Motorola GM1200E Manual de instalação - Página 28

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Detection of Noise Sources

With the ignition switch on and the breaker points closed, coil primary current builds up at a rate

determined by the coil inductance.

When the breaker points open, primary current decreases and, by self-induction, an electromotive

force is induced in the primary which is many times greater than the battery voltage. The high

voltage inducted in the coil secondary causes a spark across the distributor rotor-to-spark plug wire

gap and then across the spark plug gap for a short interval of time when the breaker points open.

The condenser reduces arcing of the points.

The secondary circuit of the ignition coil, including the distributor rotor gap and the spark gap, is the

main source of ignition interference. The lead inductance and stray capacitance provide a tuned

circuit. Because the discharge of the circuit is through a low resistance (ionised spark gap), the

circuit tends to oscillate. The frequency and amplitude of oscillation vary as current changes in the

spark gap.

4.0

Detection of Noise Sources

Detection of offending noise sources is the heart of noise suppression because, after the noise

source is identiﬁed, the solution becomes obvious. A logical methodical procedure is basic to the

effective noise suppression.

Use your available equipment to best advantage. A pick-up loop, about one inch in diameter, can be

connected to a portable CB radio or a mobile radio being operated at a frequency similar to the

installed radio frequency but from an isolated supply voltage. The pick-up loop can be moved

throughout the vehicle with the radio used as a radiated noise detector. Be sure the pick-up loop has

sufﬁcient insulation to prevent the input of the radio from coming into direct contact with high voltage

points in the vehicle ignition system.

A non-polarised bypass capacitor, which has alligator clips ﬁrmly attached, can be used on a trial-

and-error basis to locate wiring which needs additional ﬁltering. Keep capacitor lead lengths short

for best suppression. Ceramic disc capacitors are not as suitable or effective as automotive coaxial

capacitors. (See ﬁgures 2-3 and 2-4 for part numbers.)

Ignition

Switch

Battery

Notes:

1. Noise reduction can only be achieved if components are properly grounded.

2. Items not supplied in kit. See auto parts dealer.

Reference

Number

Figure 2-3 TLN8845A Noise Reduction Kit for Alternator-Equipped Vehicles.

2-4

Ignition

Coil

Breaker

Points

Quantity

Motorola

Part Number

1V839913

8C82571B02

30A502396

1V80782A52

Reducing Noise Interference in Mobile Two-Way Radio Installations

Resistive wire

(note 2)

Distributor

Description

Lead & Lug Assembly

Capacitor, Coaxial (0.1µF, 100V)

Ignition Coil Suppressor Cable

Hood Wipers (2) and Mounting

Hardware Kit (not illustrated).

Resistor

Spark Plugs

(note 2)