Re: [Discuss-gnuradio] Coherent 100-600 Mhz reception

[ldoolitt]

Friends -

On Sun, Oct 29, 2006 at 05:42:46PM -0800, Johnathan Corgan wrote:
> On Sun, 2006-10-29 at 20:03 -0500, Kim Toms wrote:
> > I want to do a direction finding application in the 462 Mhz spectrum
> > (FRS, GMRS), and need a coherent set of receivers to process the
> > signals.  Preferably from 3 or four antennas.  The bandwidth is only
> > 200 Khz for the whole band. 
> > My questions are:  Can I use 4 receivers with a USRP?
>
> [conceptual solution using two basic RX boards]
> 
> Unfortunately, that's a lot of analog electronics in front of the ADCs
> that must be closely matched in amplitude and phase delay in order to
> preserve the accuracy of phase relationships between the DF antennas.

The usual approach in this case is to add "pilot tone" injection,
electrically close to the antenna (or even as part of the antenna
system).  The pilot tone can be continuous or switchable.  Then you
can calibrate all that analog electronics in-situ.

As long as you have to build custom electronics, you will get much better
results doing an actual downconversion, than you will relying purely
on undersampling.

    - Larry