Influence of Phase Detector Frequency on Linearity of Frequency Modulated Continuous Wave Signal
In this paper, a scheme for generating a frequency-modulated continuous wave signal by rapidly changing the phase-to-phase ratio of phase-locked loop is studied. The influence of the frequency of the phase detector in the phase-locked loop on the linearity of the signal is discussed. Using ADS for modeling and simulation, through the analysis of the simulation results, it is concluded that at a suitable small frequency division ratio hopping interval, a high-precision, high-linearity FM signal can be generated; the higher the frequency of the phase detector is, the resulting The FM linearity of the FM signal is better.
Keywords frequency modulated continuous wave; phase detector: phase-locked loop: linearity
With the improvement of people's living standards, the number of private car holdings is increasing year by year. At the same time, with the improvement of vehicle safety performance, anti-collision radar system and adaptive cruise system have become the standard configuration of medium and high-end vehicles. Based on the requirements of the anti-collision radar system and the adaptive cruise system for short-range ranging and speed measurement, the system generally adopts the FMCW system. In the actual use of on-board radar, the signal is required to have high frequency and ultra-wideband characteristics. At the same time, in order to ensure measurement accuracy, the signal source is generally required to have good frequency-modulated linearity, but the conventional chirp signal generation scheme is difficult to meet these requirements. Therefore, this paper introduces a frequency synthesizer that generates a chirp signal by rapidly changing the frequency division ratio of the phase-locked loop divider, and analyzes the factors affecting its scanning linearity. This method has the characteristics of high frequency accuracy, easy debugging and good linearity, and is suitable for use as a vehicle radar signal source. In this paper, the ADS software is used to simulate the system. Based on the simulation results, the influence of the frequency of the phase detector on the frequency-modulated linearity of the generated signal is analyzed.
1 Design of frequency synthesizer The phase-locked loop circuit consists of four modules: reference frequency source, phase detector, loop filter and voltage controlled oscillator (VCO). As shown in Figure 1, if the phase-locked loop circuit is used as a frequency synthesizer, a program divider is also required. Its output frequency fout=Nfr, where fr is the phase detector frequency; N is the frequency divider division ratio.
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Therefore, the output frequency can be controlled by changing the division ratio N. By programming the program to change the value of N periodically, linearly and quickly, the FM continuous wave signal can be obtained. Create a simulation model in ADS, as shown in Figure 2.
In the simulation of the model, when the jump interval Δt of N is long, an approximate step-like sweep curve is obtained; as Δt gradually decreases, the sweep curve of the generated FM continuous wave signal gradually increases. It becomes smoother and eventually close to a straight line, at which point a linear sweep signal can be obtained; but as Δt is further reduced, the linearity of the FM curve will deteriorate, as shown in Figure 3.
2 The influence of phase detector frequency on the linearity of frequency-modulated continuous wave The frequency band commonly used in China's vehicle radar is 24 GHz. The frequency range of the integrated phase-locked loop frequency is 23.875~24.125 GHz and the sweep bandwidth is 250. MHz, taking the appropriate transition interval Δt = 2μs, the loop bandwidth is 250 kHz. The simulation was performed for the case where the phase-detection frequencies were 1 MHz, 5 MHz and 10 MHz, respectively. The simulation results are shown in Figures 4-6.
From the comparison of the simulation results, the accuracy of the FM signal is higher. At the same time, as the frequency of the phase detector increases, the frequency from the start-up to the lock-up time decreases, and the peak overshoot at the frequency extreme point becomes smaller.
In FMCW radar, FM linearity is defined as 
Where fcmax is the maximum frequency difference between the actual frequency value and the ideal value during the sweeping process; B is the sweep bandwidth. Since only the linearity after the FM lock is concerned in the application, it can be seen from the figure that after the frequency is locked, fcmax appears near the extreme point, so only the frequency linearity in the interval from 150 to 350 μs is calculated. The frequency modulation linearity of the phase detectors at 1 MHz, 5 MHz and 10 MHz, respectively, is shown in Table 1.
As can be seen in Table 1, as the phase-detection frequency increases, the frequency-modulation linearity is continuously optimized.
3 Conclusion This paper introduces a frequency synthesizer that produces a chirp signal by rapidly changing the phase-locked loop division ratio N. The model was simulated and analyzed with ADS software.
The simulation results show that by rapidly changing the frequency division ratio of the phase-locked loop divider, the chirp signal with better accuracy and linearity can be obtained. When the appropriate interval of the frequency division ratio N is selected, as the frequency of the phase detector increases, the peak overshoot at the frequency extreme point becomes smaller, and the frequency modulation linearity is also optimized.
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