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SERVICING
70-1526
ELIMINATING RADIO INTERFERENCE
Occasionally, you must contend with interference
from somewhere in the automobile. Interference
problems are solved by understanding the interfer¬
ence and its path into the transceiver, locating its
source logically, then eliminating it in the simplest
way available.
Interference may be conducted into the transceiver
directly, or induced into it, or both. Conducted inter¬
ference passes through the DC power leads or the
accessory wiring of the radio. Radiated interfer¬
ence, which can originate from anywhere in the
vehicle, simply produces noise voltages on conduc¬
tors inside the radio or its antenna. See Figure 4-7.
Conducted interference is simple noise voltage pre¬
sent in the vehicle electrical system. With many
electrical devices turning on and off in a vehicle,
current spikes produce voltage drops across wire
resistances, causing voltage transients to appear
throughout the electrical system. Connecting the
radio power leads to this noisy electrical system
applies the noise voltage directly to the radio. Most
noise voltage is attenuated by power-line filters
within the radio; but spikes that are severe enough
may become audible.
Antenna Jack) to overcome any high frequency
noise signal that could invade below. If noise re¬
mains, interference is at low frequencies that can
enter only by proximity coupling to radio wiring or
direct conduction.
Next, power the radio with an independent 12 V
power source (such as another car battery). Isolate
by moving wiring and/or the radio while listening for
changes in the noise level. If the noise stopped
when you connected the independent power
source, noise voltages are conducting through on
the positive circuit or the ground (see ELIMINATING
CONDUCTED NOISE).
ELIMINATING CONDUCTED NOISE
If noise voltage is present on the power leads, there
may be defective equipment in the vehicle electrical
system that needs repair. An alternator with a bad
diode has a large current ripple on its output, which
produces a whine in the transceiver that varies in
pitch with engine speed. Its current capacity is
limited, but vehicle operation will not be noticeably
impaired. Lights that dim during large current de¬
mands are a good sign of such a defect.
While interference conducted through power leads
affects only transceiver audio circuitry, induced in¬
terference often invades the receiver through the
antenna by imitating receiver IF frequencies or
channel frequencies. Induced interference occurs
when an electromagnetic field penetrates the radio.
If an electromagnetic field is strong enough, it can
induce noise currents on the radio accessory and
power wiring.
IDENTIFYING THE INTERFERENCE
The first step toward eliminating interference is to
identify and characterize it. Listening to the noise
can reveal a lot. For example: if the noise heard
varies with engine speed, its source must relate to
the engine, such as the alternator, ignition system,
or tachometer.
Because you are dealing with frequency-modulated
equipment, determining if the noise is at receiver-
sensitive frequencies is easy. With all squelch cir¬
cuits open, simply apply an unmodulated signal to
the transceiver that is strong enough (10 mV at the
Another possible source of conducted interference
is a fan motor in the same circuit to which the radio
is connected. Because a fan also induces interfer¬
ence, confirm that noise is conducted into the radio
(see IDENTIFYING THE INTERFERENCE). If the
interference is conducted into the DC power leads
of the radio, find a power connection point in the
electrical system for the transceiver that is further
from the fan circuit.
Noise voltages can also be added to the radio DC
power input via the ground path. This is a condition
where a high, noisy current shares the ground path
of the radio equipment. For example:
Ground current of a fan motor finds its way to the
vehicle battery through segments of metal body
A-frame assemblies (see Figure 4 - 8). If the elec¬
trical bond between two parts is weak, and the radio
ground current must also travel through this weak
joint, a voltage drop induced across the joint by the
fan current will appear at the radio power plug.
To avoid a noisy ground, connect radio ground
closer to the vehicle battery.
-73
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