A spectrally clean transmitting system for solid-state phased-array radars

Navy Radar operations are being curtailed in a littoral environment. This is due to two factors: The encroachment of the cell phone systems into the Navy radar bands and in-band interference from other radars. The Spectral width of most pulsed radars is significantly wider than necessary with the present modulation schemes. Most radars utilize some form of constant envelope pulse with phase or frequency modulation. This causes the spectrum to broaden several times the information bandwidth. If both the amplitude and phase of the transmitted signal are allowed to change a significantly narrower bandwidth can be achieved. In this paper a method to create waveforms with instantaneous bandwidths of 20 MHz confined within -100 dB are presented. The theoretical spectral results of three popular phase modulation schemes (phase shift keying, minimum phase shift keying, and derivative phase shift keying) are compared with the spectrally clean results. In addition the Chireix out-phasing method is presented as an alternative to generating amplitude and phase modulated waveforms. The Chireix method provides a way of improving the efficiency compared to the conventional class A power amplifier. Preliminary results are shown for a spectrally clean waveform.