Identification Of Noise Source

1. Radio Description

(a) Radio frequency band

(1) Radio broadcasts use the radio frequency bands shown in the table below.

Frequency 30

kHz 300

kHz 30

MHz 30 f

Hz 300

MHz

Designation

LF

MF

HF

VHF

Radio Wave

AM

FM

Modulation

Amplitude modulation

Frequency modulation

LF: Low Frequency

MF: Medium Frequency HF: High Frequency VHF: Very High Frequency

LF: Low Frequency

MF: Medium Frequency HF: High Frequency VHF: Very High Frequency

E108734E01

FM (Stereo)

FM (Stereo)

FM (Monaural)

(b) Service area

(1) The service areas of AM and FM broadcasts are vastly different. Sometimes an AM broadcast can be received very clearly but an FM stereo cannot. FM stereo has the smallest service area, and is prone to pick up static and other types of interference such as noise.

(c) Radio reception problems HINT:

In addition to static, other problems such as "phasing", "multipath", and "fade out" exist. These problems are not caused by electrical noise, but by the radio signal propagation method itself.

E108735E01

E108735E01

(1) Phasing

AM broadcasts are susceptible to electrical interference and another kind of interference called phasing. Occurring only at night, phasing is the interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off the ionosphere and the other signal is received directly from the transmitter.

(2) Multipath

Multipath is a type of interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off buildings or mountains and the other signal is received directly from the transmitter.

Fade Out aar DF/,

Fade out is caused by objects (buildings, mountains, and other large obstacles) that deflect away part of a signal, resulting in a weaker signal when the object is between the transmitter and vehicle. High frequency radio waves, such as FM broadcasts, are easily deflected by obstructions. Low frequency radio waves, such as AM broadcasts, are much more difficult to deflect. (d) Noise problem

Technicians must have a clear understanding about each customer's noise complaint. Use the following table to diagnose noise problems.

Radio Frequency

Noise Occurrence Condition

Presumable Cause

AM

Noise occurs in a specified area

Foreign noise

AM

Noise occurs when listening to an intermittent broadcast

An identical program transmitted from multiple towers can cause noise where the signals overlap

AM

Noise occurs only at night

Music beat from a distant broadcast

FM

Noise occurs while driving in a specified area

Multipath or phasing noise resulting from a change in FM frequency

HINT:

If the noise does not match the examples above, refer to the descriptions about phasing and multipath.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Post a comment