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Figure 4-1. Audio signals subjected to various interference effects.
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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Figure 4-2. Examples of oscilloscope traces of I kHz test tone signal to white noise interference levels.


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
6.
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
of
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


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