Re: [Discuss-gnuradio] Why 4 ADC's?

[Eric Blossom]

On Thu, Jul 19, 2007 at 06:30:57AM -0700, Johnathan Corgan wrote:
> Jeff Brower wrote:
> 
> > The LFRX has two separate antenna inputs, supplied to Vin-A and
> > Vin-B.  These are not I-Q data.  USRP board FPGA logic would apply
> > one mixer or two (30 MHz or less).
> 
> [...]
> 
> > The RFX2400 mixes with a 2.4 GHz osc and creates one set of I-Q 65
> > MHz range signals, +/-I on Vin-A and +/-Q on Vin-B.  To get baseband
> > signals, the logic has to apply two mixers.
> 
> The FPGA DDC logic uses a single but *complex* mixer to do frequency
> conversion of the signal from the ADCs.  That is, it treats the data
> from Vin-A and Vin-B as a complex number in I and Q format, and
> multiplies this complex number by a complex sinusoid of a configurable
> frequency.  This is very different from having two mixers.

In fact the routing from the ADCs to the inputs of the DDCs is
controlled by the input mux in the FPGA.  You can route any ADC
into any DDC input, or you can stick a constant zero into the Q input
(details below).

The _default_ wiring depends on the software that controls the given
daughterboard.  By default the Basic Rx and LF RX daughterboard code
treats the two inputs as two separate real inputs (not I & Q), one
into DDC 0, one into DDC 1.  You can easily override this by calling
u.set_mux(...)  The RFX-* daughterboards treat the signals as I & Q,
since that's the only configuration that makes sense given the
quadrature output of the RFX-* boards.


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

  /*!
   * \brief Set input mux configuration.
   *
   * This determines which ADC (or constant zero) is connected to 
   * each DDC input.  There are 4 DDCs.  Each has two inputs.
   *
   * <pre>
   * Mux value:
   *
   *    3                   2                   1                       
   *  1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
   * +-------+-------+-------+-------+-------+-------+-------+-------+
   * |   Q3  |   I3  |   Q2  |   I2  |   Q1  |   I1  |   Q0  |   I0  |
   * +-------+-------+-------+-------+-------+-------+-------+-------+
   *
   * Each 4-bit I field is either 0,1,2,3
   * Each 4-bit Q field is either 0,1,2,3 or 0xf (input is const zero)
   * All Q's must be 0xf or none of them may be 0xf
   * </pre>
   */
  bool set_mux  (int mux);



Eric [ decloaking after vacation ;) ]