Motorboating (electronics)

Motorboating is one of many problems that can afflict radio transmitters and similar devices. Radio transmitters are vulnerable to unwanted feedback; one possible symptom of unwanted feedback are rapid changes in power output, which repeats about 20 to 20,000 times per second; this is called motorboating because when the radio transmission is received, it resembles the sound of a motorboat engine.

Technical description

Motorboating is a term for a self-inflicted EMC problem where Radio frequency (RF) from a device leaves through one route and then re-enters the unit through another conduit where it causes an increase in the level of the RF generated by the unit. When this RF output climbs above a given level a catastrophic change occurs which causes the RF power output to drop rapidly. This causes a series of audio frequency pulses to be generated, in an AM system these can be heard clearly as a sound which is like the engine of a motorboat.

In general, as the intended RF power output of the device is increased the possibility of motorboating is increased. If motorboating is occurring in a system then the rate of the low frequency oscillation will increase.

Theory

If an electronic device is considered as a black box whose output is related to a series of inputs by a mathematical function, then if one of the inputs to the device is leaking RF into the system then the output will be a function of the RF input.

If the RF leaks into the device which generates the RF it can alter the RF power output, it might cause an increase in the RF power level, which in turn leads to an increase of the effect which causes the effect increasing the RF output to increase. This, if unchecked, will lead to a greater and greater increase in power output.

Assuming that the device does not fail due to this power surge, then it is likely that the system will reach a state where it is either insensitive to any further increase in RF input (then the power increase will cease), or a sudden catastrophic change will occur to state where the RF power output is smaller. The second of these cases can lead to an oscillation.

Example

Imagine a 27 MHz CB radioset connected to an external 12 volt DC supply, if the decoupling capacitors which link the supply wires to the RF ground (case) are missing then it is possible for RF to enter the radio set through the supply wires. This then causes the motorboating to occur.

References