A Self-Powered 50-Mb/s OOK Transmitter for Optoisolator LED Emulation

Isolators are used to eliminate ground loops, and also to protect circuits that are sensitive to high voltages. Optoisolators are normally deployed for these purposes, but they suffer from several limitations, such as low speed of operation and temperature instability. In this paper, an ON-OFF keying (OOK) transmitter (TX) is designed so as to emulate and serve as an alternative to optoisolators, without significant changes to the rest of the system. The TX primarily consists of a voltage clamp circuit to convert input current into voltage, a discharge circuit to avoid metastability, and a 300-700-MHz spread-spectrum (SS) oscillator. The start-up and die-down times of the oscillator are optimized for maximum data speed. SS modulation of the oscillator output restricts radiative emissions to within permissible limits. The TX derives power from the input data (current) signal itself, thereby operating without any external power supply, and supports speeds of up to 50 Mb/s. The TX was fabricated in a BiCMOS semiconductor process and tested using an OOK receiver.