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) testing which, for civil aircraft, includes High Intensity Radiated Field (HIRF) and lightning tests. HIRF and lightning testing involve:
- High level whole aircraft testing
- Hybrid of low level aircraft testing and high level equipment testing.
For both test methods, 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 electromagnetic (EM) environment, typically from 10 kHz to 1 GHz.
Sentinel 3 Fibre Optic Link test arrangement (PPM Test Image)
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 (e.g. currents that degrade the airframe shielding). Interconnecting cables that pass from the external environment to the internal aircraft environment will impact measurements, as RF current can flow on the cable shields. It is therefore important 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 measurement, unless the cable is fibre optic (as used by PPM Test). This is particularly significant over the 10 kHz to 1 GHz frequency range, where cable coupling dominates the leakage mechanism into the aircraft under test.
Avoiding signal loss at high frequencies
The external test instrumentation must be outside the measurement area which, with large aircraft, may lead to a separation distance of tens of metres or more. Signal loss then becomes a significant factor at higher frequencies. This signal loss is completely mitigated by the use of FOLs. A typical low level swept current measurement in an aircraft would require signal cables of 40 to 50 metres, introducing significant losses. FOLs are typically used with fibre link lengths of 100 metres or more, permitting signals to be coupled (almost losslessly) from the transducers installed on the aircraft to the remote measurement equipment.
HIRF Test Methodology
The final stage of aircraft clearance may involve limited illumination of the aircraft with threat level RF fields, typically over the range 10 kHz – 18 GHz. The following methodology is being increasingly used as an alternative approach, as it has lower facility costs than testing the total aircraft with threat simulators.
1. Measure the coupling of EM energy (transfer function)
The object of this test is to measure the coupling of EM energy into the interior of the aircraft over the total frequency band of all the environments by illuminating the aircraft with low-level swept continuous wave (CW) radiated fields. These measurements are normally made in “free field” conditions. The Low-Level Swept Current (LLSC) test (Figure 1) measures the level of induced currents and voltages on system components as a result of radiated fields below 400 MHz. Above 400 MHz, Low-Level Swept Field (LLSF) testing (Figure 2) is used to determine the transfer function relating the external field to the internal bay fields at the location of the equipment under evaluation.
Figure 1: Test arrangement for the LLSC test (nose antenna omitted)
Figure 2: Test arrangement for the LLSF test
2. Compute currents or internal fields from coupling measurements
Use suitable signal processing algorithms and compute the currents (100 MHz) from the coupling measurements at the equipment’s location, which would be induced by the HIRF environments on the wiring systems.
3. Directly inject predicted threat currents or irradiate the equipment and its wiring
Directly inject the predicted threat currents on the wiring systems or, at higher frequencies (>400 MHz), irradiate the equipment and its wiring being assessed with predicted threat fields. Appropriate modulation should be applied to simulate emitter parameters. This testing can be applied at system rig level (or the systems integration facility), providing the rig is an accurate representation of the aircraft system.
4. Use current probes and broadband antennas
Cable bundle currents can be measured using small ferrite current transformers or probes. The internal fields can be measured using small broadband antennas, such as the “Top Hat” biconical 1- 18 GHz receive antenna shown in Figure 2. Signals from the current probes/antennas are coupled back to the remote instrumentation using analogue FOLs to as high a frequency as possible. Multi-channel equipment, such as the PPM Sentinel 3 FOL System, covers frequencies up to 3 GHz. Above this frequency, cables are used – with great care – to ensure they do not compromise the integrity of the airframe shielding being assessed.
5. The future of HIRF testing with FOLs
Future fibre optic technology will likely permit reliable FOLs to be developed to cover the full low level swept coupling range of 10 kHz to 18 GHz. This would dramatically improve the measurement dynamic range, as high frequency FOLs would remove the cable losses which become more significant above 1 GHz due to the cable lengths involved. As an example, even low-loss microwave signal cables have loss figures of typically 1 dB/metre at 18 GHz.
Aircraft lightning testing is concerned with two aspects of the direct attachment of lightning to the aircraft:
- Direct damage, including fuel explosion
- Induced electrical transients causing upset or damage to critical electronic systems, known as indirect effects.
Direct damage tests take place on individual items of the airframe. FOLs have not typically been used for this in the past. Unlike HIRF, currents are directly injected into the airframe either at the threat level (200 kA) or at a sub threat level and the resulting induced currents and voltages are extrapolated to the threat and reinjected at equipment level. The injection is achieved by making the aircraft the centre conductor of a coaxial return conductor system. Return conductors are then placed around the airframe. Various configurations are used to simulate the different attachment points and current path of the lightning to the aircraft e.g. nose to tail, nose to wing. Figure 3 shows a coaxial return conductor rig fitted around a typical aircraft (Jaguar T2).
Figure 3: Lightning return conductor rig simulating nose to tail lighting strike
In this type of test, FOL sensitivity is not so much of an issue, unless testing at low level and extrapolating to high threat levels. The main challenge however, is achieving adequate shielding of the FOL transmitter against the high level, low frequency, magnetic H-fields surrounding the aircraft. This prevents breakthrough swamping the wanted signals within the FOL transmitter. This is where good H-field shielding is required, utilising high permeability materials, such as µ-metal shields.
Previously, the shielding effectiveness of FOLs has proven inadequate for reliable accurate lightning testing. The Sentinel 3 FOL system has been specifically designed to exceed the shielding effectiveness requirements for HEMP and Lightning testing and is proven to perform well in these testing environments.
Practical aspects to consider when using FOLs
The requirements of a FOL system vary depending on whether low or high level testing is being conducted. For example, when conducting LLSC/LLSF measurements on an aircraft immersed in a low-level field, very low levels of current or voltage are measured. Typically, sensitivities of 10 µV or lower are required when measuring continuous wave (CW) signals using bandwidths of 1-100 kHz. The latest generation of FOLs now provide significantly improved levels of sensitivity and dynamic range (over legacy FOL systems) and combined with higher shielding performance are now able to perform better in both low level measurements as well high level EMP/ lightning testing environments.
1. Spurious Resonances
It is extremely important to ferrite load the signal cables from any attached probe to the FOL transmitter head to prevent spurious resonances when exposed to the RF environment. Figure 4 shows a schematic of how the ferrites are employed on the measurement sensor system.
Figure 4: Schematic showing addition of ferrite rings to the screened cable
Figure 5 is an E-field measurement system used for LLSC testing. The D-DOT antenna is an electrically short antenna normally used for time domain transient field measurements, but it is also excellent for CW E-field measurement with a linear response with frequency (where the antenna factor gradient is 20 dB/decade).
Figure 5: Sensor set-up for LLSC field calibration. Note ferrite loaded cable.
Ferrite loading can be used on the cable between any current sensor and the FOL transmitter head when making induced current measurements to suppress spurious resonances, with a frequency related to the cable length.
Figure 6 shows the effect of introducing the ferrites onto measurement signal cables. The green trace is the swept CW E-field measured without ferrites clamped around the cable between the balun of the D-DOT antenna and the FOL transmitter head. The red line shows the result with ferrites clamped around the cable. It shows that the signal cable length resonance at around 48-50 MHz has been damped out.
Figure 6: Field measurement plot showing effect of introducing ferrites to the signal cable.
Figure 7: Ferrite loaded cables to the current sensor.
It is important to insulate the FOL transmitter head and attached current sensor from any surrounding metalwork, because a loop formed by the FOL transmitter head, cable screen and sensor body can cause spurious results. Figure 8 shows an installation for measuring induced currents in an aircraft bay with ferrite loaded cables to the FOL transmitter head and “bubble wrap” insulating the head from the surrounding metalwork.
Figure 8: Use of the FOL in practice with ferrite loaded cables and insulating “bubble wrap”.
Aircraft trials can require FOLs to work over an extreme range of operating temperatures from -30 °C to +50 °C. Early FOLs could often see changes in optical gain of up to 3-4 dB during a day of outside aircraft testing. These older generation FOLs (without thermal compensation or automatic gain control) required repeated calibration sweeps between measurements to compensate for (zero out) the changes in optical gain.
The latest generation of FOLs can maintain their relative gain accuracy from the use of an active control loop that monitors and adjusts the laser temperature and optical output power. The later generation of FOLs, such as Sentinel 3 do not require repeated calibration sweeps during testing.
3. Mechanical Strength
Aircraft test areas are hazardous places with aircraft tugs, access stands and test personnel passing back and forth over the area. Additionally, fibres must be fed into the aircraft. A high degree of flexibility and mechanical resilience is required to minimise damage. There is often a trade-off between the flexibility of the fibre cable required to access restricted areas of an aircraft (perhaps through a tight aperture) and the ruggedness required for real test deployment in the field. Fibre cannot be encased in a metal jacket for protection as this would remove the benefit of using FOLs to minimise the impact on the aircraft’s transfer function.
For this reason, The Sentinel 3 fibre interconnect has options for both ruggedised field deployable cable (usually on a cable reel) and a lightweight (bend insensitive) cable for routing into tight areas within the aircraft.
4. Measurement of Dynamic Range
FOLs should have a minimum dynamic range of 120-130 dB/Hz without changing any attenuator/preamplifier settings. This will allow adequate dynamic range when using wide bandwidths without excessive increase in measurement time. The Sentinel 3 FOL has a dynamic range of 150 dB/Hz.
5. Phase linearity with frequency: (e.g. constant group delay)
Phase linearity is important in correcting time domain measurements such as lightning testing, as any non-linearity can cause waveform distortion and is difficult to address during post processing.
The FOL receiver is typically remote from the aircraft and in a benign environment. However, transmitters must have a high degree of shielding to prevent RF pickup swamping the signal from the current probe/antenna. Good shielding for HIRF threats may not be adequate for lightning threats, where extremely high levels of low frequency magnetic H-fields are present. Issues have occurred where FOL transmitters have worked satisfactorily in the HIRF environment but been unusable in the simulated lightning environment. This has been considered in the design of FOLs for use in high H-field environments by the introduction of high permeability shields.
The Sentinel 3 has been proven in real testing to perform well in high field EMP and Lightning testing without any ‘breakthrough’ being observed.
Fibre optic links are essential for Aircraft EMC testing, as they:
- prevent the testing instrumentation from influencing the aircraft transfer impedance
- significantly reduce signal loss
- have a significant increase in measurement dynamic range, particularly at higher frequencies
- provide full galvanic isolation and EM immunity.
In the future FOLs are likely to be capable of much higher bandwidths with an upper frequency of up to 18 GHz. This will enable measurements with a significantly higher dynamic range at the higher frequencies, compared to measurements using coaxial signal cables.
With this ability to detect smaller signals, testing facilities can then reduce the signal strength used in their EMC testing and still retain measurement accuracy.
Content written in conjunction with Nigel Carter, Technical Manager for E3, QinetiQ UK.
All images credited to QinetiQ Ltd, except where specified.