A Matched Filter and Coherent Digitizer for Pulsed Doppler Radar Systems

In this dissertation, a matched filter and coherent digitizer will be presented for pulsed Doppler radar systems. The matched filter is used to filter as much out-of-band thermal noise in the received signal as possible while maintaining pulse shape integrity for various transmitted pulse widths. The coherent digitizer is used to digitize the filtered pulse and recover the Doppler frequency tone, which is often buried in noise and in the presence of large blockers. A configurable bandwidth filter is presented to be used as a matched filter in a pulsed Doppler radar system. To eliminated dispersion effects in the received waveform, a finite impulse response (FIR) topology is proposed which has a measured standard deviation of in-band group delay of 11.0 ns that is primarily dominated by the inherent delay introduced by the sample-and-hold. The filter is designed to operate at an intermediate frequency (IF) of 40 MHz while being tunable in bandwidth from 3 to 30 MHz, making it optimal for radar systems with varying pulse widths. Employing a total of 128 taps, the FIR filter provides greater than 50 dB sharp attenuation in the stop-band in order to minimize all out-of-band noise in the low SNR received radar signal. Due to the FIR filter being discrete-time in nature, an anti-alias filter must be used to avoid out-of-band frequency components folding back in-band after sampling. A continuous-time filter based on current-reuse differential difference amplifiers, which is used as an anti-alias filter, will be presented. The two differential pairs in the differential difference amplifier process two independent input signals but share the same output and bias current. To demonstrate the achievable power savings, a 6th order lowpass Butterworth filter was designed, achieving a 65-MHz -3-dB frequency, an in-band input-referred third-order intercept point of 12.0 dBm, and an input referred noise density of 40 nV/Hz^1/2 , while only consuming 8.07 mW from a 1.8 V supply. Following the matched filter is a coherent subsampling digitizer. Prior to transmission, the radar system modulates the RF pulse with a known pseudorandom BPSK sequence. Upon reception, the radar digitizer uses a programmable sample-and-hold circuit to multiply the received waveform by a properly time-delayed version of the known BPSK sequence. This operation demodulates the desired echo signal while suppressing the spectrum of all in-band non-correlated interferers, making them appear as noise in the frequency domain. The resulting demodulated narrow-band Doppler waveform is then subsampled at the IF frequency by a ∆Σ modulator. Because the digitization bandwidth within the ∆Σ feedback loop is much less than the input bandwidth to the digitizer, the thermal noise outside of the Doppler bandwidth can be effectively filtered prior to quantization, providing an increase in SNR at the digitizer’s output compared to the input SNR. In this demonstration, a ∆Σ correlation digitizer is fabricated in a 0.18 µm CMOS. The digitizer has a power consumption of 1.12 mW with an IIP3 of 7.5 dBm. The digitizer is able to recover Doppler tones in the presence of blockers up to 40 dBm greater than the Doppler tone