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Re: [Discuss-gnuradio] TVRX gain troubles

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] TVRX gain troubles
Date: Tue, 07 Apr 2009 14:24:53 -0700
User-agent: Thunderbird (X11/20090320)

Andrew Lutomirski wrote:

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).

The gain vs. gain setting is always going to be somewhat nonlinear, as you have seen. There are 3 stages of gain control -- RF, IF, and PGA. The only one you can expect to be perfectly linear is the PGA (the last 20 dB).

The reason you see the first two plateaus in the gain curve is that at the high end of the range of each of those gain settings, the gain amps become nonlinear is twofold --

First, the gain range becomes more nonlinear near the top of each variable gain amp. You could compensate for this in the gain setting function in the daughterboard code if you like, by reducing the amount of gain we try to get from each stage.

Second, if your signal is strong enough, it will cause saturation in the frontend.

The formula we use in the daughterboard code for the TVRX is by no means optimal. What it does is first add gain to the RF, until that is maxed out. Once that happens, it adds gain to the IF. Once that is maxed out, it puts the remaining gain in the PGA.

Every application will have different requirements. For example, if there is a strong inband interferer, you will need to reduce the gain. You might also want to distribute the gain differently, instead of putting it all up front.

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.
 > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio

There is nothing automatic unless you implement it. What you are seeing is compression of your signal from strong nearby stations when you attach the antenna. Since you are in the FM band, it is likely that one or more very strong stations are nearby.


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