INTRODUCTION Telemedicine requires low-cost RF / sensing hardware suitable for implementing duplex biotelemetry links; the potential market for home monitoring (with radio telemeters linking bodyworn patient sensing circuitry to the PSTN) is rising as day surgery procedures become commonplace and patients are released from hospital with only a minimum of direct medical backup. The remote acquisition of heart-rate and body temperature is rapidly becoming essential in such circumstances, with the transmission of diagnostic quality ECGs and blood pressure being a future aim. These measurements will ultimately be effected under full remote control, without the patient’s knowledge. The critical parameter for a tetherless clinical monitor is the 2-5 m range typically found between the recuperating patient and the nearest cable point able to support a duplex base station: such distances may be bridged using low power UHF radio.

METHOD We have adopted a transponder design based around a low-sensitivity "video" detector operating in pulsed mode at 2.45 GHz for the base-to-patient link, an on-board microcontroller for patient coding / identification and sensor control, and a 418 MHz SAWR-controlled FSK data transmitter for the patient-to-base path. This paper will discuss in detail the design and operation of the 2.45 GHz hardware.

The transmit path has a free-space path loss of 54 dB at 5 m range; excess loss can amount to an extra 25 dB. For 0 dBi link antennas and a detector sensitivity of -53 dBm, a transmit power of +27 dBm is implied. This is developed using a Pacific Monolithics PM2502 VCO driving a PM2104 power amplifier via a PM2601 RF switch which provides about 18 dB on-off isolation. The complete transmitter is powered from a 3.6 V 19.5 Ah lithium inorganic D-cell.

RESULTS A 50 W microstrip stub-matched zero-bias diode detector forms the heart of the receiver, achieving a tangential sensitivity of -56 dBm. Three-pole RF bandpass filtering provides rejection of out-of-band signals, typically 28 dB in the 900 MHz cellular band. The prototype receiver has a 1.6 kHz bandlimited baseband pulse amplifier, draws 20 m A from a 3 V lithium coincell and occupies 6 cm² of surface-mount board space. Evaluations using the complete system have resulted in a maximum indoor range of 13.5 m.