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Aviation History
1964
1964 - 0079.PDF
62 FUGHT International, 9 January 1964 The Suppression of Radio Interference BY D. R. SALMON, AIMIEE* RADIO interference is a global term which, though relatedoriginally to electrical interference with radio reception, hasgrown to cover the whole field of interference experienced over a wide frequency range covering communications, navigation and radar equipment. The nuisance value of interference on dom- estic radio and television is well known. In professional equipment the effects of interference may be far more serious and present a hazard to safety. The problem of the suppression of interference in aircraft and ships has been widely investigated during past years, and in most cases it has become a contractual obligation to ensure that installations conform to specified limits of interference. The need for suppression of radio interference has developed as communications and navigation equipment has become more sensitive, and the scope of application of electrical and electronic apparatus has increased. In wartime aircraft conventional choke- capacitor filter networks housed in metal cases were mounted near to apparatus causing interference, but detailed consideration of acceptable limits of interference and methods of measurement immediately after the war in both aircraft and ships resulted in the publication of specifications for noise limits. Pioneer work at RAE produced EL 1716 in 1951 as a draft document laying down acceptable limits of interference over the frequency range 150kc/s to 150Mc/s and, while these limits were quite arbitrary, the speci- fication was successfully applied for both civil and military air- craft equipment until the more recent introduction of BS2G100. The ARB adopted the limits and methods of EL 1716 in its own civil airworthiness requirements. But it soon became evident that clearance of the complete air- craft would be necessary to ensure that noise appearing at com- munications and navigation aerial terminations had been reduced to a satisfactory level. A code of practice committee therefore developed BSI Code of Practice CP 1012 (1961) The Abatement and Measurement of Radio Interference from Electrical Installations in Civil Aircraft. The final step has recently been taken to revise the frequency range over which measurements of conducted inter- ference are made, to bring the specification up to date in its cover- age of current navigation equipment, as well as to revise the com- ponent performance standards in line with the environmental conditions experienced in modern aircraft. Interruption or sudden variation of the electrical state of certain NOISE SOURCES X electrical and electronic equipment, whether operated on low frequency a.c. or on d.c, will result in electromagnetic disturbances capable of interfering with communications or imposing undesirable signals in navigation or radar equipment over a wide frequency range. The most common form of interruption is clearly that of the commutator of a rotary machine or the operation of a contactor, but the effect is present in many other forms arising from recti- fication, mercury switches, solid state switching, and so on, which are all common in modern electronic apparatus. Interference will take effect either by conduction along the connecting cables or by radiation and subsequent reinduction in adjacent cable runs. Suppressor components are therefore applied to present a low- impedance path for the noise voltages at their source, to introduce an inductor presenting a high impedance at radio frequencies to the conducted noise voltage—or to apply a combination of these methods—or screening of the noise source. In, Fig 1 a low impedance path is provided by a capacitor con- nected to earth, the connection to the apparatus ensuring that the noise voltages do not appear in the supply cables. Fig 2 shows chokes connected in the supply lines. Except for specific functions requiring the use of inductors, it would be more normal to employ a combination of chokes and capacitors, as in Fig 3, if it was impos- sible to cover the required frequency range with capacitors only. The most efficient method of suppression is to apply the com- ponents directly at the source, and high-efficiency, small suppressor capacitors are essential to facilitate suppression within the appar- atus. Overall weight penalty is thus reduced and external suppressor assemblies with screened connectors and possible loss of efficiency are avoided. Considerable development work has taken place to produce small suppressor components. Performance of a suppressor capacitor may be assessed by evolv- ing a transfer impedance frequency characteristic. Transfer impedance (ZT) may be defined as the ratio of output noise voltage to input noise current (see in Fig 4). The lower the output noise voltage from the suppressor, the lower the transfer impedance and the more efficient the suppressor. For a perfect two-terminal cap- acitor of 0.5/xF the transfer impedance frequency characteristic will be a straight line (-^c), as in Fig 5a, but, in practice, a capacitor has inductance and resistance because of its construction and internal wiring and a practical characteristic will be as in Fig 5b. A conventional capacitor thus has a limited performance range as a suppressor because, above one Ohm, its efficiency will deteriorate. * Standard Telephone & Cables Ltd, Capacitor Division. SUPPLY figs 1-3 (left) Capacitance, ference suppression systems choke and combined networks for inter- NOISE SOURCES fig 4 (lower left) Schematic diagram of suppressor system Fig 5 (below) Transfer impedance at various frequencies of the different suppression networks SUPPLY NOISE SOURCES C C _L TI _L T L Hi"- L x>otfoo SUPPLY NOISE <, SOURCE i s . INPUT ( NOISE CURRENT SUPPRESSOR t OUTPUT NOISE VOLTS1 SUPPLY UNIT Frequency (Mc/s)
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