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Re: [Discuss-gnuradio] Quadrature direct-conversion receiver design

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Quadrature direct-conversion receiver design
Date: Fri, 05 Mar 2010 04:08:52 -0800
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On 03/04/2010 06:24 PM, Marcus D. Leech wrote:
OK, so this probably seems like a fundamental sort of question, but I've
noticed that there seem to be
   a couple of different places on the net describing quadrature receiver
toplogies, and I want to know something
   fairly simple.

For a direct-conversion quadrature receiver, must you phase split *BOTH*
the RF and LO inputs to the
   mixers?  That is, do you need a 90 degree hybrid on both the RF and LO

The way I think of it is that you need quadrature on 2 of the 3 ports -- The way we use this on USRPs is that the RF is split but not phase shifted, the LO is phase shifted, and you get the 2nd phase shift by using I and Q samples.

In what amateurs call phasing receivers, instead of using I and Q sampling, you phase shift one channel using analog components and then add that to the other channel.

In both cases you could do the phase shift on the RF instead of the LO, but that is usually harder to do cleanly.


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