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Re: [Discuss-gnuradio] Info regarding full duplex operation of RFX2400

From: Greg Troxel
Subject: Re: [Discuss-gnuradio] Info regarding full duplex operation of RFX2400
Date: Thu, 22 Mar 2007 09:54:48 -0400
User-agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (berkeley-unix)

"Rohit Gupta" <address@hidden> writes:

> I have question regarding the full duplex operation of RFX2400 board.
> Looking at the circuit diagram of the board, I understand that each RFX2400
> board has seperate receive AND transmit chain. Hence, if I connect one
> antenna to "RX" port and another different antenna to "TX/RX" port and use
> this  system  to  make  full duplex  transceiver, is it possible that there
> will be crosstalk between different RF components of RFX2400 board when both
> transmit/receive chains are operating at the same time. IF yes, how bad is
> its effect??

A fair question, and I haven't seen any measurements.  Yes, you will
have problems, but I'm not sure how bad.

Presumably you will be intending to be on different frequencies (or
more generally, have non-overlapping signals).

Narrowband FM repeaters try to achieve on the order of 170 dB
difference between tx power and rx sensitivity (e.g. 5 MHz offset at
445 MHz).  This requires tuned cavity filters on transmit and rx
paths, and also good isolation in the radio, usually with separate
metal compartments and power supply isolation.

So, issues will be common power supply, noise sidebands from
the transmitter showing up at the receive frequency (repeaters pass
tx/notch rx in the tx line, and pass rx notch tx in the rx line), and
downconverter LO phase noise causing transmit components to show up
in the receive passband (reciprocal mixing).

All that said, I don't really have a handle on the noise sidebands in
USRP/RFX2400.   You certainly aren't going to be able to build a
narrowband FM repeater with 50W transmit and -120 dBm receive
sensitivity :-)

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