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

From: Roshan Baliga
Subject: Re: [Discuss-gnuradio] Why 4 ADC's?
Date: Wed, 18 Jul 2007 13:43:56 -0700
User-agent: Thunderbird (X11/20070604)

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 
-- filters to separate the I and Q signals which were summed at the point of 
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).


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