A∑-Δ Fractional-n Frequency Synthesizer Applied To FMCW Generator In Automobile Anti-collision Radar

Automobile anti-collision radar is an important part of adaptive cruise control(ACC). FMCW(Frequency Modulated Continuous Wave) radar, which presents significant advantages, such as high-resolution, low-emission power, more compact size, has been wide used. In order to achieve high system performance, it requires a well-designed FMCW generator. This paper presents a Σ-Δ fractional-N frequency synthesizer for FMCW generator operating in an Automobile radar.This paper briefly analyses the theory of anti-collision radar and discussed several means of generating ramp wave, in order to meet the needs of collision avoidance radar, we choose to generate FMCW by using a fractional-N frequency synthesizer. The Top-Down design methodology is applied to design the fractional-N frequency synthesizer. By using Matlab’s Simulink module to create a mathematical model description of the system, we determined the the frequency synthesizer system parameters. The frequency synthesizer consists of a none dead-zone PFD circuit, a charge pump circuit with charge and discharge current highly matched, a multi-modulus divider and a Σ-Δ modulator which can achieve fractional divider from 16 to 31, a bandgap reference, and a loop filter. While the current of charge pump is adjustable from 300 ?A to 5m A, the PFD delay can also be adjusted in order to meet the requirement.Simulated with off-chip VCO modelled by Verilog-A,the charge pump current mismatch rate was less than 1% when the output voltage changed from 0.5V to 2.8V; the loop lock time was about 12 ?s from power on while the reference frequency is 100 MHz. The performance of the FMCW generator based on this Σ-Δ fractional-N frequency synthesizer has reached the requirements needed by the ACC radar system.