Fiber Optic Servo Extensions By James Goss      This is something new to our hobby and will probably catch on for the larger planes first and then the smaller ones as well. What we are talking about here is having a servo connected to the receiver, but without any direct connected electrical wiring between them. Instead of having copper wire connecting the servo to the receiver, fiber optic cable will be used. With fiber optic cable being used to transfer data between the receiver and servo there is no electrical current flow. It is current flow that causes all the glitch problems we face in our hobby. Current flow through a wire generates a magnetic field that starts at the center of the wire and expands out around the wire depending on how strong the flow of current is. This magnetic field can be picked up by other wires in the form of an induced voltage and create an unwanted glitch. The servo wires can also receive an induced voltage as well as generate one. This actually happens with all servo systems no matter how long the wires are, but on a small scale, and you do not notice the tiny glitches.     When the wires get around three feet in length or larger, the induced voltages may begin to be really noticeable in the form of glitches. Three feet because our frequency is 72 mhz and at that frequency the wavelength is 4.1 meters long. Our antenna system is based on a ¼ wavelength and three feet is in that range. In other words ¼ of 4.1 meters is almost 1 meter and 1 meter is 39.37 inches. So this is why the three-foot rule applies here. With extremely long wires the servo may become so erratic that you can't use it at all.      By using fiber optic servo extensions the length becomes immaterial and the servo can be placed at any distance you desire from the receiver. If you have three servos on each aileron you still only need one fiber cable going to each group of three. Remember that light pulses are transmitting the servo data. We will come out of the receiver as always with a standard servo wire. This wire will go to the optic transmitter, which changes the electrical data pulses into burst of light. This transmitter will be located near the receiver and is very tiny in size. The fiber optic cable will leave this transmitter and can be any length you desire to reach the servo area. At the servo location you will have a tiny optic decoder that decodes the encoded light pulses and converts the servo signal back to electrical pulses. The servos must have electrical pulses to operate, remember that the light pulses do not reach the servo. At each servo location you will also have a servo battery pack to drive the servos. In a regular servo installation the three wires going to the servo carries the servo dc power on two wires and the third wire is the servo signal wire. So in the fiber extension system we are eliminating the signal wire and having a devoted battery for each extended servo area. These batteries can be much smaller in size because they only supply a fraction of the total servo current. This also provides more failsafe for our planes because if one of the batteries goes out on one of the ailerons, or elevators, we still have the other half still in operation for landing our plane.      You can see that in large giant scale planes there is going to be a demand for these fiber optic extension systems and in the future I think we will find them in use as part of out regular equipment on small planes as well. At this time the optic extensions are selling for around $65. I am sure they will fall in price as they become popular. You may wonder why there is not a system that will allow us to still use one battery for all the servos and use the fiber optic line for the signal wire. The problem we have with a design such as this will be that as the dc lines carry the power to the servo they are acceptable to generating and receiving induced voltages as mentioned above. Even though these lines do not go directly into the input of the servo, as does the signal line, any amplitude changes on these power lines will be picked up in the amplifiers in the servo and treated the same as a incoming signal. Even with voltage regulators found at the input of the servo these glitches will still get through because the regulators can't react fast enough to keep them out of the amplifiers.