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Alexander Chemeris alexander.chemeris at gmail.comThank 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 >>>> >>>> >>>> >> >> > -- Regards, Alexander Chemeris. CEO, Fairwaves LLC / ООО УмРадио http://fairwaves.ru