Electronic – Noise issues when driving DC motor with PWM

To answer all your questions:

1. See the explanation by @JonRB, in short saying: You are making a wave going from 0V to 3V with the arduino, because the arduino does not possess a negative rail to work with. So you need to make the signal relative to a ground, in stead of its current DC offset of about 1.5V

How do you do that?

You can do two things:

1. A: You can generate a virtual ground, at about 1.5V and relate all your measurements to that and have your diode conduct into a resistor connected to that virtual ground to make it act as a rectifier. Note, however that you'll still lose about 0.7V off the signal, because that's the voltage the diode drops, or "needs to work".
2. B: You can decouple the DC away from the signal with a capacitor:

^{simulate this circuit – Schematic created using CircuitLab}

The resistor is there to pull the capacitor on the other side to a DC level of ground. The capacitor doesn't allow DC voltages to impose anything on the other side, but to a moving signal it becomes a frequency-dependant impedance. Note, that this will dampen lower frequencies, because the impedance of the capacitor is higher at lower frequencies. The larger your capacitor is the better lesser it will inhibit lower frequencies, so if you can get 10uF or 47uF. If you want 50Hz to be the lower limit (for audible for example) the drawn values will easily suffice.

2. You need an amplifier for that. For such large increases you should look into audio amplifiers or power amplifier design manuals. But I'd advice the latter only if you have a very firm grasp of signals, frequencies, transistors, op-amps, capacitors and inductors. Alternatively you could increase the voltage you are switching with your PWM stage. For this you need an external power source at the voltage you want your peak-peak swing to be and a decent transistor half-bridge (BJT, MOST or IGBT) rated at the working voltage and currents.

3. That depends on the circuits you want to power and what for. I'm getting the impression that you want to build some sort of Power Frequency Driver/Chopper. I am not going to re-itterate a full how-to on building those, but you should really consider googling those if you want to not do something very dangerous (in terms of damage or health). Also note this is a difficult and long road compared to hooking up an Arduino to a set of filters.

4. See point 2: Amplification before or after AC filtering.

The hobby servo needs a signal like this:

In a 20ms window, (50 times a second) a pulse with a width between approximately 1ms and 2ms (may vary, depending on servo) with a middle point of 1.5ms. So in the 20ms window, there is almost 90% of the time being "silence" with no signal level. The pulse is only high for a very short (relatively) period.

The Arduino pins are default output to either 500Hz and 3.9Khz. You cannot directly use the pin in PWM mode (with analogWrite), because it will be receiving pulses too fast, and will bug out/malfunction/won't do anything useful.

The Servo library available for inclusion to an Arduino sketch uses the built-in timers of the ATMEGA328P (or other chip, depends on which arduino you are using) and some fancy software and interrupts to get the proper 50Hz timing, and uses an ordinary digital output pin to send the required pulse.

You can do this yourself in a simple loop, if you only control 1 servo and have a simple program, by simply setting a digital pin HIGH and then delaying (using DelayMicroeconds) for between 1000 and 2000 microseconds depending on what position you want to move to, then setting the pin LOW and delaying for (20,000 - the time you delayed for the servo).

You can send the same signal constantly and the servo will stay at that point, or you can send it until it reaches there and stop sending a signal. The servo should stay at that point, but I do not think it will remember the point (you can move it with your hand and it will stay at the new point, until a new signal is sent again).

It's better not to try and use the PWM hardware outputs for the Arduino to control a servo, even if you do manage to pre-scale the timers down to the required 50Hz (I think this would be hard to get, but I believe you can do it!) as the PWM output registers will need to be set with 8 bit resolution in a tiny range of 10-20%, which only gives you 25 total positions available to the servo (that is pretty bad, 25 positions for a whole +-90 degree movement range!). So go with the Servo Library, basically :)