A NARROWBAND, CRYSTAL CONTROLLED BIOMEDICAL TELEMETRY SYSTEM

Abstract

A miniature, single-channel, crystal-controlled transmitter has been developed for biomedical applications. A narrowband frequency modulation (±7 kHz) of the RF is used to achieve maximum operating range with minimum transmitting power. The radiated power is limited to stay within the 50 μv/m at 15 m FCC requirement for low power transmitters (Part 15.212 FCC Regulations) in the 88 to 108 MHz band. This technique offers a number of advantages. First only manufacturer’s type approval is required for the device and the user does not need a license. Second maximum operating range can be achieved within the FCC power limitations by using a narrowband that is consistent with the required medical information bandwidth. A third advantage in using the 88 to 108 MHz band is that the commercial FM stations are relatively widely spaced and carefully regulated so that by selecting the transmitter’s frequency in the guard band between stations, minimum interference from other transmitter sources is encountered. For EKG and EEG applications, where an information bandwidth of 0.1 to 150 Hz is adequate, a subcarrier with a fixed frequency (approximately 1 kHz) is used with differential pulse width modulation (DPWM). This type of subcarrier has the advantage of simple circuitry for both the transmitter and demodulator. Since DPWM is self-aligning, it can be handled effectively by magnetic tape recorders having relatively high tape speed errors. For EMG and other high frequency data signals the RF can be directly frequency modulated without the use of a subcarrier.