Detection Of Noise Sources - Motorola GM1200E Installation Manual

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
1
2
3
-
Figure 2-3 TLN8845A Noise Reduction Kit for Alternator-Equipped Vehicles.
2-4
Ignition
2
Coil
1
Breaker
Points
Quantity Motorola
Part Number
1 1V839913
1 8C82571B02
1 30A502396
1 1V80782A52
Reducing Noise Interference in Mobile Two-Way Radio Installations
Resistive wire
(note 2)
3
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)