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[Discuss-gnuradio] IQ imbalance...

From: Ian Holland
Subject: [Discuss-gnuradio] IQ imbalance...
Date: Fri, 9 Apr 2010 13:46:41 +0930

Hi All

I am using a pair of USRP2s, each equipped with a XCVR2450, for transmission 
over-the-air of an RRC-filtered BPSK signal. The Tx antenna has 3dBi gain, and 
the Rx antenna has 18 dBi gain. The transmitted signal is at maximum amplitude, 
with gain set to 30 dB. The clocks on each end of the link are running from the 
internal oscillators - i.e. the clocks are not locked.

At the receive side, using an MM synchroniser and Costas loop, I am able to see 
a BPSK constellation at the receiver when the Rx Gain setting is 30 dB.
The amplitude of the constellation points is around 0.15 in this instance. 
However, when I increase the Rx Gain beyond 33 dB (in which case the 
constellation is centered around +/- 0.2 on the scope sink), there seems to be 
a large IQ amplitude balance, whereby the I signal is much stronger. Indeed, 
the Q signal disappears entirely when the Rx Gain is above around 36 dB.

Is this expected behaviour, and if so, can anyone please explain why this is 
expected to occur?



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