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

From: Alexandru Csete
Subject: Re: [Discuss-gnuradio] Number of TX/RX channels on a USRP
Date: Sun, 23 Jan 2011 12:03:39 +0100

On Sun, Jan 23, 2011 at 2:56 AM, Arya Santini <address@hidden> wrote:
> 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:

Hi Arya,

Thanks for reading the doc - it makes explaining easier :-)

> "..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!

This multiplexing refers to the data stream on the USB interface, i.e.
after FPGA processing where the real signal is converted to
quadrature/complex signal. So the 4 RX channels that use real samples
are converted to complex signal in the FPGA.

> 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 am not entirely sure about this so someone may correct me if I'm wrong...
The LFRX and BasicRX are not quadrature receivers so the use real
sampling and conversion to quadrature signal is done in the FPGA. You
can however also chose to input quadrature signal in which case you
use RX_A_A and RX_A_B as I and Q input. This works in the two receiver
configuration. If you use the 4 receiver configuration then 2
additional receivers have to be mounted on the TX_A and TX_B slots.
Therefore, if we chose to use 4 receivers we can not have transmitters
at the same time.


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