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

From: Jeff Brower
Subject: Re: [Discuss-gnuradio] Why 4 ADC's?
Date: Thu, 19 Jul 2007 01:19:15 -0500


> > 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 LFRX has two separate antenna inputs, supplied to Vin-A and Vin-B.  These 
are not
I-Q data.  USRP board FPGA logic would apply one mixer or two (30 MHz or less).

The OP was wondering he could run the same antenna to both inputs, then offset
sampling clocks on the A/Ds could by 90 deg (for example invert one clock) and
effectively double the sampling rate.  I think the resulting signal would have 
likely to increase BER.
> 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.

The RFX2400 mixes with a 2.4 GHz osc and creates one set of I-Q 65 MHz range 
+/-I on Vin-A and +/-Q on Vin-B.  To get baseband signals, the logic has to 
apply two


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