In 2009, the Federal Communications Commission (FCC) debuted the Medical Device Radiocommunications Service (MedRadio) for transmitting data using implanted biomedical devices. As a result, wireless implantable biomedical devices gained 5 MHz of bandwidth from 401 to 406 MHz. To succeed, however, these devices had to overcome a major challenge: limited battery lifetime. In wireless transceivers, one of the most power-hungry components is the frequency synthesizer, which generates the carrier frequency for wireless transmission. At Purdue University, a low-power, low-voltage frequency synthesizer for implantable medical devices has been created by Wu-Hsin Chen, Wing-Fai Loke, and Byunghoo Jung.
Their 0.5-V medical-band frequency synthesizer consumes just 440 μW while exhibiting phase noise of -91.5 dBc/Hz at 1 MHz offset from the carrier. A number of design approaches were utilized to give this synthesizer its performance edge. To provide a high driving current with a low standby current, for example, the charge pump relies on dynamic threshold-voltage and switch-coupled techniques. In addition, a ring-based voltage-controlled oscillator (VCO) uses a dual resistor-varactor tuning method to compensate for process-voltage-temperature (PVT) variations and the exponential voltage-to-current curve. See “A 0.5-V, 440-μW Frequency Synthesizer for Implantable Medical Devices,” IEEE Journal Of Solid-State Circuits, Aug. 2012, p. 1896.