Thanks Zach.
I noticed the spinning too, and noticed it was not what others have shown in their videos, so figured it had to do with my manual tuning, so thanks for verifying.
I read the first page and that doc looks like a really good primer on digital RF. Some of those diagram might as well be art.
73, John Brier KG4AKV
On Sun, Feb 2, 2020, 20:44 Zach Metzinger via AMSAT-BB amsat-bb@amsat.org wrote:
On 02/02/20 18:50, John Brier via AMSAT-BB wrote:
One thing I didn't mention in the video, at the end of the pass around 10 degrees elevation and lower I decoded a lot of frames because the signal was still strong and by that point the frequency wasn't changing much anymore, as the satellite's relative speed stopped changing.
The video is instructive as one can see the plotted constellation "spinning" before symbol lock. Even after locking, the constellation rotates, indicating a frequency error between the transmitter and receiver presenting itself as a continuous phase rotation.
With an antipodal signal, the constellation diagram will tell you a lot about received signal quality, including SNR (fuzzy points), interfering signals (donuts), out-of-lock (one big circle), frequency/phase errors (smeared clusters of points in a radial pattern).
A great technical read on vector modulation analysis:
http://literature.cdn.keysight.com/litweb/pdf/5091-8687E.pdf
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