Stations and systems affected.

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

the

correct

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.

are-

(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

weather.

frequencies.

(f) 'Whether any station reported

higher than normal receive levels.

In this case,

used.

waveducting (a propagation phenomenon) may be

(d) Design

deficiencies

cable

involved.

terminations.

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

from.

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

4-13