I have also been asked for more details on my motor suppression circuit that I first blogged in dc-to-daylight, so here goes :-
The Solarbotics GM3 generates large amounts of RF noise from 20MHz up to at least the TV band, which is 470- 850MHz in the UK. I know this because I can see the 20 MHz on my scope and it was also affecting our TV reception.
This is the circuit I used :-
The 1nF capacitors were axial ceramics and the 10nF was a radial ceramic, mainly because that is what I had to hand. I don't know the spec of the ferrite beads because I salvaged them from an old disc drive. Here is what they look like though :-
They should be a low Q type rated for at least 1A. The current rating is not so much about how much current they can carry but about the point where the magnetic field saturates the ferrite and the inductance disappears.
We want them to have a high impedance from 20 MHz to 800 MHz. I don't have much knowledge in this area but think this is quite a big ask for a ferrite and that I fell lucky with these. To get more impedance at the low frequency end it is normal to increase the number of turns to increase the inductance which is proportional to their square. The problem with that is that it increases the capacitance, reducing the attenuation at the high frequency end.
These beads are a good compromise: they have nearly a whole turn compared to a straight through bead which is half a turn, hence four times the inductance, but the wires maintain 0.1" separation so minimizing the capacitance.
The first two 1nF capacitors are soldered to the motor case. This is easier than you might imagine because steel is such a poor conductor of heat compared to copper, although it has to be said I am using a 50W temperature controller soldering iron. I cleaned the area first with a PCB cleaning block.
This is the rest of the circuit before it was soldered on top of the two capacitor leads. Spot my mistake!
Ignore the back emf diode, it is specific to my controller and should really be part of it. I used twin screened cable with the braid grounded at the controller end and left unconnected at the filter end.