Publications
Generating precision nonlinear FM chirp waveforms
It is well-known that Non-Linear FM (NLFM) chirp modulation can advantageously shape the transmitted signal's Power Spectral Density such that the autocorrelation function (i.e. matched filter output) exhibits substantially reduced sidelobes from its Linear FM (LFM) counterpart. Consequently, no additional filtering is required and maximum Signal-to-Noise Ratio (SNR) performance is preserved. This yields a 1-2 dB advantage in SNR over the output of a LFM waveform with equivalent sidelobe filtering. However precision NLFM chirps are more difficult to design, produce, and process. This paper presents design and implementation techniques for Nonlinear FM waveforms. A simple iterative design procedure is presented that yields a NLFM phase/frequency function with the desired inherent sidelobe response. We propose to then generate the NLFM waveform by using a cascaded integrator/accumulator structure. Several specific architectures are examined to meet target performance criteria, including bandwidth constraints and sidelobe reduction goals. We first examine a fixed parameter set to generate a fixed polynomial phase function. Polynomial coefficients are selected to be constant during the pulse. Alternatively, a NLFM waveform can be generated via integrating a stepped parameter set, whereby parameters are constant over specific intervals, with the pulse width encompassing multiple intervals. The parameter changes in steps during the course of the pulse as a function of time. Alternatively yet, the parameter steps can be made a function of the pulse's instantaneous frequency.