Page 293 - Fundamentals of Radar Signal Processing

FIGURE 4.38 FM waveform having same linear component as that of Fig. 4.33,
but no nonlinear component: (a) magnitude of Fourier spectrum, (b) magnitude
of matched filter output.

An example of a hybrid technique that combines a similar frequency
modulation function with amplitude tapering of the matched filter impulse
response is described in De Witte and Griffiths (2004). It is claimed there that
the far sidelobes are controlled primarily by the maximum instantaneous
frequency, while the near-in sidelobes are controlled by the amplitude

weighting.
In addition to the more difficult phase control required, the major
drawback of nonlinear FM pulses is their Doppler intolerance. Figure 4.39
shows the matched filter output for the waveform of Fig. 4.37 when a Doppler
mismatch of 7/τ Hz is present. While the general sidelobe level remains largely
unchanged, the mainlobe is seriously degraded, exhibiting both range-Doppler
coupling (a shift of the peak) and severe spreading and ambiguity caused by

very high near-in sidelobes. The major advantage of NLFM over linear FM with
receiver weighting is that the receiver filter for the NLFM waveform is a
matched filter so that lower sidelobes are achieved with is no reduction of the
matched filter output peak.

FIGURE 4.39 Output of NLFM matched filter when F = 7/τ Hz.
D