Self-Injection Locking (SIL) Radar System for Vital Sign and Heart Motion Detection

Self-injection locked oscillators have been used in recent years in many applications such as motion sensing and have demonstrated good performance. Applications of this motion sending capability can be expanded to health care and biological signals. In fact, radar has been increasingly utilized to monitor human heart health in recent years, as it has many advantages compared to other available systems. Radar can be miniaturized unlike ECG, CT or MRI systems and so radar imaging can be very compact in size and therefore can be used in in-home care and for ambulatory monitoring. Also, radar sensors can actually detect the cardiac volume, as it is correlated with the heart motion, which can be utilized to monitor the heart and detect some known heart problems, such as heart arrhythmia; in addition, radar is a promising tool to be used for adjusting medications needed for heart failure patients. Several radar monitor products in the market prove to be accurate and efficient for monitoring sleeping quality and avoiding sleep apnea. Also, many research groups leveraged radar technology to successfully monitor heartbeat in both contact and non-contact scenarios. However, the systems used usually require two antennas, one each for the receiver and transmitter, and need more circuitry for leakage cancellation. Since the need for a portable heart monitor is increasing, specifications such as area and power need to be minimized. This work investigates an effective way to use self-injection locking in a radar system to detect cardiac motion with minimal hardware and reduced power. It expands the idea of using a phase-locked loop (PLL) system and a single antenna, which reduces system complexity. This work investigates and compares a LC-based voltage-controlled oscillator (LCVCO) and a ring voltage-controlled oscillator (RVCO) mainly in terms of sensitivity for motion detection and phase noise. A study of injection-locking of both the LCVCO and RVCO is also established. The study has proven that the RVCO is superior to the LCVCO in terms of sensitivity while the phase noise won’t be much different as the frequency of interest is very small, in fact heart rate and respiration frequency is band-limited up to a few Hertz. The sensitivity of both systems was studied, and was shown to be directly proportional to the locking range of the oscillators, or inversely proportional to the quality factors of the oscillators. However, while analytical expressions for the locking range and the quality factor of the LCVCO are well established, similar expressions for the RCVCO are not. In this work, a derivation of both the locking range and the quality factor of a 4-stage ring oscillator is presented.The theory demonstrated that a radar system based on a RVCO PLL has a better sensitivity and consumes less power and area, which makes it very suitable for a portable heart monitor. To verify the proposed idea, a design of both systems, LCVCO-based PLL and RVCO-based PLL was done in a 65\,nm CMOS process at 1.5\,GHz. The simulation and measurement results confirm the idea proposed. The RVCO shows better performance across all metrics: higher sensitivity by 4\,dB, lower power consumption by 30\,\%, and smaller area, almost half, compared to the LCVCO