RF-telemetry transmitter features minimal parts count

Requirements for portable, short-range telemetry systems frequently include low power consumption, small size, and low cost. Although dedicated to conditioning the low-level signal a strain-gauge bridge produces, the circuit can operate with almost any resistive transducer based on a Wheatstone bridge. The circuit comprises a voltage-to-frequency converter (VFC) that produces a pulse-position-modulation (PPM) output, and an on/off keyed (OOK) RF transmitter. Based on direct digitization, the VFC comprises IC1, IC2, IC3A, and IC3B. A Wheatstone bridge containing strain gauge RX produces an output of approximately 5 mV. An integrator stage comprising C6 and IC1, a Linear Technology LTC1250 offset-compensated, low-drift operational amplifier, connects directly across the bridge. According to European Telecommunication Standard I-ETS 300 220, the transmitter requires no license and can operate at any carrier frequency within the 74- to 87.5-MHz band.