Null point elimination using RF phase shifter in continuous-wave Doppler radar system

Abstract

A voltage-controlled tunable phase shifter is introduced in the architecture of a microwave Doppler radar to eliminate the null point problem in the non-contact vital sign detection. With a simple control mechanism, the phase of the demodulated signal can be effectively adjusted, providing an opportunity to always lock the radar system to the optimum point. Experiment demonstrates that such a method results in a significant improvement in measurement accuracy.Introduction: Since the early 1970s, microwave continuous-wave Doppler radar (CWDR) has been widely used or studied in various wireless motion sensing applications, such as vital sign detection [1, 2], disaster rescuing [3], and wireless measurement of periodic mechanical movements [4]. A lot of progress has been achieved in this area. However, there is still a challenge to provide a robust system, the detection accuracy of which is insensitive to target position, because of the null point issue. Null points occur in the CWDR system when the received signal and local oscillator (LO) are either in-phase or 1808 out of phase, which would severely decrease detection accuracy [3], especially for systems using very short wavelengths. Researches aiming to solve the null point problem have been conducted for years. In [3], channel selection is proposed by utilising quadrature Doppler radar architecture. By doing so, the null point problem can be eliminated by properly choosing the channel that is not at a null point. However, the detection accuracy is not always satisfactory because whether the system can work at an optimum point still depends on the target position. In [5], frequency tuning based on double-sideband transmission was proposed. However, it requires a two-stage downconversion structure with a tunable intermediate frequency, which increases the hardware complexity. In [6], an arctangent demodulation with DC offset compensation is proposed. However, system calibration is required before each measurement, which brings inconvenience to the continuous detection. In this Letter, we propose a simple but novel method to systematically overcome the null point problem. By introducing a voltage-controlled RF phase shifter to the CWDR, the null point problem can be eliminated. Moreover, the transceiver is always able to work at the optimum point when an adaptive phase control loop can be properly established. An experiment has been conducted and theoretical analysis is clearly verified.

Noncontact Distance and Amplitude-Independent Vibration Measurement Based on an Extended DACM Algorithm

“…First, the small-angle approximation-based measurement is distance-dependent: when the distance between the target and the radar sensor changes, the detection sensitivity of the receiver will also change, resulting in alternately appearing "optimum" and "null" points where the receiver exhibits the best and the worst noise performance [7]- [9]. To solve this problem, multiple solutions have been proposed and investigated.…”

“…By choosing a proper frequency separation, the severe null point problem was relieved. In [9], a voltage-controlled RF phase shifter was employed to tune the transmission delay, which is equivalent to the adjustment of the detection distance. However, to maintain the sensor always working at the optimum point without involving distance dependency, extra costs of system complexity and adaptive feedback loop have to be paid.…”

Appropriate reflected power control for vital signal radar adopting phase shifting method

“…Nevertheless, it is found in Fig. 3(b) that there is only very small phase region, which is so called the optimal detected point in [5,6], might be used to detect the vital signal and the high received power will saturate the mixer quickly. Therefore, the appropriate reflected power of the modulated RF signal is also an essential issue for successfully detecting the vital signals located at different locations.…”

“…Recently, a new feature of vital signal detection radar is proposed to overcome the null point problem with adopting the phase shifter and use only one channel of DAQ for various distances between the target and radar [5~7]. In that reported in [5] and [6], the variable phase shifters were controlled by the software of the computer to find the optimum operation point for the vital signal detection. Although, the problem of null points can be solved with adjusting the required phase shifting by computer software, however, it will limit the development of vital signal radar with phase shifting technique to be a stand-alone system.…”

Direct conversion Doppler radar vital sign detection system using power management technique

“…). The phase shifter is used to alleviate the DC offset and prevent the null detection point . The dual antennas are used to improve the isolation between the transmitting and receiving paths .…”