Re: [Discuss-gnuradio] IQ imbalance...

[Matt Ettus]

On 04/08/2010 09:16 PM, Ian Holland wrote:
> 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?


I'm not sure exactly what you're describing here, but I am pretty sure 
it is not what I would call IQ imbalance.  IQ imbalance would show up 
before any frequency translation, so at the Costas loop output is not 
where you would see it.

The purpose of a costas loop is to track the phase of the incoming 
signal.  That means that the majority of the energy in a BPSK signal 
will be in I and little will be in Q when the loop is locked.  The 
stronger the signal and the better the SNR, the smaller the Q amplitude 
will be relative to the I signal.

Matt