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

[Matt Ettus]

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
> ports?


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.

Matt