Receiver - TM-11-490-50035

TM 11-490-5

fails to clear up the problem, equipment problems, such

multiple subscribers are reporting problems. To affect

as receiver alignment or receive module malfunctions

rapid isolation of the source of the interference, proceed

are possible. Isolate the trouble by substitution.

as follows:

(a) On the orderwire circuit, request the

their transmissions are interferring with the receipt of

distant-operator to check his transmission from the

signals by another station, certain steps may be taken at

transmit voice frequency telegraph system. This is

the suspected station to determine whether this is

accomplished through appropriate patching and

actually the cause of the problem:

monitoring and the results are reported. If the report is

that clear copy is leaving his station, then the trouble

1. Obtain approval to take the transmitter off

lies either along the propagation path, in antennas and

the circuit.

connections, in any interconnecting cables, or within the

2. Terminate the transmitter into a dummy

local operator's own station.

load, and proceed to check whether transmitter is

(b) Monitor adjacent channels to

operating normally as indicated in paragraph 4-3c(2)(b).

determine whether they are also affected. If these

(2) Major system isolation procedures.

channels are affected, service may be restored by

indicated previously, a major point where interference

connecting to available spare channels. Notify the

can enter a communication system is the radio link.

distant operator to do the same, after informing the

This radio link normally consists of a transmitter with

subscribers that a momentary interruption of service

some type of modulator, the receiver, the transmit and

may be necessary while patching is accomplished.

receive antenna system, and the transfer media

Equipment (possibly multiplex) malfunctions are

between the two antennas.

indicated; or if only a few channels are affected,

Although the exact methodology will vary depending

possible EMC problems are involved. If a single

upon the equipment and type indicators or test

channel only was found to be affected, this indicates

instrumentation available, the procedure is similar for all

that other than EMC problems are involved and

components.

corrective maintenance must be accomplished.

(a ) Receiver.

1. With the test instrumentation available,

determine whether a carrier channel is at fault by

check the receiver output signal to determine if it is

reconnecting the channel at both stations to a spare

normal for the particular system. If the system is

channel. Care must be taken that such reconnecting is

transferring digital signals and the pulse recognition and

coordinated to avoid undue disruption of service.

pulse restoration circuits (that is, the circuits that

(d) If connection to spare channels

determine when a pulse is present and then generate a

does not restore service, the interference may be

true pulse at that time interval) are an integral part of

caused by a local transmitter or ancillary, or there may

the receiver, the signal should be monitored between

be a problem arising from proximity of the transmission

the detector and these pulse circuits. If the signal is

line/antenna system to the circuit suffering the

normal, the interference is not entering through the radio

interference. In this case, a detailed check of the

link.

antenna and transmission line would be required,

2. If the signal is not normal at the receiver

including measuring the voltage standing wave ratio,

output, the receiver input signal should be checked

unwanted ground connection, corrosion, insulation

using available signal indicators, test instrumentation,

breakdown, and the like.

and test points. If the RF signal cannot be checked, the

(e) If problems are still apparent,

IF signal can provide an indication of signal level and

checks should be made of the RF signal levels

deviation from normal. A spare receiver or broadband

throughout the system, back to the originating station.

portable radio may also be used to check the incoming

Levels which are too high will usually cause overdrive

RF signal. If the input signal is normal, as determined

problems with accompanying nonlinear distortion.

by tests on the actual receiver, the spare receiver, or the

Those which are too low cause unnecessary degradation

portable radio, the distant transmitted signal is

of the signal-to-noise ratio. Adjustments must be made

interference free. If not, then the distant transmit

to ensure that levels are correct. Refer to paragraph 4-

system may be at fault. If there is no fault at the distant

3d (8).

station and there are no apparant propagation problems,

local testing will be required to determine whether the

(f) It is possible that the problem could

problem is associated with the actual receiver or

stem from nonlinearities within the voice frequency

receiver antenna system. This can most readily be

telegraph terminal, from the carrier system, or within the

accomplished by replacing the receiver. If the

interconnecting cables. A distortion analyzer may be

used to detect nonlinearities in the system.

(g) If reconnection to a spare channel

4-11