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TM 11-490-5
occur when the interference is introduced into the
(b) Whether the transmit signal power
baseband portion of the system, if introduced before
levels are normal.
translation is completed, it is possible for interference
(c) Whether
from one broadcast station to adversely affect four or
attentuation for the number of channels in operation has
five channels in each basic supergroup in the system.
been introduced.
For a 600-channel system, there are 10 basic
3. Stations and systems affected.
supergroups. Consequently, a single broadcast station
(a) Whether  similar  systems  in the
has been known to adversely affect 40 to 50 FDM
station are affected.
channels. Introduction of broadcast band interference
(b) Whether similar systems in other
can result from many different factors. some of which
stations are affected.
(c) Whether a few stations or many
(a) Equipment design deficiencies: poor
stations are affected.
grounding inside the equipment, filters not effective at
(d) In  the  case  of  dual-redundant
broadcast band frequencies, and poor selection of
systems. the effect of turning "A" side and "B" side off.
interior cables.
(e) Whether the noise is affected by
(b) Equipment improperly grounded or
weather: for example, whether it is greater during dry
general grounding system not effective at higher
(f) 'Whether  any  station  reported
(c) Isolation  transformers  improperly
higher than normal receive levels.
In this case,
waveducting (a propagation phenomenon) may be
(d) Design
4. Special EMC considerations.  Because
(e) Cables improperly installed.
the frequencies used by the internal processing portion
(e) Security devices.
Internal checks on
of FDM multiplexing systems fall within the standard AM
security devices are beyond the scope of this manual. If
broadcast band (535-1605 kHz), interference may result
the input of a security device appears normal and the
when FDM systems are located near a broadcast
output abnormal, unit substitution or checking ground
station.  Typically, there are two different types of
wiring is the normal isolation procedure. If the device
interference  which  may  be  experienced
is multiple channel,  interference associated with the
broadcast station sources. The first type results when
unit would normally affect all channels.  If a single
the broadcast station assigned frequency falls in the
channel is being interfered with, the problem is probably
middle of an FDM channel.  In this case, a serious
not associated with the security device.
single-tone interference caused by the mixing of the
(3) Major component isolation procedures.
broadcast  station  and  FDM  channel  carriers  is
(a) Switching or patching facilities. Since the
experienced on one channel. However, the modulating
switching  and  patching  facilities  are  normally
frequencies of the broadcast signal can cause high
nonautomatic, interference problems are generally due
noise also in the two channels on either side of the
to loose connections, worn cables, dirty contacts,
channel worst affected. As there is a frequency offset
improper grounding, incorrect patching or jumpering,
between the two carriers, the noise will usually be
and similar problems that can be isolated and resolved
garbled. Further, the noise on the channels will vary with
by careful operation and proper maintenance.
the broadcast station modulation. The second type of
(b) Interconnecting cables.
The key to
interference results from the broadcast station being
isolation of multipair cable problems is to check for any
assigned a frequency that falls between two FDM
new cables which may have been laid which could have
channels, resulting in a coherent signal being heard as
resulted in damage to existing cables, newly assigned
noise on one channel.  A second channel will have
circuits in existing cables, and any vehicular traffic,
approximately the same noise reading, but being on the
usually in the vicinity of the station, which could have
other side of the broadcast frequency, will have inverted
damaged the cables. Such traffic can cause damage to
frequencies. Usually, the higher modulating frequencies
the cable insulation and -puncturing of the protective
at the broadcast station (above 4 kHz) will cause some
sheath, resulting in moisture seepage into the cable.
noise on one high and one lower channel, resulting in
Eventually, this moisture can cause internal (interpair
high noise on four channels. When interference from the
and interpair) corrosion which leads to partial shorts,
broadcast band is considered, it might be expected that
leaks, or a path to ground which may result in ground
a single broadcast station would affect four or five
loops, crosstalk, and hum. The test procedure for
channels in a 600-channel FDM system. While this may

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