09-07-2019 | By Rob Coppinger
Predictive adaptive antenna tuning for inductive charging and data transfer could allow wireless bandwidth rates to increase.
Inductive links such as radio-frequency identification (RFID) and wireless charging normally use the same carrier frequency, used primarily for power transfer. The induction needs a magnetic field strong enough for a practical operating range and for this a high-Q resonant transmitter coil is required. High Q refers to the quality of the inductance, which is the level of inductance losses due to the circuit resistance. However, with a high-Q resonant transmitter coil there are limits to the available bandwidth, which can also reduce the power transfer level by as much as 50%. High-Q coils are also sensitive to environmental factors.
The bandwidth limits are due to the fluctuation in the high-q coil’s resonance. If the resonance can be maintained, the data rate and power transfer can be improved. Resonance is the cycle between the inductor's magnetic field collapsing and the inductor's winding's current charging a capacitor that recreates the magnetic field again in the inductor.
This is achieved with the automatic tuning. Multiple tuning systems ensure that the resonance, and therefore the bandwidth, can be improved. The adaptive tuning system has at least three control loops to maintain continuous resonance. A smart power 50-volt CMOS processor was developed for controlling this tuning. This system also uses an extra capacitor. The tuning technology has been developed by University of Southampton researchers, from its school of electronics and computer science.
“We could scale this up to quite high powers, we’ve done it in the few watts range, a hundred milliwatts, but there is nothing to stop this going to the kilowatt region at lower frequencies,” says University of Southampton emeritus professor William Redman-White. He adds that this automatic tuning technology, which has been patented, could improve the function of inductive technologies like cooking hobs.
The automatic tuning of the high-q resonance inductive system operates all the time, without having to go off-line and carry out a test. Even when a magnet is introduced into the immediate vicinity the re-tuning can defeat any interference. Redman-White explained that: “If you have frequency shift keying, what you do is you maintain a second tuning loop that remembers what the tuning was when it was at that last frequency. I jump to one frequency and pick up tuning loop one and it adjusts itself automatically and then I have a data bit and want to switch to frequency two.”
The operator can then switch to tuning loop two and the previous value it had means it will adjust the frequency to return to that; irrespective of any frequency drift that occurred in the meantime. Back and forth, “I can jump frequency, talking 30% in frequency, big jumps, and I will be totally in resonance. I can do it in such a way that current flowing through the inductor is never interrupted,” said Redman-White. There is no abrupt change in the magnetic field or loss of the power flow being transmitted to, for example, a mobile phone or an RFID tag. This predictive adaptive antenna tuning method was demonstrated by University of Southampton electronic engineers at the Institute of Electrical and Electronics Engineers’ International Solid-State Circuits Conference.