LC filter is one of the most basic filters in electroincs, which comprises of just one inductor and one capacitor in its simplest form. This circuit is widely used by FPV hobbyists to suppress noise in power source created by ESC and motors.
What Is LC Filter And How Does It Improve FPV Experience
When we break down a simple LC filter, we have an inductor and a capacitor.
An inductor resists changes to the current flowing through it, while a capacitor resists changes to the voltage across it. See this post for a more sophisticated version of power filter.
Apart from the video transmitter power, antenna gain, the FPV range and quality also depend on how noise your power supply is. If you have an independent power source, e.g. a separate battery for your video transmitter, you generally should not have noise issue.
But generally speaking when you share the main battery with the ESC/Motor, the power supply becomes noisy. It might cause horizontal white lines/ripples appearing across the screen when we are flying FPV.
When the multirotor is airborne, the motor speed is constantly changing in order to stay balanced in the air. The change of motor speed means change of current drawn by the motors, and fluctuations in voltage level – eventually this becomes noise.
Most common video transmitter requires 12V power supply, which can be powered from the 3S battery directly. If you are running 4S Lipo battery. To have a stable 12V we normally use a voltage regulator. Voltage regulators have some degree of power filtering, but we can further enhance it’s filtering by coupling a LC filter.
Building a LC Filter
The value of capacitor and inductor affects the ability of noise suppression at certain frequency, so if you are really going to use a LC filter, you normally need to identify what frequency noise you want to filter out. But I have been hearing a lot people just use any random capacitor and inductors and they do just fine, so maybe it’s not a big deal after all.
When choosing capacitor, ensure the capacitor voltage rating supports the voltage of your application. In my opinion 100uF to 2000uF is a good range.
As for the inductor, you can make one yourself with a Ferrite ring or just use a rod/pencil. The length and diameter should be large enough to get half a dozen winds of wire through it or around it. However try to use Ferrite rings and really, they are very cheap. Or you can just hack one out of an old PC mother board or radio power supply. Same for the capacitor: a cheap electrolytic type (polarized or non-polarised) hacked from an old radio, or PC part is going to do just fine. But make sure you use low ESR type of caps, they work better for filtering purposes.
If you are making your own inductor, I find it works better with just the positive wire wound on the toroid. When winding the cable round the ferrite ring, do NOT loop any of the wires back to the point they originate from, i.e. what is behind the filter stays behind the filter. Try to place the capacitor on the side where the transmitter and camera are relative to the inductor.
LC filters are great because they are so simple and cheap to make/buy. They are likely to help eliminate/attenuate “noise” even when they are not well designed. I always put one in regardless.