Recognition Procedure-continued

TM 11-490-5

oscilloscope. A portable radio or high impedance

analyzer, if available, to see how it compares with a

earphones are also useful tools for an experienced

normal receive signal.

operator to determine the presence and nature of RF

3. If necessary, have the distant desired

interference.

transmission disabled, to allow a better visual and aural

examination of the receiver output to see whether

(e) If it is determined that cochannel RF

cochannel interference is occurring.

interference is present, the possibility of deliberate

interference or jamming by unfriendly forces must be

4. If necessary, change the receiver

investigated.

antenna and/or alter the antenna azimuth slightly (if

practicable) to observe any effects on the extent of the

(f) If it is suspected that the interference is

interference.

jamming, do no use the orderwire or any other form of

clean-text transmission to advise another station of the

5. If the above steps are inconclusive,

severity of the interference, or even that interference is

disconnect the receiver antenna from the receiver and

present, until it has been determined that jamming is not

connect in its place the carrier output of a suitable signal

involved or the proper authority has been informed and

generator adjusted to the receive frequency, at an

a go-ahead has been obtained.

appropriate level. (If the receiver has a beat frequency

oscillator, it should be disabled.)

(g) If the interference is not being introduced

through the RF medium, rapid isolation: , to either

6. Apply a suitable degree of modulation

baseband (audio) or the multiplex carrier will assist in

to the signal generator and observe the receiver output.

recognition of the type of interference.

If the

This is to ensure that the receiver is not introducing any

interference was introduced into the baseband audio

interference due to nonlinearity.)

signal or the multiplex system prior to transmission, it

7. The next step is to arrange an RF

may be difficult to recognize at the receiver unless the

"back to back" test, keeping the receiver isolated from

operator has experienced this type of interference

its antenna, as follows:

previously. Therefore, the origin of the interference to

(a) Retain the connection from the

either the baseband audio or multiplex system can

signal generator to the receiver input, but switch off the

normally be traced more rapidly by checking the

modulation.

multiplex system at both receiver and transmitter sites.

(b) Connect the composite transmit

Detailed procedures for isolating the entry point into the

output of a multiplex/submultiplex system to the

system are contained in (2) below; however, a check of

"external modulation" -jacks or terminals of the signal

the input and output signals of the multiplex equipments

generator. (Insure that the depth of modulation is such

can provide a rapid determination of the multiplex

as to provide linear operation.

performance. If this performance is determined to be

normal, then further isolation to cabling and switching

which were suffering interference.

circuits as detailed in c (3) (a ) and c (3) (b ) below may

(d) Adjust the receiver output to

be required.

normal receive multiplex/submultiplex system levels. In

(2)

Determine nature of interference.

this way a "back to back" arrangement is achieved

(a) Monitor the channel or channels in

which incorporates all receiving equipment except the

question with high impedance earphones or other

antenna system. If the interference is still present, it

available test equipment to determine the nature of the

originated within the station. If it is no longer present, it

interference. Table 4-1 provides a listing of the audible

was introduced by RF means. Remember, however,

output to be expected from a receiver under various

that it could be originating at the desired distant station

interference conditions and may be used to assist in the

due to a fault condition there.

recognition of interference sources. Note that the

conditions listed in this table will be in addition to the

remembered that even if interference is introduced by

signal normally heard, which itself may be in the form of

RF means, it could be locally generated. However, the

tones, recognizable to experienced operating personnel,

determination greatly facilitates isolation procedures.

but from which it is impossible to extract the intelligence

of the signal by merely listening. That is, other end

(d) If the interference is entering the system by RF

instrumentation would be required for signals such as

means, a check must be made to establish the source.

teletype, digital, and facsimile, in order that the manner

Cochannel interference might require the distant wanted

in which the intelligence of these signals is affected can

station to disable its transmitter output to allow the local

be determined. As an alternative, the desired signal

station to demodulate and print (if possible) the

could be disconnected at the far station so that the

interfering signal as a means of identification. Adjacent

interference alone could be heard. Figure 4-1 shows

channel interference is more easily discernible and is

usually easy to see on a spectrum analyzer or

4-4