Avoiding instrumentation interference and signal losses at high frequencies
As part of an aircraft’s certification for flight, aircraft must demonstrate safe operation within a range of environmental conditions. This includes Electromagnetic Environmental Effects (E3) which, for civil aircraft, include High Intensity Radiated Fields (HIRF) and Lightning. Both HIRF and lightning testing involves either (i) high level whole aircraft testing or (ii) a hybrid test method of low level aircraft testing with high level equipment testing.
For either test method, fibre optic links (FOLs) are essential to prevent compromising the measurements. For example, during low level swept current and field measurements, FOLs are employed to provide signal gain and isolation from the generated EM environment, typically from 10kHz to 1GHz.
Prevent instrumentation influencing the aircraft’s transfer impedance
Instrumentation should not influence the aircraft’s transfer impedance whilst it is exposed to RF fields or simulated lightning – for example, currents that degrade the airframe shielding. Interconnecting cables that pass from the external environment to the internal aircraft environment will impact measurements, for example RF current can flow on the cable shields. It is important therefore that signal cables are not used to connect the external instrumentation to the field/current/voltage probes installed within the aircraft, whilst performing frequency or time domain measurements. This is particularly significant over the 10kHz to 1GHz frequency range, where cable coupling dominates the leakage mechanism into the aircraft under test.
Avoiding signal loss at high frequencies
The external instrumentation must be outside the measurement area which with large aircraft may lead to a separation distance of tens’s of metres. Signal loss then becomes a factor at higher frequencies. Such signal loss is also avoided by the use of a FOLs. A typical aircraft low level swept current measurement would require signal cables of 40 to 50 metres, introducing significant losses. FOLs are typically used with 100 or 200m link lengths, permitting signals to be coupled from the transducers installed on the aircraft to the measurement equipment for even large transporter/passenger aircraft, requiring fibre lengths of over 100m.