New oscillator for low-power implantable transceivers

Due to the limitations of implanting a device in the human body, the devices have limited power storage. Using CMOS technology, different circuits and systems design techniques have to be in place to achieve an energy efficient communication interface. This CMOS technology, created by Arash Moradi and Mohamad Sawan, helps to get rid of most of the external bulky components. Two of the main challenges the creators faced were to reduce the size and power consumption of the transmitter front-end to maintain real-time, long-term data transmission. The main issues of this article are the size and battery power of a voltage controlled oscillator as a medical implant. This voltage controlled oscillator was created to provide frequency deviation of frequency-shift-keying modulation in medically implanted RF transceivers. Crystal-less and inductor-less voltage controlled oscillators will help implement this type of device as an implantable and create VLSI systems with different frequency ranges.

The designers’ voltage controlled oscillator is a low-power differential rail-to-rail quadrature voltage controlled oscillator that is composed of a 2-stage quadrature oscillator. Each stage of the oscillator acts similar to a bi-stable circuit triggered by the output of the other one. No external clock signal is required and the device only needs a small silicon area and a low current consumption. As to not affect the oscillation, the startup circuit is designed to initiate the oscillation and is then automatically disconnected from the oscillator’s circuit. The designers claim that this is the first design of a fully-integrated low-power quadrature voltage controlled oscillator that is capable of providing rail-to-rail, differential and quadrature version of a variable-frequency signal, simultaneously.

Challenges, such as determining the parasitic capacitance and resistance within the prototype, played major roles in changing the behavior of the expected output signals. Producing the input signals, including the enabling or disabling of different blocks, with proper falling and rising time and providing the power supply will have to be properly planned. The designers may add more control signals in future prototypes so that it can be used for several communication applications with different frequency and current ranges, such as providing the optimum carrier frequency for wireless power transfer using inductive links.

The designer’s results and design used TSMC 90nm CMOS technology. The value of the current consumption for the transmitter was mainly due to the output driver including a power amplifier. The poly phase filter stage and modulation need no DC current consumption. A low power budget was needed for the whole transmitter to provide a data rate of 20Mb/s which is better than other similar circuits. The accomplished power amplifier was not fully optimized and there was a tradeoff between power consumption and provided output voltage amplitude. The proposed circuits, which were implemented in 90nm CMOS technology, enhance the data rate capability and power efficiency of wireless implantable devices and improve the figure of merit comparing with previously published works.

In implantable medical devices, wireless link technology is increasing. However, wireless connections may consume 90% of a device’s total power. Development within CMOS technology can provide a potentially better circuit that may consume less power due to having to implement less components inside of the device. An optimum solution would be to have an implantable medical device in which the device consumed as minimum amounts of power as possible.

Consuming small amounts of power could lead to not having to replace the device as much or undergoing invasive procedures in order to charge the battery of the device. According to the researchers in a paper published for IEEE, the quadrature voltage controlled oscillator operates at 915 MHz and consumes 580 uA from a 1V power supply while showing good phase noise immunity and a fast settling time. There are many positives to the proposed design of this low power oscillator. All over the globe, patients with an implantable medical device could go potentially years without having to undergo surgery to replace their device or to have their device recharged. Along with gathering data on medical patients, the device could also be used to monitor the habits of animals. The animals could be monitored for much longer periods of time with a device that would be less likely to be unattached due to fighting or swimming. If possible, one solution could be the usage of wireless charging in which the patient could charge their medical device without having to visit a doctor or hospital. Other solutions could include thermocoupling devices as well as kinetic battery chargers. The only problem with thermocoupling devices is their poor efficiency. The more intricate the device becomes, the more the medical costs of the device, surgeries, and replacements become, and more power will be consumed by the SOC. Thus, as the device becomes more complicated, thermocouple devices might not be the best solution. For the case of animals, a kinetic charger could be used. The difficulty with using a kinetic charger would be the use of a magnet which could interfere with the intended operation of the device.

Though there seem to be many benefits, it also comes with some disadvantages. This great device could not be used around the world due to the price of medical attention, such as the surgeries, that would be needed to replace or recharge the device. Along with price factors, the more intricate these devices become, the more issues that may arise. These issues would likely have to do with the rise and falling time of the device, the parasitic capacitances, and the delay times associated with the components of the device. The wireless transmitter and receiver would need to be implemented in such a way that it should not interfere with the human body or any other device a person typically uses, such as a phone or possibly a radio. Also, designers should be aware of potential issues such as interfering with radio and other types of waves in the case that a wireless charging device could be used. Using CMOS designs and tactics, these types of devices and components could be implemented using minimal power usage, minimal components, and minimally invasive procedures for implementation.

Article in question for reference: New oscillator for low-power implantable transcievers

By Tyler Bigham and Coleman Irby

The University of Mississippi Electrical Engineering Department introduced a Digital CMOS/VLSI Design course this semester. As part of this course, students researched a contemporary issue and wrote a blog article about their findings for presentation on SemiWiki. Your feedback is greatly appreciated.