22 Dec
2016
22 Dec
'16
6:41 p.m.
Hi Alexander,
On Thu, Dec 22, 2016 at 6:37 AM, Alexander Chemeris
<alexander.chemeris(a)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