Wideband Chirp-Transform Adaptive Filter

Abstract

Abstract : A wideband chirp-transform adaptive-filter system is described that is capable of providing both spectral monitoring and the excision of multiple narrowband signals. The 80-MHz-bandwidth system processes a wideband continuous 300-MHz center-frequency input signal in 10-microsec. segments, using two identical subsystems to achieve 100-percent duty cycle. Spectral analysis is achieved with a multiply-convolve-(multiply) configuration in the forward section of each subsystem. Spectral components can then be excised by time gating of the forward transforms. Subsequently, inverse-transform sections employing a (multiply)-convolve-multiply configuration recreate 12.5-microsec-long segments of the filtered time-domain signal. Using a principle described previously, these output segments from the two subsystems are overlapped by 25 percent and combined coherently in order to cancel artifacts introduced by the input segmentation. The coherence required is of the order of 7 ps between adjacent segments and is obtained by using the reflective array compressors bilaterally so that both subsystems share common devices. The system provides one 100-kHz-resolution spectrum every 1-microsec at a delay of 20 microsecs. The overall signal delay of the adaptive filter is 40 microsec. Dynamic range in excess of 30 dB has been achieved. Excision of frequency subbands as narrow as 400 kHz by up to 30 dB will be described.