IEEE 140-1990 pdf free.IEEE Recommended Practice for Minimization of Interference from Radio-Frequency Heating Equipment.
IEEE 140 behavior of high-frequency power gives the impression that it does not follow electrical laws applicableat lower frequencies, where power capability is typically measured in kilowatts and radiation allowance is only a fewmilliwatts. Actually, there is no fundamental difference. The difficulty lies only in the fact that effects that werenegligible at 60 Hz become very important as the frequency is raised. It becomes increasingly difficult to confineelectric currents to desired circuits. Dangerous potentials may exist between apparatus cases that can cause burns tooperators as well as serious radiation emissions. High-frequency currents may circulate in control devices and makethem inoperative or otherwise impair their functions. Circuits and structures that are not associated with the equipment in any way may pick up high-frequency currents. The solution to all such hazards is the use of shielding and filteringto confine the currents to safe channels.
High-frequency energy can propagate through free space and through wires, causing interference to equipment at greatdistances from the source. The energy may travel along the earth’s surface or be conducted on wires directly to theremote point, or it may be reflected from the ionosphere to be returned to earth at tremendous distances from its source.Again, the solution lies in adequate shielding and design of the high-frequency circuits. Surprisingly small amounts ofradiated power can cause considerable trouble of this nature.There are always high-frequency currents inside a cabinet. They must be kept inside and not allowed to escape.A poorjoint (one that is dirty，loose, etc.) or an opening are places from which currents may escape, so they must beeliminated.
The oldest type of generating equipment is the rotating machine.However, this device is not generally used above 10kH.In the range of 10 to 500 kH, common power sources were the spark gap and Poulsen arc. Interference from sparkequipment is distributed over a very wide frequency range.For example, a 20 kH generator will radiate a continuousspectrum of noise that is very similar to automobile ignition interference, which may extend well into the very high frequency regions.
Even if the operating frequency of the ISM equipment will not cause interference, harmonics can be generated bynonlinear elements in the system, and parasitic oscillations may be generated on nonharmonically related frequencies.
Nonlinear electrical components distort waves, generating frequencies that are harmonics， as well as sidebandfrequencies that bear no harmonic relation to the fundamental. In addition, such phenomena as moding and parasiticoscillations cause trouble.Moding is a sudden frequency shift or “skip”” of the generator during the loading procedure.lt is a normal tank circuit characteristic that is present when a tuned load is tightly coupled to the tank. Parasiticoscillations are spurious frequencies generated by a feedback path other than the normal path, resulting in unwantedoscillations in the generator.IEEE 140 pdf download.