Hi Thomas, On Thu, Dec 22, 2016 at 9:41 PM, Tom Tsou <[email protected]> wrote: > On Thu, Dec 22, 2016 at 6:37 AM, Alexander Chemeris > <[email protected]> wrote: >> We're porting osmo-trx to our XTRX and we see some weird >> constellation (https://goo.gl/photos/1KycUsv26Z6T6aN97). It looks like >> a normal GMSK circle is there, but there is also a square like if a >> signal is clipped. Do you have any idea what could cause this from >> your previous experience? > > Squaring of the constellation is typically associated with > overpowering some part of the chain. What does the spectrum look like?
Spectrum looked fine, as far as I could tell. It looks like that was a result of interpolation inside of LMS7 - we probably misconfigured it somehow. Have to go through documentation for it again. When we disabled all interpolation within LMS7, we've got a proper circle (https://goo.gl/photos/QY9dEgMLLkYcvmCb9), but phase noise is still too high. Maybe because of an uncompensated LO leakage. We'll probably look into this next. >> We're running osmo-trx at 4 SPS. Then we double sample rate in >> software (XTRX can't handle sample rates below 2 MSPS yet) and then >> transmit. See first image above. >> >> Then we tried to increase sample rate and enabled 4x interpolation in >> LMS7 - that led to a distorted cicrle + square constellation >> (https://goo.gl/photos/zMZRecxmZsqiCYMHA). > > Again, check spectrum. Severe bandlimiting and overpower conditions > will have visible spectrum effects. Also, the I/Q offset is high - > consider offset tuning to remove DC and quadrature imbalance from the > test scenario. Yes, we haven't tuned IQ offset and DC at all yet. Now is probably a good time to look into this. -- Regards, Alexander Chemeris. CEO, Fairwaves, Inc. https://fairwaves.co
