Re: [Discuss-gnuradio] Why 4 ADC's?

[Jeff Brower]

Ian-

> Ok, that makes sense.  Looking back at this site
> (http://www.nd.edu/~jnl/sdr/docs/tutorials/4.html#tth_sEc2.1) I see
> that the DDC is combining the I and Q inputs to do complex
> multiplication.  I originally thought it was multiplying the I and Q
> components separately, so that no input to Q would mean zero output
> for Q also.

The schematic for the BasicRX daughterboard (lfrx.pdf) shows the board is *not*
bringing out I and Q data separately.  There are two possible antenna inputs, 
brought
to Vin-A and Vin-B.  I and Q data is generated in the USRP board logic by 
complex
multiply less than 30 MHz.  I haven't verified in the logic yet, but the results
should be either one set of I-Q data (for one antenna) or two sets (if both RF 
inputs
are used).

-Jeff

> -Ian Larsen
> 
> On 7/18/07, Johnathan Corgan <address@hidden> wrote:
> > Ian Larsen wrote:
> >
> > > I assume this also true for the BasicRX daughterboard?  (Doesn't do
> > > downconversion, but does do analog multiplication to get I and Q?)
> >
> > Almost. The boards don't do anything with the signal, but have separate
> > I and Q inputs that go to the ADCs as is.
> >
> > On the FPGA, however, there are digital mixers that you can use to
> > convert a center frequency down to baseband, so you get a complex I and
> > Q signal out of that, even if you only supply a signal to the I input.
> > That is what is actually happening when you "tune" a BasicRX or LFRX board.
> >
> > --
> > Johnathan Corgan
> > Corgan Enterprises LLC
> > http://corganenterprises.com