l Sliding-contact interference. This condition originates in an alternator and in a series-wound motor. Generally, an ac motor without brushes does not create interference. Inverters An inverter is a dc motor with armature taps brought out to slip rings to supply an ac voltage. The ac output contains some of the interference voltages generated at the dc end, as well as the brush interference at the ac end of the inverter. SWITCHING DEVICES A switching device makes abrupt changes in electrical circuits. Such changes are accompanied by transients capable of interfering with the operation of radio and other types of electronic receivers. The simple manual switch (occasionally operated) is of little consequence as a source of interference. Examples of switching devices (frequently operated) capable of causing appreciable or serious interference are the relay and the thyratron. Relays A relay is an electromagnetically operated remote-control switch. Its main purpose is to switch high-current, high-voltage, or other critical circuits. Since the relay is used almost exclusively to control large amounts of power with relatively small amounts of power, the relay is always a potential source of interference. This is especially true when the relay is used to control an inductive circuit. Relay-actuating circuits should not be overlooked as possible interference sources. Even though the actuating currents are small, the inductances of the actuating coils are usually quite high. It is not unusual for the control circuit of a relay to produce more interference than the controlled circuit. Thyratrons A thyratron is a gas-filled, grid-controlled, electronic switching tube used mainly in radar modulators. The current in a thyratron is either ON or OFF; there is no in-between. Since the time required to turn a thyratron ON is only a few microseconds, the current waveform in a thyratron circuit always has a sharp leading edge. As a result, the waveform is rich in radio interference energy. The voltage and peak power in a radar modulator are usually very high, and the waveforms are intentionally made as sharp and flat as possible. Although these factors are essential for proper radar operation, they also increase the production of interference energy. PULSED ELECTRONIC EQUIPMENT Pulse interference is generated by pulsed electronic equipment. Types of systems that fall within this category include radar, beacons, transponders, and coded-pulse equipment. Radar In radar equipment, range resolution depends largely on the sharpness of the leading and trailing edges of the pulse. The ideal pulse is a perfect square wave. Target definition is also dependent on the narrowness of the pulse. Both the steepness and the narrowness of a pulse determine the number and amplitudes of harmonic frequencies. With respect to the shape of a radar pulse, the better the radar is working, the greater the interference it is capable of producing. Most of the interference is produced at frequencies other than those leaving the radar antenna, except in receivers operating with the radar band. Radar interference at frequencies below the antenna frequency severely affects all receivers in use. Principal sources of such interference are the modulator, pulse cables, and transmitter. Transponders, Coded-Pulse Equipment, and Beacons This group includes IFF, beacons, TACAN, teletype, and other coded-pulse equipment. The interference energy produced by this group is the same as that produced by radar-pulsing circuits. The effects of this interference energy are lessened because the equipment is usually self-contained in one shielded case, and uses lower pulse power. The effects are increased because the radiating frequencies are lower, which allows fundamental frequencies and harmonics to fall within the frequency bands used by other equipment. Each piece of equipment is highly capable of producing interference outside the aircraft where it can be picked up by receiver antennas. 10-4