximise the SNR (which'll be really, really low, circa -40 dB, owing to the returned signal being bounced off air...). I'm currently trying to set up a CORDIC mixer in the FPGA between 26 MHz and 32 MHz so that I can throw this to the DUC and have it produce a 70 MHz output wave. There's an external analog 5 MHz filter at the transmitter output. > > Do I need to rewrite the C++ code (e.g., > > > usrp/host/lib/legacy/usrp_standard.cc) if I want to continue to use > > > usrp_source_?() / usrp_sink_?() and such from Python? Any tips on how I > > > go about doing it? Can / should I just rewrite the DAC configuration > > > from within the FPGA when it's initialised? > > Depending on how much you're changing around the DAC configuration, > you may be able to do all the AD8962 reconfiguation from python on the > host. usrp_basic has _write_9862 and _read_9862 methods that are > exported to python. You can probably reuse the usrp_basic and maybe > usrp_standard, depending on your situation. The stuff you don't need > you can ignore. Good stuff, cheers. I was a bit worried about using these low-level functionality methods after initialising the standard code. > Johnathan may have additional comments... You've both been very helpful! > Eric -- Nick Withers email: [email protected] Web: http://www.nickwithers.com Mobile: +61 414 397 446
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