Auxiliary circuit for the adjustment of transmitters for powering implants by coupled coils

IT HAS been shown (DONALDSON, accompanying paper) that, when a regulated voltage supply is to be provided in an implant to which power is transferred by inductive coupling between coils, the efficiency is dependent on the type of regulator, the regulator voltage, the coupling coefficient and also the transmitter frequency. With a typical transmitter/receiver pair, working at a frequency of about 3 MHz, it is desirable to adjust the transmitter to within _+10kHz to avoid unnecessary loss. Even if the receiver output is not regulated, for instance when it is connected straight to stimulating electrodes, the transmitter still has to be close to the right frequency. This correct frequency ego, according to the theory of DONALDSON and PERKINS (1983), is that at which the impedance for the implanted receiver, coupled into the transmitter resonant circuit has no reactive component at any value of coupling coefficient. This condition is met when the phase of the current in the transmitter circuit does not change as the coils' separation varies. Although this can readily be seen, using a dual-beam oscilloscope which shows the voltages across the tuning capacitor, and of a reference phase simultaneously, this is rather inconvenient if one merely wishes to check the frequency after the receiver has been implanted. An alternative to the use of an oscilloscope is the phase detector circuit shown in Fig. 1. The meter indicates the angle between the two voltages in the transmitter circuit; it is