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RE: [Discuss-gnuradio] IQ imbalance...
From: |
Ian Holland |
Subject: |
RE: [Discuss-gnuradio] IQ imbalance... |
Date: |
Mon, 12 Apr 2010 09:31:44 +0930 |
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
- [Discuss-gnuradio] IQ imbalance..., Ian Holland, 2010/04/09
- Re: [Discuss-gnuradio] IQ imbalance..., Matt Ettus, 2010/04/09
- RE: [Discuss-gnuradio] IQ imbalance...,
Ian Holland <=
- Re: [Discuss-gnuradio] IQ imbalance..., Matt Ettus, 2010/04/12
- RE: [Discuss-gnuradio] IQ imbalance..., Ian Holland, 2010/04/12
- Re: [Discuss-gnuradio] IQ imbalance..., Matt Ettus, 2010/04/15
- RE: [Discuss-gnuradio] IQ imbalance..., Ian Holland, 2010/04/15
- Re: [Discuss-gnuradio] IQ imbalance..., Per Zetterberg, 2010/04/16
- Re: [Discuss-gnuradio] IQ imbalance..., Matt Ettus, 2010/04/16
- Re: [Discuss-gnuradio] IQ imbalance..., Jeff Brower, 2010/04/16