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[Discuss-gnuradio] TVRx and BasicRx gain questions

From: Sebastiaan Heunis
Subject: [Discuss-gnuradio] TVRx and BasicRx gain questions
Date: Wed, 25 Feb 2009 16:56:06 +0200

Hi all

Can someone please provide me with some guidance?  We are working on a
passive radar project and the aim is to implement a pcl radar receiver
on the USRP1 using TVRx daughterboards.  I have the Microtune 7901.

I did some tests to see what the effects of the RF and IF amplifiers
in the tuner are (linear region,etc.).  I noticed that the maximum
output of the TVRx is -15dBm regardless of the input signal and the
gains.  For this measurement, I've got the test circuit that they have
in the Microtune datasheet with the Minicircuits RF transformer to
match the 200 ohm output of the TV tuner to 50 ohms.  Also, I captured
a signal from the TVRx and looked at it in Matlab.  The maximum I
value is 17 and the maximum Q value is 16, which means that we are not
close to using the whole dynamic range.

I also did a test with the BasicRx db.  This time I injected a 13dBm
signal into RX-A.  The maximum value in Matlab is 7919 which looks
much better.

Now I have 2 questions:
-Does anyone know anything about the TVRx and why I only get an output
of -15dBm?  The Microtune datasheet is a bit vague.  It only says that
the signal level for 1dB RF gain compression is 72dBuV (-113dBm) with
AGC deactivated.  Maybe the TVRx wasn't designed to utilise the entire
dynamic range of the ADC??
-What is the maximum signal level after the DDC+filtering,etc?  My
guess is around 2047*8 because of the bit-shifting at the ADC


Sebastiaan Heunis
Radar and Remote Sensing Group, University of Cape Town
Tel:  +27 83 305 5667

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