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

[Roshan Baliga]

Jeff Brower wrote:
> I think the OP is confused at what "signal processing" is happening on the RF
> daughterboard.  Although some recent posts have said "none", there actually is 
> some
> -- filters to separate the I and Q signals which were summed at the point of 
> original
> transmission to make a single airwave.

I disagree with this; it depends on which daughterboards you're talking 
about. The BasicRX and LFRX daughterboards are I/Q agnostic - the "I" 
path and the "Q" path are identical. They do contain hardware* to 
convert single ended signals to differential signals for USRP, and 
handle biasing. But they don't care which signal is I and which signal 
is Q. In fact, the two signals could be completely unrelated (as far as 
the daughterboard cares). It's only the USRP that needs to know if the 
signals are I and Q or unrelated signals.

* LFRX uses the AD8132, and the BasicRX uses the ADT1 transformer.

The daughterboards with tuners obviously do push I and Q down to the 
USRP, and I presume do something different to each. I don't have any 
such daughterboards, so I can't say specifically what each does.

> This is why the two ADCs are actually sampling separate, different signals, even
> though their sampling rate clocks are synchronized.

Generally, yes, the ADCs are sampling separate, downconverted to IF 
signals (I and Q).

-Roshan