Analog FIR Filter Used for Range-Optimal Pulsed Radar Applications

Abstract

Matched filter is one of the most critical block in radar applications. With different measured range and relative velocity of a target we will need different bandwidth of the matched filter to maximize the radar signal to noise ratio (SNR). Conventional matched filter designs incorporate surface acoustic wave (SAW) filters. However, it is not inherently tunable and will need multiple SAW filters with to change the bandwidth resulting in costly solutions.

In this work, a novel method of implementing the matched filter with an analog FIR filter is proposed. The FIR filter provides a linear phase response which is suitable for radar applications. Analog FIR filters can be implemented in the discrete domain, requiring operational amplifiers, switches and capacitors. In this work, the FIR filter is implemented using a highly programmable operational transconductance amplifier with tunable transconductance gain.

The operational amplifiers designed for the filter uses a fully differential source degeneration topology to increase the linearity; also capacitive degeneration was placed to compensate its high frequency response. An active continuous-time common mode feedback (CMFB) circuit is also presented. This circuit presents a much smaller load capacitance to the output of the amplifier, yielding a higher frequency response.

To satisfy system specifications a 128-tap FIR system is implemented, which require over 128 amplifiers, 136 unity capacitors of 1pF each and 4760 switches. The functionality of the proposed architecture has been verified through schematic and behavior model simulations. In the simulation, the robustness of the FIR filter to process and temperature variation is also verified. The circuits were designed in the TowerJazz 180nm CMOS technology and fabricated on November 2013.