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). Thanks Ian. -----Original Message----- From: Matt Ettus [mailto:m...@ettus.com] Sent: Monday, 12 April 2010 3:04 PM To: Ian Holland Cc: discuss-gnuradio@gnu.org 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. Matt 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 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