[Discuss-gnuradio] Questions on Tx path

[jjw]

I have been reading about how the Tx path works in the USRP and was slightly
confused.   I know that some of the interpolation is done in the FPGA and
some of it is done in the DUCs on the external chips.  Noting the following
code from class usrp_standard_tx:

  /*!
   * \brief Set output mux configuration.
   *
   * <pre>
   *     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
   *  +-------------------------------+-------+-------+-------+-------+
   *  |                               | DAC3  | DAC2  | DAC1  |  DAC0 |
   *  +-------------------------------+-------+-------+-------+-------+
   *
   *  There are two interpolators with complex inputs and outputs.
   *  There are four DACs.
   *
   *  Each 4-bit DACx field specifies the source for the DAC and
   *  whether or not that DAC is enabled.  Each subfield is coded
   *  like this:
   *
   *     3 2 1 0
   *    +-+-----+
   *    |E|  N  |
   *    +-+-----+
   *
   *  Where E is set if the DAC is enabled, and N specifies which
   *  interpolator output is connected to this DAC.
   *
   *   N   which interp output
   *  ---  -------------------
   *   0   chan 0 I
   *   1   chan 0 Q
   *   2   chan 1 I
   *   3   chan 1 Q
   * </pre>
   */ 

It seems that there is only a possible setup for 2 transmit paths.  This has
me wondering a few questions.  Is there any way to transmit more than two
paths at once?  How many CIC interpolators are there in the FPGA (2 or 4?) 
I'm assuming that each CIC gets complex data from the USB and also outputs
complex interpolated baseband data to the DUCs where it is taken to IF. 
Does the mux do it's work after the CICs but before going out to the DUCs on
the external chip or before the CIC?  Thanks for any help, I know these must
seem like stupid questions. 

John
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