Wideband rectifiers for IoT energy harvesting have been extensively researched for decades. The main challenge in designing a wideband rectifier is achieving impedance matching for diodes across a broad frequency range. In conventional engineering practices, the circuit topology and matching network are meticulously engineered to achieve extended bandwidth. However, the input frequency bandwidth is also limited by the matching network, which generally results in an increase in the physical dimensions of this network. This paper proposes a novel parallel multistage voltage multiplier with frequency-insensitive input impedance, comparing three common rectifier diode topologies. To validate the objectivity of the proposed design methodology, a comparative analysis is conducted among three conventional rectifier diode topologies under a standardized matching network structure. This analysis assesses the stability and consistency of each topology. Leveraging this feature effectively, an uncomplicated matching network can still achieve ultra-wide bandwidth performance. A specialized model has been designed for verification. The test results show that the rectifier power conversion efficiency remains above 50% from 0.2 to 2.6 GHz, with a relative bandwidth of 171.4%. The total dimensions can be as compact as 28 × 10 mm2. When the input power is set to 12 dBm, the peak efficiency exceeds 82%.

Voice assistants can be manipulated by various malicious voice commands, yet existing attacks require a nearby speaker to play the attack commands. In this paper, we show that even when no speakers are available, we can play malicious commands by utilizing the capacitors inside electronic devices, i.e., we convert capacitors into speakers and call it CapSpeaker. Essentially, capacitors can emit acoustic noises due to the inverse piezoelectric effect, i.e., varying the voltage across a capacitor can make it vibrate and thus emit acoustic noises. Forcing capacitors to play malicious voice commands is challenging because (1) the frequency responses of capacitors as speakers have poor performance in the range of audible voices, and (2) we have no direct control over the voltage across capacitors to manipulate their emitting sounds. To overcome the challenges, we use a PWM-based modulation scheme to embed the malicious audio onto a high-frequency carrier, e.g., above 20 kHz, and we create malware that can induce the right voltage across the capacitors such that CapSpeaker plays the chosen malicious commands. We conducted extensive experiments with 2 LED lamps (a modified one and a commercial one) and 5 victim devices (iPhone 4s, iPad mini 5, Huawei Nova 5i, etc.). Evaluation results demonstrate that CapSpeaker is feasible at a distance up to 10.5 cm, triggering a smartphone to receive voice commands, e.g., "open the door''.