Designing Doppler signals to overcome transmission nonlinearities

Doppler processing requires a radiating signal with low spectral sidelobes over the range of frequencies at which Doppler shifts are expected. This is conventionally achieved by windowing the driving sine wave. The effectiveness of windowing depends upon the transmitter amplitude response, and is therefore affected by nonlinearity. This problem is overcome by gating the sine wave with a series of nonuniformly spaced pulses of constant duration and amplitude. The pulse start times then control the shape and level of the radiated spectrum; the most closely controlled sidelobes are those near the transmitted frequency. The resulting spectrum is not affected by nonzero threshold or limited linear range (typical sources of nonlinearity). Furthermore, amplitude shading (within the linear range) and pulse-duration variation can be introduced to control sidelobe levels at higher Doppler frequencies. Pulse-to-pulse phase shifts allow one to design a signal with a central spectral peak shifted from the perceived transmitted frequency