New-Tech Europe | March 2017 | Digital Edition

FODLs offer constant delay versus frequency, are immune to vibration, are largely resistant to electromagnetic interference, and fibre delays do not radiate energy. Repeatability of simulation, low system cost and time-savings are key advantages. Tests where excellent close-in carrier phase noise performance is necessary, such as the fixed target suppression (FTS) test, can be performed very well. However, FODLs cannot generate time-variant range-Doppler targets, nor do they offer continuous range settings or arbitrary signal attenuation and gain. Unlike optical delay lines, DRFMs manipulate the radar signal digitally – down-converting, filtering and digitising the received RF signal before storing and modifying it. Signals are then reconverted to analogue, and mixed to RF frequency using the same local oscillator (LO) used for down-conversion. A final amplification and retransmission finalises the processing chain. Developed for electronic countermeasures in military applications, DRFMs that create false targets to mislead the enemys radar and can also be used to simulate real targets for test purposes. Naturally, there is scant commercial and public information available about this classified technology. It is nevertheless known that these systems can cover frequencies up to 40 GHz, offer up to 12-bit digitisation with up to 1.4 GHz of instantaneous bandwidth, up to –65 dBc spurious- free dynamic range with a minimum delay of several dozen ns. Technical constraints limit the ability to combine all these specifications in a single DRFM. Typically, wide bandwidth means a trade-off in signal fidelity or

Fig. 1: Simplified block diagram of a fibre optical delay line (FODL)

Fig. 2: Simplified block diagram of a DRFM system

and material. To ensure an acceptable accuracy and resolution, detection and false alarm rate of the radar system for these functional tests, targets have to be generated over the entire unambiguous range, unambiguous radial velocity interval and azimuth/ elevation coverage with different radar cross sections. Traditional solutions, such as fibre optical delay lines (FODL) or digital radio frequency memory (DRFM), all have their advantages, but also drawbacks such as being specifically designed for only this purpose.

COTS measuring equipment can overcome such disadvantages with the ability to perform multiple test and measurement tasks. Traditional radar target generators FODLs are relatively flexible, phase coherent and small systems that convert the RF signal of the radar to optical and delay it by means of a fibre optical line of a certain length. The signal is then reconverted to RF and retransmitted to the radar. Some systems are also able to introduce Doppler frequency shift.

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