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

[Eric Blossom]

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.  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.)

> 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.

Johnathan may have additional comments...

Eric