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Re: [Discuss-gnuradio] usrp_sink_s and other quick questions

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] usrp_sink_s and other quick questions
Date: Mon, 21 May 2007 23:22:19 -0700
User-agent: Mutt/1.5.9i

On Mon, May 21, 2007 at 04:19:58PM -0400, Steven Clark wrote:
> If I need pin-level access to the DAC chip, sink_s is my only option AFAIK.
> I understand that it's interleaved I&Q, don't worry about that.
> My original questions remain.
> On 5/21/07, Eric Blossom <address@hidden> wrote:
> >
> >On Mon, May 21, 2007 at 10:48:04AM -0400, Steven Clark wrote:
> >> Hi all-
> >>
> >> Several quick questions.
> >> 1) For usrp sink_s, which bit of the short activates the MSB of the DAC
> >> chip? I.e., if I send 0bABCDEFGH IJKLMNOP, where each letter is either a
> >1
> >> or a 0, which letter matches the MSB?

What you sent to sink_s doesn't go directly to the DACs, but rather is
first interpolated and possibly upconverted.

See http://gnuradio.org/trac/wiki/UsrpRfxDiagrams

The DACs run at a constant 128 MS/s.  They are fed from the FPGA at
32 MS/s, and are interpolated by a factor of 4 in the AD9862.  See the
AD9862 datasheet for details.

The I & Q data written to sink_s should be 16-bit 2's-complement
host-endian data in the range -32768 to 32767.  The max and minimum
values that make sense actually depend somewhat on the associated RF
daughterboard.  Analog clipping may occur at values smaller in
magnitude than +/- 32,767.


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