Re: [Discuss-gnuradio] Re: Pulse-doppler RADAR wind profiler project and how to bypass interpolation, DAC filtering, etc.

[Nick Withers]

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: address@hidden
Web: http://www.nickwithers.com
Mobile: +61 414 397 446

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