Implantable biomedical devices (IMDs) capable of recording electrophysiological signals effectively facilitate medical treatment, but they also face strict volume requirements [1]-[6]. An effective way to miniaturize the IMDs is to eliminate the bulky components such as battery and crystal. Wireless power transfer (WPT) helps to remove the battery [1]-[5], while a bulky crystal is still required to provide a precise clock to ensure the performance of signal-acquisition and communication blocks (Fig. 1 left, top). To eliminate the crystal, prior work [1] uses an on-chip oscillator as the clock generator (Fig. 1 left, middle), while suffering from off-chip tuning and SNR degradation of analog front-end (AFE), ADC, and wireless transmission. Recently, clock recovering from power-harvesting tone has become a promising solution to further reduce the volume of battery-less systems (Fig. 1 left, bottom) [2]-[5]. However, it's difficult to deal with a trade-off: A high power-harvesting frequency leads to power-hungry clock-recovery circuits [4], while a low frequency requires a large-size antenna [5].