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

From: Ian Holland
Subject: RE: [Discuss-gnuradio] IQ imbalance...
Date: Mon, 12 Apr 2010 15:39:46 +0930

Hi Matt

Can you please confirm by input level you are referring to the input to
the transceiver daughterboard? I am using the XCVR2450, for over-the-air
reception. The input level (to the XCVR2450 at the receiver) would have
been roughly:

Tx Power (max. 20 dBm as per
http://www.ettus.com/downloads/er_ds_transceiver_dbrds_v5b.pdf) + Tx
Ant. Gain (3dBi) - Free Space Loss (at least 46dB for 2m separation and
2.4 GHz freq.) + Rx Ant. Gain (14 dBi) 

As far as I can tell based on the above (presuming the 20 dBm transmit
power is based on max. gain setting for the Transmitting XCVR2450), the
largest signal I could have at the Rx port after the Rx antenna is:

20 + 3 - 46 + 14 = -9 dBm

So, if this is the case, I presume all was safe regardless of the chosen
Rx gain setting for the receiving daughterboard.

Can you please confirm if this would be the case, as I am encountering
inconsistent behaviour with my equipment (such as the unrepeatable error
mentioned earlier, and occasional fails to lock at 5 GHz without first
trying to lock to a much lower frequency).



-----Original Message-----
From: Matt Ettus [mailto:address@hidden 
Sent: Monday, 12 April 2010 3:04 PM
To: Ian Holland
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] IQ imbalance...

As long as the input level is in the safe range, having too much gain 
would probably not damage anything.  On the WBX, however, too much gain 
with a strong but normally safe level might be a problem.


On 04/11/2010 05:01 PM, Ian Holland wrote:
> 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
> 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
> 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
> will be relative to the I signal.
> Matt

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