Automotive radars and weather radars transmit and receive chirp signals whose frequencies change in a triangular wave shape over time. These radars detect the distance and speed to an object by measuring the frequency of the difference between the transmitted and received signals. The detection accuracy depends on the linearity of the transmitted chirp signal. Therefore, generating a linear chirp signal is needed to improve the detection accuracy. Phase Locked Loops (PLL) are widely used signal sources for generating chirp signals. The PLL can output a signal with a frequency N times that of the reference signal (where N is a real number). N can be arbitrarily controlled by an external control signal. The PLL can generate a chirp signal by transitioning the value of N in a triangular wave shape. However, the PLL distorts the chirp signal around the turn-around point by its transient response due to its low pass type transmission characteristics.

In this paper, a linearity improvement method of detecting the output frequency of the PLL and controlling the value of N from the detected results was proposed. First, the proposed method detects the output frequency of the PLL whose linearity has degraded due to overshoot. From the detection result, the value of N is calculated to reduce the error from the desired frequency. Next, the linearity is improved including the turn-around points by controlling the PLL using the calculated value of N. The proposed method constantly detects the output frequency of the PLL during operation and improves the linearity. The linearity can be improved even if the characteristics of the PLL change due to temperature changes or aging. Therefore, the proposed method can generate linear chirp signals without stopping the radar operation. Simulation and measurement results show that the maximum frequency error is improved by 87% or more compared to that without the proposed method.

This paper contributes to improving the detection accuracy of radars that transmit and receive chirp signals, and contributes to practical application. Therefore, this paper is highly evaluated as a paper that deserves the Best Paper Award.