High Linearity RF Amplifier Design
High Linearity RF Amplifier Design
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Transmission of spatially diverse orthogonal waveforms in radar to support enhanced illumination control and surveillance timeline management has been recently discussed in the radar literature. Such use of orthogonal waveforms in radar (sometimes referred to as MIMO radar) is potentially attractive in modern HF over-the-horizon (skywave) radar systems. However, HF radar implementation of orthogonal waveforms present some special challenges to waveform design. HF radars operate over several octaves of RF frequency, have array inter-element spacing which deviates significantly from one half-wavelength, use nonlinear power amplifiers, and have stringent spectral occupancy requirements. Implementation of specific orthogonal waveform types present their own particular design requirements, but some issues are generic and span a wide range of waveform classes. In particular, operation with over-sampled arrays can give rise to excessive reactive power. This can be mitigated by the use of a unitary transform into a separate orthonormal beamspace, but this also transforms constant modulus waveforms favored in HF skywave radar into waveforms with significant peak to mean variation. Such amplitude variation results in loss in high-power amplifier efficiency. The impact of operation with those waveforms with HF skywave radars is examined.