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:address@hidden
Sent: Friday, 9 April 2010 11:37 PM
To: Ian Holland
Cc: address@hidden
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