Aviation Electronics Technician 1 - Historic Naval Ships Association

l Sliding-contact interference. This conditionoriginates in an alternator and in a series-woundmotor.Generally, an ac motor without brushes does notcreate interference.InvertersAn inverter is a dc motor with armature tapsbrought out to slip rings to supply an ac voltage. Theac output contains some of the interference voltagesgenerated at the dc end, as well as the brushinterference at the ac end of the inverter.SWITCHING DEVICESA switching device makes abrupt changes inelectrical circuits. Such changes are accompanied bytransients capable of interfering with the operation ofradio and other types of electronic receivers. Thesimple manual switch (occasionally operated) is oflittle consequence as a source of interference.Examples of switching devices (frequently operated)capable of causing appreciable or serious interferenceare the relay and the thyratron.RelaysA relay is an electromagnetically operatedremote-control switch. Its main purpose is to switchhigh-current, high-voltage, or other critical circuits.Since the relay is used almost exclusively to controllarge amounts of power with relatively small amountsof power, the relay is always a potential source ofinterference. This is especially true when the relay isused to control an inductive circuit. Relay-actuatingcircuits should not be overlooked as possibleinterference sources. Even though the actuatingcurrents are small, the inductances of the actuatingcoils are usually quite high. It is not unusual for thecontrol circuit of a relay to produce more interferencethan the controlled circuit.ThyratronsA thyratron is a gas-filled, grid-controlled,electronic switching tube used mainly in radarmodulators. The current in a thyratron is either ON orOFF; there is no in-between. Since the time requiredto turn a thyratron ON is only a few microseconds, thecurrent waveform in a thyratron circuit always has asharp leading edge. As a result, the waveform is richin radio interference energy. The voltage and peakpower in a radar modulator are usually very high, andthe waveforms are intentionally made as sharp andflat as possible. Although these factors are essentialfor proper radar operation, they also increase theproduction of interference energy.PULSED ELECTRONIC EQUIPMENTPulse interference is generated by pulsedelectronic equipment. Types of systems that fallwithin this category include radar, beacons,transponders, and coded-pulse equipment.RadarIn radar equipment, range resolution dependslargely on the sharpness of the leading and trailingedges of the pulse. The ideal pulse is a perfect squarewave. Target definition is also dependent on thenarrowness of the pulse. Both the steepness and thenarrowness of a pulse determine the number andamplitudes of harmonic frequencies. With respect tothe shape of a radar pulse, the better the radar isworking, the greater the interference it is capable ofproducing. Most of the interference is produced atfrequencies other than those leaving the radarantenna, except in receivers operating with the radarband.Radar interference at frequencies below theantenna frequency severely affects all receivers inuse. Principal sources of such interference are themodulator, pulse cables, and transmitter.Transponders, Coded-Pulse Equipment, andBeaconsThis group includes IFF, beacons, TACAN,teletype, and other coded-pulse equipment. Theinterference energy produced by this group is thesame as that produced by radar-pulsing circuits. Theeffects of this interference energy are lessenedbecause the equipment is usually self-contained inone shielded case, and uses lower pulse power. Theeffects are increased because the radiatingfrequencies are lower, which allows fundamentalfrequencies and harmonics to fall within thefrequency bands used by other equipment. Eachpiece of equipment is highly capable of producinginterference outside the aircraft where it can be pickedup by receiver antennas.10-4