Monolithic circuits for a pulse-powered micropower command receiver

THE STRINGENT REQUIREMENTS imposed on implantable telemetry systems often preclude continuous operation . In such cases, the operating life of battery powered implants must be extended by the use of a radio-controlled power switch, or command receiver, to disconnect the battery when physiological data are not needed. Micropower monolithic circuits for an implantable command receiver have been demonstrated, but their use has been limited by short range and availability problems . Continuous micropower operation limits this 1 5 PA receiver to a carrier frequency of approximately 1 MHz which results in a range of less than 3 m when a hand-held transmitter is used. By using a new minimum duty-cycle pulse powered command receiver, the carrier frequcncy can be increased to 30 MHz with virtually no increase in current drain compared with the theoretical 30 times increase which continuous powered operation requires. This results in a 1 0 times greater range of 30 m and markedly reduced frequency of false triggering with a receiver drawing only 20 pA of current. The range increase results from improved antenna efficiency and reduced antenna noise figure at the higher frequency3. Receiver availability is substantially enhanced through the use of a Kitchip requiring only one custom metal mask to implement the receiver using a dic with a fixed layout of monolithic elcmcnts4. A block diagram of the pulse powered receiver is shown in Figure 1, and its specifications are given in Table 1. The RF amplifier and detector (Figure 2 u ) are pulse powered reducing power consumption from 1 2 mW to 30 pW of power. While monolithic circuits can be built readily with useful gain at 30 MHz, the inductor tuning or feedback methods typically required to reduce power dissipation and bandwidth in a receiver are not practical in a monolithic realization. However, operation of the receiver in a pulse powcrcd or sampling mode allows the average power dissipation to be reduced5 along with signal bandwidth very easily. Figure 3 shows how this occurs. Previously this pulse-powered technique has been applied to receiver 1