Power for neurological prostheses: a simple inductive R.F. link with improved performance

A variety of methods has been used or proposed to stabilize the amplitude of signals received by implanted neurological prostheses from external equipment via R.F. inductive links of uncertain geometry. These include: use of Zener diodes, operation at critical coupling, operation inside a Helmholz pair, recoding amplitude data into time intervals or pulse trains and automatic adjustment of transmitter power to compensate for fluctuations in coupling coefficient. Rather surprisingly, a pair of inductively-coupled series-resonant circuits is inherently insensitive to variations in coupling coefficient over a wide range (e.g. 0.03-1.0) if the transmitter coil is part of a self-oscillator. Exploitation of this insensitivity may, in some applications, render the use of special amplitude-fixing strategems unnecessary.