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[Discuss-gnuradio] TVRX gain troubles

From: Andrew Lutomirski
Subject: [Discuss-gnuradio] TVRX gain troubles
Date: Tue, 7 Apr 2009 16:50:53 -0400


I'm having trouble with the gain control on a TVRX (actually, on a
whole bunch of TVRX's).  There are two problems:

1. The effect of the gain setting is far from linear.  I measured this
with the attached program, at 1 dB increments of requested gain, with
antennas (with LNAs) attached to both channels.  (I'm not completely
sure I have the full scale right, but that's just an offset.)  I get
essentially identical results using usrp_rx_cfile.py and a little
Python script to do the same calculation (also attached).

2. The system acts like there's some kind of automatic gain control.
My setup is an antenna (with built-in LNA), connected through a
combiner, to the USRP.  On the other input to the combiner is a signal
generator.  The entire setup is duplicated on sides A and B.  I did
two runs.  First, I had the signal generator on and the antenna
connected.  Then I replaced the antenna with a terminator and left the
signal generator on.  (The USRP was tuned to 106MHz and the signal was
a sine wave at 106.25MHz.)  The power I see in MATLAB at 0.25MHz from
center is dramatically (12 dB) less with the antenna connected.

Any ideas what's going on?

This occurs with gnuradio trunk r10579.  I'm updating now and will try
again with a newer trunk shortly.


Attachment: gain_test_090407.png
Description: PNG image

Attachment: usrp_receive.cc
Description: Text Data

Attachment: avg_power
Description: Binary data

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