Tech Tips

No. 4 - Minimizing Radio Frequency Interference

Back in 1997 Mike Fairfield of Kelowna, B.C. wrote:
"I took a look at your Web site and I am interested in your speed controls. I have a home-made proportional control in my tug, but I get a fair bit of interference on other functions on the radio - horn sounds, rudder twitches, motor reverses for a fraction of a second, etc. I'm using a Hitec Focus 4 AM radio. I don't know if that make has problems, but I don't think so. I tried one of my Futabas with pretty much the same results."

The problem Mike described with the rudder twitching, etc., is your typical case of locally generated RFI (Radio Frequency Interference). Changing speed controls will not have much effect on this kind of problem. Below are some guidelines to help avoid these problems.

Local RFI Sources

When I say local I do not mean somewhere in the neighbourhood, I mean right on board your model. The most likely suspect is your main drive motor (or motors). Other possibilities are winch motors, radar-turning motors, relays or any other type of electro-mechanical gear. Also any piece of electronic gear with a microprocessor (speed controls, sound generators, etc. ) is a potential source, if the clock frequency of the micro is high enough. Not on the list is the motors inside the servos, these are generally too small and and are properly suppressed.

Pick the Right Motor

Assuming the main drive motor is the cause, it may be possible to eliminate the problem by using a different motor, here are some tips. Bear in mind this applies only to scale boats, fast electrics are a different story.

RFI Suppression for Motors

Assuming you are using a single drive motor, proper suppression requires 3 capacitors. I prefer to use a value of .001 microfarads with a voltage rating of 50 volts or better.

The first capacitor goes directly across the motor terminals, the other two go from each of the terminals to the metal motor casing. It is important that these capacitors be physically as close as possible to the motor terminals and that the lead wires be as short as possible. Don't be afraid to solder directly to the metal motor casing. You will need either a soldering gun, or a high power iron (100 watts or better).

To complete the suppression network, run a ground wire from the motor casing to the main battery negative terminal.

Receiver Location

Try to locate the receiver as far possible from any RFI sources. The interference generated by these sources decreases by the inverse of the distance squared. In plain English, if your change the distance to the drive motor from 1 foot to 2 feet, the amount of interference is reduced by a factor of 4, not 2.

Ground Noise

This is not the same as local RFI but can be of concern in models running at high currents. This noise is generated by the drive motor and gets back to the receiver through the ground wiring. Large bursts of noise are common when starting a motor or changing direction quickly.

The solution to this problem is to buy a speed control in which the output section is optically isolated from the input section. In this kind of system there is no connection between the ground on the main battery and the receiver battery ground. The disadvantage is that you need separate batteries. Note also, this will not help to solve any RFI problems. Our new SCO control is optically isolated.

Go Back to Tech Tips Index

Go Back to Main Menu