[Discuss-gnuradio] Multichannel systems FAQ

[Matt Ettus]

Many people have asked this about the USRP, so I am posting this to the
list.

> - looking at the specifications for this USRP, I see it has 4 channel
>  receive.  Are these channels configured in a way that the samples
> are collected at the same time, or in a way that the phase
> relationship between the channels is known and remains fixed?

Yes, all samples are synchronous.  There is one master oscillator.

> - you offer a bunch of receive "daughter boards";  are the LO's on
> these boards synchronous, or can they be synchronized in some way
> (phase differences again known and fixed)?  How sensitive are these
> boards? Can one feed in 4 separate RF channels to the USRP?

Depends on which daughterboard you are using.  You can use 2 RX
daughterboards at a time.

LFRX (DC-30 MHz) and BasicRF (1 MHz to 400 MHz)  -- These boards have no
 LOs on them.  They put your signals straight into the ADC converters.
You need your own amplifiers, filters, and mixers/LOs if desired.
Everything you put into these boards is totally coherent.  Each board
gives you 2 connections, so with 2 you get 4 channels at once.

TVRX (50 MHz to 860MHz) - This board is not suitable for coherent
applications since it is based on a TV tuner which has its own
oscillator in the can.

DBSRX (800 MHz to 2.4+ GHz) -- Completely coherent, is currently being
used for radio astronomy interferometry.  Each board has one antenna
input, so with 2 boards you get 2 antennas.  Has its own LNA, VCO,
mixer, filter, etc.  Just add antenna.

Flex400 (400-500 MHz transceiver) -- completely coherent.  Every time
you tune to a new frequency, there is a 180 degree phase ambiguity in
the receive side because of a divider after the VCO/PLL.  You can easily
figure out the relative phase (0 or 180) between 2 antennas by using the
TX side with a dummy load.  Has own VCO, PLL, filters, LNA, mixer, etc.
 Just add antenna.  Each board has 2 receive antenna inputs, although
you can only use one at a time.  You can switch between them in less
than 1 us.  With 2 boards you get 2 antennas full time, or 4 switched.

Flex2400 (2300-2700 MHz transceiver) -- completely coherent.  No phase
ambiguity in this one.  Otherwise the same as the Flex400.

Flex900 (800-1000 MHz transceiver, coming soon) -- 180 degree phase
ambiguity, same as the Flex400.

See http://comsec.com/wiki?USRPClockingNotes for info on the simple mods
to make the Flex boards coherent.

Matt