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[Discuss-gnuradio] Multichannel systems FAQ
From: |
Matt Ettus |
Subject: |
[Discuss-gnuradio] Multichannel systems FAQ |
Date: |
Thu, 16 Mar 2006 17:34:52 -0800 |
User-agent: |
Thunderbird 1.5 (X11/20051201) |
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
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