wvmarle: Close. The scientific method is more like: propose a theory, design experiments that can test it. Definitely not "just try something". Repeatability is part of this.
In your case, you concluded "not enough power due to the BT module". The simplest experiment to prove/disprove this would be to remove the BT module from the circuit, see if the jitter goes away. That's an experiment I don't see in your power supply tests. I expect it won't make a difference, if only because BT is designed to be low power, secondly because the power requirement of a BT (or NRF24) module is far less than that of a typical servo.
yeah, okay - my method of conclusion was suspect - but that was when using separate supply, and i did measure current draw of the control circuit with and without the HC-05 under the "working" situation, so there was no jittering in that setup.
i failed to take into consideration the new factors when i then combined the supply, because the symptoms looked the same.
wvmarle: Your PCA9685 should be perfectly fine as it just provides the signal for the servo, not the power (unless you wire it totally wrong). If used with LEDs it can probably drive LEDs directly. An LED takes 10-20mA typically, a signal ideally no current, in reality maybe a few µA, should be <1mA. No problem there.
okay, thanks for the confirmation - i did think that the PCA9685 would "just" be dealing with the signalling currents and so surmised the servo (power) currents themselves wouldn't be catered to in that datasheet.
wvmarle: Pi filters work well. I'm using them mostly on the MCU side, to keep the noise of the motors out. That's when using a 12V circuit supplying power to a number of motors and projects. 12V supply, diode (Schottky for lower voltage drop but a 1N400x will do just fine), then Pi filter before the 12V-5V buck converter. The diode is to prevent back flow, really helps in my case even though I'm using 1.5 mm2 wires.
i guess i should make one then - if not Mike's ideal suggestion of one per servo, at least one for the control circuit as a whole.
will have to start learning how to make my own choke (air-core coils) and set up a proper power circuit for the project from a single supply.
i think the PCA9685 can't handle V+ above 6V and i probably don't want to run the SG90s with more than that anyway - which doesn't leave much room for a series(back flow) diode (drop) and then the LD1117A(3v3) also.