FM-CW Radar Modulation Circuit for VCO Linearity Correction
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Solution Overview
Problem
Conventional FM-CW radar systems face challenges in achieving high linearity of frequency modulation due to the nonlinear frequency characteristics of Voltage Controlled Oscillators (VCOs), which affect measurement accuracy and require complex correction mechanisms.
Innovation Solution
A frequency modulation circuit that generates differential signals and uses a microcomputer to calculate time errors through polynomial approximation, updating modulation control voltages to ensure high linearity, thereby correcting the VCO's frequency modulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a VCO is used for frequency modulation in FM-CW radar, then the circuit configuration remains relatively simple, but the frequency modulation linearity deteriorates due to nonlinear voltage-frequency characteristics
Solution Approach 1:
The patent introduces an intermediary frequency division mechanism that divides the VCO output frequency by a variable division ratio before feedback. This intermediary step linearizes the overall frequency modulation characteristic by compensating for the VCO's nonlinear voltage-frequency relationship, allowing the simple VCO circuit to achieve high modulation linearity without complex correction networks.
Solution Approach 2:
The patent implements a feedback mechanism where the divided frequency signal is fed back to control the VCO's modulation voltage. This closed-loop feedback automatically corrects the nonlinear frequency characteristics of the VCO, maintaining high modulation linearity while keeping the circuit configuration simple and avoiding the need for complex pre-correction networks.
2Manufacturing precision
If feedback control is implemented to correct VCO frequency characteristics, then modulation linearity improves, but inspection time increases due to additional measurement and adjustment work
Solution Approach 1:
The patent enables the VCO system to perform self-correction through automatic feedback control. The divided frequency signal automatically adjusts the modulation voltage to compensate for nonlinearities, eliminating the need for manual measurement and adjustment during inspection. This self-service mechanism maintains high modulation linearity while significantly reducing inspection time for mass production.
3Ease of operation
If quadrature demodulation is used to measure frequency from phase information, then the measurement process is simplified, but measurement accuracy deteriorates
Solution Approach 1:
The patent replaces the conventional quadrature demodulation method with a frequency division-based measurement approach. Instead of measuring phase information through complex demodulation, the system uses a divider to directly obtain frequency information through frequency division. This substitution maintains ease of operation while dramatically improving frequency measurement accuracy by avoiding the inherent inaccuracies of phase-based measurement.
Data Source
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AI summary
A frequency modulation circuit 110-1 includes a VCO 5, a DIV 19, a MIX 20, a single-phase differential converter 18, and a signal processing circuit 6. The signal processing circuit 6 performs differential arithmetic processing of an intermediate frequency signal with a program of a microcomputer according to a quadrature demodulation scheme and, thereafter, measures a frequency from phase information, performs n-th order polynomial (n is an integer equal to or larger than 2) approximation on time-frequency data of an IF signal output by a chirp modulation control voltage after inverse function correction, and performs modulation correction for correcting a time error.