Ignition noise

TM 11-490-5

procedures are pertinent to both DCS and non-DCS

high-pitched whine from rotating machines.

facilities.

5. Cables. Interference problems attributable to

1. Ignition noise. Privately owned vehicles in

cables may result from damage which has caused

the vicinity of a communications site normally do not

insulation breakdown, or breaks in shields. Interference

contain extensive noise suppression devices and are

may be noted as hum which results from broken

therefore sources of ignition noise.

The noise is

shielding, improper shielding, or improper shield

characterized by a popping sound at a low periodic rate

connections; or crosstalk as a result of signal-carrying

which varies with the speed of the engine. Gasoline-

pairs being in proximity, particularly when high signal

powered lawn mowers are also potential noise sources

levels are carried in proximity to low signal levels as

around communications sites. Ignition noise from these

when improper cabling, patching, or jumpering for a

machines results in high levels of radiated emissions

given installation task occur.

because of their relatively open construction and

Equipment grounding.

Defective or

consequent lack of shielding. The level of interference

will vary, depending on the type of signal being

improperly installed earth grounds and equipment

received. For example, since ignition noise has a rather

grounding systems are a major source of EMI problems.

broad RF spectrum, it will usually affect all channels of

Primarily, the cause is an unwanted resistance common

an FDM system. In the case of PCM, which is time

to one or more circuits as a result of ground paths longer

dependent, the number of channels affected would

than necessary or poor ground connections. These

depend on those channels which were in the time slot

conditions are more frequently encountered in multiple

when the interference pulses occurred. At low operating

ground point systems rather than common point

speeds, the interference pulses occurred.

At low

systems. The indications that grounding insufficiencies

operating speeds, the interference may appear to be

are the origin of EMI are varied and dependent upon the

random popping on a single channel. At high operating

circuit involved; however, the usual symptom is

speeds, however, the PCM system may be totally

powerline hum. In other instances, the problems may

affected.

be simply that equipment malfunctions occur because of

2. Radar. A pulse-modulated radar can cause

high impedance return paths which limit current flow and

cause reduced operating voltages.

broadband interference similar to vehicular ignition

noise; however, the periodic rate is directly related to

(3) Identification of possible jamming sources.

the PRF of the radar. If aural means (earphones) are

Jamming

stations

use

various

types

used to monitor this type of interference, it will be

emission/modulation.

When such interference is

noticeable as a definite and recognizable tone, as

encountered, the possibility of jamming must be

opposed to the popping characteristic of ignition noise.

considered. Methods of jamming typically employed are

The intensity of the interference will also vary as the

as follows:

radar antenna rotates, if the radar is a scanning type.

(a ) White noise.

So-called "white" noise

The visual indication will be that of a pulse moving

derives its name from its counterpart in the optical

across the oscilloscope display (fig 4-1).

spectrum, white light, which results from all frequencies

3. Office equipment. If offices are colocated

in the visible spectrum. Likewise, white noise consists

with the communications site, office equipment,

of many radio frequencies combined whose energy

especially calculators and data processing equipment,

content is relatively constant per unit bandwith across

present a potential interference problem.

The

the RF spectrum. Figure 4-2 contains oscilloscope

interference will be manifested as a random rate clicking

traces which show the effect of white noise interference

sound which affects most types of signal modulation.

on an analog signal for the conditions of the signal much

Improper filtering and poor grounding are common

greater than the interference, the signal slightly greater

causes for this interference affecting communications

than the interference, and the interference much greater

equipment.

than the signal.

Characteristically, white noise

4. Power lines. Power lines are a potential

interference at levels equal to or greater than the

source of many interference problems as a result of

desired signal would completely block out any audio

insulator and transformer breakdown, poor ground

signal with a continuous hissing sound. This type of

connections, lightning discharge transients, and

noise is normally associated with jammers; however,

feedback interference from equipment using a common

when it occurs, the possibility of other problems, such as

power source. The interference may be in the form of a

when semiconductors degenerate, should not be ruled

60-Hz hum, as in the case of poor ground connections,

out. At levels much less than the desired signal, white

or poor earth grounding at the power pole; frying sounds

noise affects the signal as a background hiss. White

as when transformer breakdown occurs. or insulators

noise at considerably less amplitude than the signal

become damaged; or

could point to circuit problems also and would not

normally be indicative of jamming.

4-7