I wonder if the common GND is correct in this case. Should I change something? All parts powered separately (seems for me to be good). But as I mentioned in other topic I have strong servo twitching only during the flight and I'm not sure if it's some noises, brown-outs or code-bugs.
..between APM and RX there are only Signal wires. Take a look:
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On my TradHeli I had a lot of power problems (although not twitching). While investigating that issue I heard it was a bad idea to provide power from two switching BECs so I finally bought a 20Amp Castle Creations BEC Pro and life has been good since. http://www.castlecreations.com/products/ccbec.html
Just google high current circuit design, and you will notice/should notice, that all subsystems are tied to a common grounding point.
On your standard quad, that would be the PDB.
While only one BEC is used to power the APM electronics and servos, all 4 supply ground.
Which makes it a fairly low impedance path, so if you are using small servos to drive something like a light camera mount, it should not be a problem.
In your drawing, the servo ground is somewhat removed from the system ground, are you using the standard PDP?
Why is there any connection from your servos to that first esc? They should get power from the output rail of the APM.