Hi Matt Having seen your reply, I realise I was not clear in my original post. At the time I observed this error, it was even at the output of the RRC filter, i.e. prior to the MM synch. and Costas loop. The strange thing is, now I am unable to repeat this problem. Instead, now I see clipping of both the I and Q components when I increase the Rx gain beyond a particular level.
While on this matter, is there any risk of damaging the equipment by simply setting the Rx gain too high, or is clipping the only consequence? Ian -----Original Message----- From: Matt Ettus [mailto:m...@ettus.com] Sent: Friday, 9 April 2010 11:37 PM To: Ian Holland Cc: discuss-gnuradio@gnu.org Subject: Re: [Discuss-gnuradio] IQ imbalance... 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 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
