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  • Stu WØSTU

Ignition Interference in Mobile Stations

Receiver interference is sometimes a doggedly stubborn problem with mobile stations. The RF environment of an automobile is often just nasty, with lots of different undesired emitters. Control computers, fuel injectors, fan motors, and other devices in a car can be sources of RF noise that gets onto your receive audio. But the ignition system is notorious for the popping, clicking, and whirring sounds it can impose on your receiver. Sparking plugs or coil packs, a faulty alternator or distributor, and other ignition system sources are sometimes difficult to track down and resolve.

There is a brute force method in most amateur transceivers of suppressing the regular, repeating clicks and whines typical of ignition system noise: the noise blanker. As the name implies, this control on a receiver activates a special notch filter that will detect and “notch out” regular, repeating spikes of noise such as that from an automobile ignition system. In effect, the filter “activates” on-and-off rapidly, attenuating only during the brief regular pulses of ignition noise that are imposed on the received signal.

Time domain view of regular strong spikes within an amplitude modulated signal.
Pulsing noise on a modulated signal may be attenuated with application of the noise blanker.

While use of the noise blanker is convenient and easy, it can have some (usually) mild deleterious effects on received audio. The noise blanker is a filter, so its notch attenuates the receiver, including attenuation of the desired receive signals during the brief activations. Additionally, due to effectively “mixing” a roughly square wave notch into the signal, some mild harmonic noise can actually be generated by the noise blanker itself, thereby generally decreasing the signal-to-noise ratio and reducing receiver sensitivity.

Various noise blankers use different methods of determining what actually constitutes the noise in a received signal. As such, different noise blankers can be “confused” in different noise conditions and may not always be completely effective in attenuating the annoying sounds.

Typically, a noise blanker will be activated with a button push or a menu item of a transceiver. Almost always it is a simple on-or-off situation, although some noise blankers will have sensitivity settings that may be adjusted. Be sure to check your transceiver's user manual to find out how to activate and adjust the noise blanker.

All that stated, the most efficient policy with pulsing noise is usually “just try it.” If the noise blanker does the job, you’ll know it.

-- Stu WØSTU


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