USRP 2920: the IQ components is IF signal or baseband signal?

[nkyang]

This is the USRP 2920 block diagram and explanations in the USRP Help document.

The explanations:

• The mixer downconverts the signals to the baseband in-phase (I) and quadrature-phase (Q) components.
• The digital downconverter (DDC) mixes, filters, and decimates the signal to a user-specified rate.

I'm confused about the signals after Mixer is zero-IF or not. If the I/Q components are baseband signals, that is, they are zero-IF signals, why the DDC mixes the signals again?  

[psisterhen]

The signals after the mixer are (when combined as I and Q) complex baseband signals, not IF.  The DDC in the FPGA serves three main purposes:

• decimation: e.g. reducing the sample rate from the ADC rate (100 MS/s) to whatever rate you want (100/N)
• filtering: filtering out signal beyond the bandwidth of your decimated rate
• frequency shift: applying any additional frequency shift, within the 40 MHz bandwidth

Let's say you acquired a 40 MHz signal around 1 GHz, but you want to zoom in on a specific component at 1.05 GHz.  You would let the DDC freq shift 50 MHz and decimate to give you just the signal in that sub-band that you want.

The DDC/DUC frequency shift can also be helpful to move your signal away from DC, where the LO feedthrough may be a problem.