Debugging osmo-trx constellation on XTRX SDR

historical

Hi Thomas,

On Thu, Dec 22, 2016 at 9:41 PM, Tom Tsou <tom at tsou.cc> wrote:
> On Thu, Dec 22, 2016 at 6:37 AM, Alexander Chemeris
> <alexander.chemeris at 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.

-- 
Regards,
Alexander Chemeris.
CEO, Fairwaves, Inc.
https://fairwaves.co