Motorola Radius PM400 Manuel d'installation - Page 24
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Chapter 7
Noise Sources
7.1
Introduction
Electrical noise generated by the electrical system of a vehicle, as well as local ambient noise, can
interfere with normal operation of mobile radios. Satisfactory operation of a mobile radio may require
slight or heavy noise reduction, depending upon the relative strength of the radio signal, and upon the
ability of the radio to reject undesired noise. These requirements vary from one vehicle to another,
depending upon the vehicle type and required coverage area. When operating in a strong signal area,
a certain amount of noise interference can be tolerated. In weak signal areas, however, noise
reduction becomes very important. As a rule of thumb, noise levels create greatest interference in the
25-50 MHz band and reduce with increasing frequency.
Before attempting any noise reduction procedures, determine the noise source(s). Then, follow a
logical, systematic method of elimination until the interference is eliminated or reduced to an
acceptable level.
CAUTION: Do not add interference elimination equipment to vehicles equipped with
!
electronic ignition systems before obtaining advice from the vehicle manufacturer. Addition
of some noise suppression components may interfere with proper operation of electronic
ignition systems and could seriously damage them.
Care and patience must be exercised in locating and eliminating noise sources. There may be several
sources of noise, each slightly stronger or weaker than the other. Elimination of one source may seem
ineffective because another noise source remains active at a barely discernible difference in level.
Consult a service manual for the vehicle to determine what noise reduction provisions the
manufacturer applies when AM, AM/FM, or CB radios are installed as original equipment. These
radios are also subject to electrical noise interference, and the manufacturer may install noise
suppression components only upon those vehicles which require radio equipment at the time of
manufacture. These noise suppression components should be added in any first attempt to suppress
noise.
The three major noise sources affecting mobile radio systems are: (1) radiated noise, (2) conducted
noise, and (3) induced noise. (See Figure 7-1 for typical vehicular noise sources.)
7.2
Radiated Noise
Radiated noise enters the radio through the antenna along with the desired signal and can block or
degrade desired communication. It can be generated by power lines, fluorescent lights, or by
electrical discharges from static build-up, ignition systems, or electrical motors. Radiated noise is the
most common cause of mobile radio interference.
When a spark discharge or arc takes place through air, energy is radiated at frequencies from a few
kilohertz to hundreds of megahertz. This spurious radiation may have some energy radiated at a
frequency near or even identical to the desired radio signal. The standard receiver may be unable to
distinguish between the two signals (desired and noise). Therefore they both enter the receiver,
resulting in degradation of the desired signal.