Noise Suppression For R/C Receivers

Receiver interference is hardly an unknown problem among model builders. Preventive measures in the form of ferrite beads fitted to servo cables are often seen in relatively large models and/or electrically driven models, to prevent the cables from acting as antennas and radiating interference to the receiver. If miniature ferrite beads are used for this purpose, the connector must be first be taken apart, after which the lead must be threaded through the bead (perhaps making several turns around the core) and then soldered back onto the connector. An interference source can also cause problems in the receiver via the power supply connection.

 The battery is normally connected directly to the receiver, with the servos in turn being powered from the receiver. The servos can draw high currents when they operate, which means they can create a lot of noise on the supply line. This sort of interference can be kept under control by isolating the supply voltage for the receiver from the supply voltage for the servos. All of these measures can easily be implemented ‘loose’ in the model, but it’s a lot nicer to fit everything onto a single small circuit board. That makes everything look a lot tidier, and it takes up less space.

The schematic diagram is shown in Figure 1. Connectors K1–K8 are located at the left. They are the inputs for the servo signals, which are connected to the receiver by the servo leads. The outputs (K9–K16) are located on the right. That is where the servos are connected. Finally, the battery is connected to K17. Interference on the supply voltage line due to the motors and servos is suppressed by a filter formed by L10, R1, C1 and C2. L10 is a ferrite-core coil with an impedance of 2000 ohms at 30 MHz. In combination with C1 and C2, it forms a substantial barrier to interference in the 35-MHz R/C band.

Signals with frequencies close to the 10.4-MHz intermediate frequency (which is used in many receivers) are also effectively blocked by this filter. L9 filters out common-mode noise on the supply line for the servos, which effectively means that it prevents the supply lines to the servos from acting as antennas. Finally, high-frequency currents on the servo signals are filtered out by ferrite beads in order to limit the antenna effects of these connection lines.

Resistor:

• R1 = 1Ω

Capacitors:

• C1 = 100nF
• C2 = 22pF

Miscellaneous:

• L1-L8,L10 = ferrite inductor
• L9 = common-mode coil
• K1-K8 = servo cable
• K9-K17 = 3-way SIL pinheader

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