RFI: How to Troubleshoot

— By John Payne

An example of transient waveform.

Noise sources on a boat are often classified as radio frequency interference (RFI) or electromagnetic interference (EMI). They are major enemies of electronic systems, corrupting your GPS position fixes, degrading radio communications and causing general electronics’ performance problems.
RFI is essentially interference and noise that is superimposed as a disturbance or voltage transient either on the electrical power supply or the data and signal lines. This is then processed along with the good data to corrupt or degrade the processed information.

Transient or Induced?
The voltage transient is probably the most damaging and comes from many sources. The best-known effect is the corruption of GPS and Loran data where the power is taken off an engine starting battery. When a significant load is applied to the battery, there is a momentary voltage drop creating what is often called a brown out condition and then is followed by a voltage increase. This low-voltage disturbance can exceed 100 volts in some cases, damaging power supplies, wiping memories or corrupting the data. The same applies to two battery systems where the house bank supplies items such as electric toilets and other heavy current draw equipment. A starting battery voltage can have a 3 to 4-volt dip on starting. The variation or interruption of current in the equipment power conductor also causes transients.
Induced interference is the second common RFI noise source. Electrical fields are radiated from cables and equipment and this is induced into other closely located cables or equipment. The most common causes of this are cables running parallel or within the same cable bundle, known as mutual coupling. Always run power supply cables and data cables separately and make cable crossovers at 90°. In particular run power cables to sensitive equipment separate to main power cables to reduce inductive and capacitive coupling to signal conductors.

Common Sources
Noise occurs in different frequency ranges and similarly, equipment may only be prone to problems within a particular frequency range. Multiple noise sources can cause a gradual degradation of electronics components and when the cumulative effects reach a certain point, the devices fail.
• Arcing Noise. These are repetitive spikes that are caused by commutators and sparking of brushes. The brushes on any alternator, particularly if dirty, can cause sparking and noise. Charging systems commonly cause this or when loose connections exist. The most common cause is loose or poor engine return paths for alternators, when the negative path arcs across points of poor electrical contact. This is also caused by ignition systems from distributors and spark plugs being impressed on a DC system, often through radiation to adjacent cables. When troubleshooting, always determine what is running at the same time.
• Induced Coupling Interference. Wiring that is installed in parallel with others can suffer from inductive coupling interference causing buzzes and humming sounds on the radio. This occurs like a transformer with a single turn primary and secondary coil, with the magnetic effects causing the induction. Low ground impedance and unbalanced circuits are the most prone with serial data, multi-cable control and co-axial cables being the most susceptible ones. It’s important to keep signal cables from running in parallel with main power wiring. When troubleshooting, always determine what is running at the time as this impacts which cables are involved and will help localize the problem area.
• Ripple Noise. Ripple is created in any rectifier bridges (diode, silicone-controlled rectifier (SCR), etc) such as alternators, chargers, fluorescent lights and inverters. It’s usually a high-pitched whine. Good quality equipment has suppressed electronics. Ripple badly degrades communications’ audio quality. When troubleshooting, again determine which lights are on, or whether chargers or engines are running.

A distance of at least 6" (15cm) should separate signal cables from power-carrying wires.

• Static Charges. These have a number of sources. External charge interference arises due to static build-ups in sailboat rigging. On reaching a certain voltage level, the static charge discharges to ground to cause interference. Another common cause is when dry, offshore winds occur and a static charge builds up on fiberglass decks. The problem is prevalent on large fiberglass yachts with large deck areas. A lightning protection system can ground these charges. Engine and shaft charges also cause interference and this can arise due to static build-ups, both induced and those due to moving parts in the engine. The static charge discharges to ground to cause interference. Shaft interference can arise due to static build-ups on propeller shafts. The static will cause interference when it discharges to ground on reaching a high voltage level.
• Surge (electromagnetic pulse). This can be caused by lightning activity. Pulses can be induced into electrical wiring and aerials.
• Spikes. Turn-on spike results from the initial charging of power supply input filters on power supplies. Turn-off spikes arise when reactive loads are switched and the magnetic fields collapse on inductive loads, such as transformers, relay or contactor coils, solenoid coils, pump motors, etc. MOV suppressors are often put across the coils.
• Solar Activity. This is caused by ionospheric and solar or sun spot activity. Usually it results in signal losses or major corruption. When troubleshooting this condition, always check whether storms are in the area.

Troubleshooting
Once sources are defined, RFI troubleshooting is the relatively simple method of locating the noise source. The listed causes must be verified one by one to eliminate them. It’s a matter of logic and the systematic switching off of each device to locate the noise source. When the noise stops, the cause is located. A simple process of elimination, although it isn’t always that easy. In many cases, it may consist of identifying two or more sources that are overlaid to cause a cumulative noise effect. Some RFI noise is simply intermittent, such as static discharges or lightning pulses, which may not even be visible locally.
What tools do you need? A small, cheap battery-powered AM radio is a good tool for tracking down and sniffing out radiated sources of noise on board, with static causes being easily picked up. Passing it close to the cables or equipment is the method used. Some noise is simply related to time of day and interference from solar activity and ionosphere factors on radios are sources that must be considered. These may affect GPS, SSB and HAM radio and satellite communications all simultaneously, giving the appearance of some greater problem. If you have determined that RFI is around noise and dust, you may already have the answer.
When troubleshooting a GPS or radio, first disconnect the antenna and, if the noise continues, it’s probably caused by the electrical system. If RFI increases when the GPS or radio aerial is reconnected, then the cause may be atmospheric or from some other emission or is being picked up from the antenna feed cables. It’s important to understand that RFI may originate from more than one path, so ensure all possibilities are investigated.

Metallic Sounds
Ticking on the radio is a common problem. When the ticking noise varies with the speed of the engine, then you are probably experiencing ignition noise or alternator based causes. Usually this only affects the power wire for the equipment. Sources of this noise are usually the distributor, the ignition coil, spark plug wires and spark plugs. If ignition noise is identified, move the power wire as far as practicable from any of these sources. If this does not improve things, then check all of these components. Also, check that plug wires and spark plugs are RFI shielded.

Suppression Methods
There are a number of methods that can be used to reduce or eliminate interference. The use of shielded cables along with proper installation is one. The use of suppressors, such as a filter or capacitor installed close to the noisy equipment is common and this effectively short circuits noise in the protected frequency range. Filters may take a number of forms and suppressor capacitors may need to be installed on alternators. Cable separation is another major suppression method.

Proper Grounding

By John Payne A distance of at least 6" (15cm) should separate signal cables from power-carrying wires. The proper grounding of electronic equipment is essential for optimum performance. Most RFI problems, such as noises, hums, buzzes, interference or cross talk, etc. are, in many cases, caused by bad electronic grounding. Ground signals carry currents and currents run through the wires. If there is any difference in voltage between the two ground points, this causes current flow and problems. Currents that flow through the various grounds are known as ground loops. The ground in electronics is a reference voltage point for all signals equal to 0 volts. To prevent ground loops, all signal grounds should be connected to one common point. Cable shields are designed to protect against interference from unknown or unspecified sources. The effectiveness of shields is measured in terms of transfer impedance. This is a measure of effectiveness in capturing the interference field and preventing it from reaching the conductor pairs inside. Data cables also have shields that are made from a foil-polymer laminate tape or have layers of braiding. These also may have a drain wire installed to enable termination of the screen to ground. Most equipment manufacturers also specify the termination of shields to ground. Never ground at both ends. Always ground one end only and, typically, this is at the equipment end. In many cases, shields are not connected at all, so check and connect them, as it may be a source of RFI.