Hi Alexander,
On Thu, Dec 22, 2016 at 6:37 AM, Alexander Chemeris alexander.chemeris@gmail.com 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?
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.
-TT