Hi Thomas,
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?
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).
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
Hi Thomas,
On Thu, Dec 22, 2016 at 9:41 PM, Tom Tsou tom@tsou.cc wrote:
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?
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.
-
Alexander Chemeris -
Tom Tsou