Conformity Magazine- May 2008 - (Page 45) waveform 3. The 1 and 10MHz ringing frequency has been determined by practical tests as representing the most common cable resonant frequencies. Single Stroke Single stroke events are used for damage assessment on avionic sub-systems and equipment. They can be divided into two categories: • PIN Injection The transient is applied directly to the system interface circuits and is used to assess the dielectric withstand voltage or damage tolerance of the interface components. PIN injection waveforms are defined in terms of the test signal measured in an open circuit (voltage) and a short circuit (current). • Cable Bundle Single Stroke Cable bundle tests are performed using an injection probe to couple transients. Tests are performed on fully configured functioning equipment to determine equipment survivability. Voltage and current levels have to be monitored during the test process to ensure the test limit is not exceeded and/or the test level is reached. Cable Bundle Multiple Stroke Multiple stroke waveforms are applied to determine the electromagnetic compatibility of systems, sub-systems and equipment. The multiple stroke waveform set comprises a series of transients, the first of which represents the initial stroke, followed by multiple transients at a lower level which represent re-strikes on an airframe. Multiple stroke transients are applied to cable bundles only using an injection probe. Figure 3 indicates differing usages of the same basic multiple stroke waveforms. Cable Bundle Multiple Burst Multiple burst waveforms are also used to determine the electromagnetic compatibility of systems, sub-systems and equipment. The multiple burst waveform set is characterised by randomly spaced groups of 20 low amplitude current transients (Figure 4). Each impulse contains rapidly changing currents. Multiple burst transients are derived from lightning leader progression or branching. Transient responses arising from the magnetic (H) field of the external environment give rise to the induced multiple burst sequence. PIN Injection Generator Requirements DO-160 PIN injection specifies Voc = Voltage amplitude in open circuit and Isc = Current amplitude in short circuit “at the injection point” (Figure 5). This should be interpreted to include connection cables and test tips required to deliver the impulse to the equipment under test (EUT). Only under these conditions can the generator impedance be defined. A generator for cable induction test is much more complex, and requires an understanding not only of the test requirements but an interpretation of those requirements for use in practical testing. Cable Bundle Generator Requirements DO-160 introduces a concept that requires some further explanation here. All cable bundle tests take into account the potential influence of EUT cabling on the impulse, focusing only on the amplitude by defining parameters of “I Test” and “V Test” or “I Limit” and “V Limit” values (Figure 6). A “test” value is the ideal that should be reached if possible. The “limit” value is the maximum allowable value measured in a cable bundle to prevent over-stressing the EUT. When this occurs, the test is deemed to have been completed. Often, the “test” and “limit” values are misinterpreted as defining the generator impedance. As we have already established, generator impedance is given only by the PIN injection requirements. Because the cable bundle impedance is so significant, it naturally follows that the type and routing of the cable or the mAy 2008 Conformity 5 http://www.emcc.de http://www.emcc.de
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