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:
Why is there any connection from your servos to that first esc? They should get power from the output rail of the APM.
Servos are powered from another BEC because of brown outs danger. The Ground is common for all components so I think this is a good aproach to power AMP and servos separately. Only signals come from AMP. It works but I still see some unidentified noise problems.
That's probably why you servos are jittering. There could be a slight difference in voltage between the BEC powering the servos and the APM. I recommend get a BEC that's capable of powering both the APM and the servos. Just to test, move the servos to the same rail at the APM, and see if the jittering goes away.
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?
No, I made my own frame and power system is just like on the picture.
I use standard digital servos for big camera mount. The GND for servos is the same GND (minus) from LIPO. Just check it with digital multimeter. Servos jitter in both cases: powered from AMP(and the same one BEC) and powered separately from other BEC
I'm not saying that APM is putting out completely jitter free pwm, there's always going to be some jitter. The question is how bad is your jitter. Try comparing it to just running the servos of the receiver, and see what the difference is. Also, if the bec on the esc is not sufficient to power the APM and your servos, running off one esc bec will just make you jitter worse. I'd say first thing to do is get a bec with sufficient power and run them all of that bec.
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