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 10µV or lower are required when measuring continuous wave (CW) signals using bandwidths of 1-100kHz.
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.
Schematic showing addition of ferrite rings to the screened cable
The schematic below shows 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 which is excellent for use in measuring CW E-fields with a linear response with frequency with the antenna factor gradient being 20dB/decade.
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.
The plot below 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 showing that the signal cable length resonance at around 48-50 MHz has been damped out.
Field measurement plot showing effect of introducing ferrites to the signal cable.
Ferrite loaded cables to the current sensor.
It is important to insulate the FOL transmitter head and attached current sensor from any surrounding metalwork as a loop formed by the FOL transmitter head, cable screen and sensor body can cause spurious results. The photograph below 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.
Use of the FOL in practice showing the ferrite loaded cables and the insulating “bubble wrap”
Aircraft trials work can required FOLs to work over an extreme range of temperatures from -30 deg. C to 50 deg. C. Early fibre losses increased dramatically at low temperatures. Modern fibre is capable of operating over this temperature range. However, transmitters and receivers from some vendors may not be specified to operate over this range and steps must be taken to ensure they are protected from temperature extremes.
3. Mechanical Strength
Aircraft test areas are hazardous places with aircraft tugs, access stands and large feet 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. 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.
4. Multi-modal interference
In the past, fibre cables were subject to a varying noise floor and hence varying signals when moved around e.g. when being blown about by the wind. This was caused by multi-modal interference in the fibre. This no longer occurs in modern FOLs.
5. Measurement of Dynamic Range
This should typically be 120-130dB/Hz minimum without changing any attenuator/preamplifier settings. This will allow adequate dynamic range when using wide bandwidths without excessive increase in measurement time.
6. Phase linearity with frequency: (e.g. constant group delay)
This is important in correcting time domain measurements such as lightning as any non-linearity’s can cause waveform distortion and are 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 where extremely high levels of low frequency magnetic H-field 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.