[Discuss-gnuradio] Number of TX/RX channels on a USRP

[Arya Santini]

Hi GNURadio,

I'm new to the USRP and trying to understand the number of RF
input/output channels available (USRP1). From the GNURadio wiki
(http://gnuradio.org/redmine/wiki/gnuradio/UsrpFAQIntroMobo), I gather
we have 4 possible TX and RX channels each, since we have 4 ADC's and
4 DAC's on the motherboard:

"..in principle, we have 4 input and 4 output channels if we use real
sampling. However, we can have more flexibility (and bandwidth) if we
use complex (IQ) sampling. Then we have to pair them up, so we get 2
complex inputs and 2 complex outputs."

This means we could have only 2 complex TX/RX channels.

Also from the USRP FAQ page
(http://gnuradio.org/redmine/wiki/gnuradio/UsrpFAQIntroFPGA), in
explaining the DDC implementation,  it is said:

"Note that when there are multiple channels (up to 4), the channels
are interleaved. For example, with 4 channels, the sequence sent over
the USB would be I0 Q0 I1 Q1 I2 Q2 I3 Q3 I0 Q0 I1 Q1, …etc. In
multiple RX channels (1,2, or 4) , all input channels must be the same
data rate (i.e. same decimation ratio)."

Do these multiple RX channels (1,2 or 4) refer to those coming out of
the ADC's? Then contrary to the aforementioned, how come now there are
4 complex channels, I0,Q0 - I3,Q3 coming in. when it should be
possible to have only 2 complex RX channels? Looks like I'm
understanding the architecture incorrectly!

Some help here would be greatly appreciated.

Also, on the daughterboard, each RX port has access to 2 ADC's. So in
the context of having 2 RX channels possible per daughterboard (in the
case of Real sampling, and total 4 for the system) are the inputs
RX_A_A, and RX_A_B separate/independent? I mean, does the RF
daughterboard circuitry receive two frequencies simultaneously on a
single receive antenna?

I apologize for the long email!


Thanks,

Arya