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Srdjan Milenkovic s.milenkovic at limemicro.comForgot to mention, < -60dBc image suppression has been achieved by correcting both phase and gain imbalance. On 30/10/2012 10:10, Alexander Chemeris wrote: > Thank you. We will check how well we could compensate I/Q imbalance on UmTRX. > > On Mon, Oct 29, 2012 at 5:40 PM, Srdjan Milenkovic > <s.milenkovic at limemicro.com> wrote: >> I am not sure calibration precision is affected by sample rate. >> Theoretically, we are rotating I vector by alpha/2 and Q vector by -alpha/2 >> and that is done by multiplication. Hence, the resolution of quadrature >> phase error (alpha) which we can correct is defined by the with of >> multipliers. >> >> Any way, we clocked ADCs at 15.36MHz (30.72MS/s digital IQ interface) in the >> experiment I mentioned. >> >> >> On 29/10/2012 13:26, Alexander Chemeris wrote: >>> Hi Srdjan, >>> >>> If I understand correctly, I/Q imbalance calibration precision depends >>> on the sampling rate. What sampling rate do you use to achieve <-60dBc >>> image level? >>> >>> On Mon, Oct 29, 2012 at 5:19 PM, Srdjan Milenkovic >>> <s.milenkovic at limemicro.com> wrote: >>>> Hi Alexander, >>>> >>>> It is IQ imbalance indeed. It has to be corrected in BB. I believe you >>>> have >>>> access to the document: >>>> >>>> "LMS6002D Improving transceiver performance using digital techniques >>>> -1.0r1.pdf" >>>> >>>> It may be useful. Also, we recommend to implement RX DC auto cancellation >>>> as >>>> explained in section 3.3 of the same document. This should solve RX LO >>>> leakage issue. >>>> >>>> We usually measure < -60dBc unwanted image level when IQ imbalance is >>>> corrected. >>>> >>>> Regards, Srdjan >>>> >>>> >>>> On 29/10/2012 12:33, Alexander Chemeris wrote: >>>>> Hi Srdjan, >>>>> >>>>> Have you seen a question at the end of this e-mail, regarding the LMS >>>>> calibration? >>>>> To me it looks like a result of the I/Q imbalance which we should >>>>> compensate at the baseband level. Is that correct? >>>>> >>>>> On Thu, Oct 25, 2012 at 10:16 PM, Thomas Tsou <ttsou at vt.edu> wrote: >>>>>> Hi all, >>>>>> >>>>>> I spent some time yesterday testing UmTRX with MCBTS. Aside from some >>>>>> trivial fixes (inverted sample rate), the code ran fine with no >>>>>> issues. The default bandwidth of the LPF was insufficient, so that was >>>>>> widened. I tested with 9 handsets between 3 and 7 carriers on a Core2 >>>>>> Duo (P7570) laptop. >>>>>> >>>>>> git://github.com/ttsou/openbts-multi-arfcn.git umtrx >>>>>> >>>>>> Configuration is through sqlite database, which is unchanged from >>>>>> mainline; just set the number of ARFCN (7 max) and C1 channels. >>>>>> Because of the bandwidth involved, be sure to increase the maximum >>>>>> buffer sizes for UHD. The uhd_usrp_probe utility will warn if the >>>>>> target size cannot be set. >>>>>> >>>>>> >>>>>> >>>>>> http://files.ettus.com/uhd_docs/manual/html/transport.html#linux-specific-notes >>>>>> >>>>>> Design >>>>>> ===== >>>>>> There are three resampling stages. >>>>>> >>>>>> 1. Outer resampler converts from the device rate to a multiple of the >>>>>> 400 kHz channel spacing. >>>>>> 2. Channelizer demultiplexes input stream to M channels >>>>>> 3. Inner resampler converts from 400 kHz to a multiple of the GSM >>>>>> symbol >>>>>> rate. >>>>>> >>>>>> UmTRX rates: >>>>>> >>>>>> Channelizer Rate Device Rate Decimation >>>>>> >>>>>> 1.6 Msps 1.625 Msps 8 >>>>>> 3.2 Msps 3.250 Msps 4 >>>>>> >>>>>> Options >>>>>> ====== >>>>>> >>>>>> To broadcast dummy bursts on all carriers for spectrum testing, >>>>>> uncomment the following preprocessor declaration in radioParams.h. >>>>>> >>>>>> #define ENABLE_ALL_CHANS >>>>>> >>>>>> With dummy bursts on all carriers: >>>>>> >>>>>> http://filebox.vt.edu/users/ttsou/http/7carrier_umtrx.PNG >>>>>> >>>>>> Calibration >>>>>> ======== >>>>>> >>>>>> Calibration remains a concern. With single carrier, the carrier >>>>>> leakage and and IQ imbalance appear as in-band distortion. For MCBTS, >>>>>> OpenBTS offsets C0 on to the lowest carrier. This makes the carrier >>>>>> and image quite visible, as shown in the following uncalibrated >>>>>> capture. I checked with a swept tone to verify that this issue was not >>>>>> a result of baseband DSP. The USRP also shows similar effects. >>>>>> >>>>>> http://filebox.vt.edu/users/ttsou/http/carrier_image.PNG >>>>>> >>>>>> Any idea how much carrier and image suppression we should expect with >>>>>> calibration on UmTRXv2? >>>>>> >>>>>> Thomas >>>>> >>>>> >>> > >