Number of TX/RX channels on a USRP

I’m new to the USRP and trying to understand the number of RF
input/output channels available (USRP1). From the GNURadio wiki
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
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.

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

I apologize for the long email!

Thanks,

Arya

On Sun, Jan 23, 2011 at 2:56 AM, Arya S. [email protected]
wrote:

I’m new to the USRP and trying to understand the number of RF
input/output channels available (USRP1). From the GNURadio wiki
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

explaining the DDC implementation, it is said:
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.

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

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.

Alex

Hi Alex, List,

On Sun, Jan 23, 2011 at 3:03 AM, Alexandru C. [email protected]
wrote:

Also from the USRP FAQ page
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.

Okay, there are a total of 4 pairs of IQ signals, each of the 4 real
outputs from the ADCs being converted to an IQ pair in the FPGA.

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.

Suppose we’re dealing with real signals alone, then we have 4 input
channels viz. RX_A_A, RX_A_B, RX_B_A, RX_B_B. My question is, are each
of these channels independent of each other in the sense that their
frequencies are different i.e are they 4 different RF signals? In
other words, on each daughterboard, can the circuitry be configured to
receive on 2 separate frequencies from the same antenna, and feed them
as inputs to each of the 2 ADCs its RX port is connected to?