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[Discuss-gnuradio] RFX900 daughterboard gain

From: Harvind Samra
Subject: [Discuss-gnuradio] RFX900 daughterboard gain
Date: Mon, 04 Aug 2008 10:13:49 -0700


I'm having problems getting the correct (well..what I believe to be
correct) gain out of the RFX 900 transceiver.

I have a input signal of around -50dBm going into the TX/RX port of the
RFX900.  I've bypassed the ISM filter.  I have to turn the RF gain all
the way to its maximum of 70dB, just to get a 0dBm signal at the A/D
converter.  Changing the PGA gain has no effect.  As I turn down the RF
gain, the input level drops accordingly. 

I've been using the C/C++ interface, so I used the write_aux_dac
function to max out the RF gain as follows:

m_uRx->write_aux_dac(1,0,(int) ceil(0.2*4096.0/3.3)).

and the set_pga function to set the IF gain.

So with 70dB total of combined front gain, I would expect that the input
to the A/D would be saturated, i.e. full-scale, with a A/D input level
of 10dBm.

Some questions:

-- I'm assuming the input impedance to the RFX900 is 50ohms.  Is that

-- The RFX900 schematic shows 2 gain stages.  The first is an LNA that
provides 22dB, the second is the DDC which provides a max of 45dB and is
adjustable via the aux DAC.  So the RF gain is adjustable b/w 22dB and

I'd appreciate any answers, questions, or suggestions anyone might have.



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