AN912

AN912 Designing LF Talkback for a Magnetic Base Station Author: Ruan Lourens Microchip Technology Inc. MAIN BUILDING BLOCKS Figure 1 shows the main building blocks that make up the LF Talkback system described in this document. The base station generates a strong magnetic field by setting up resonance in a serial resonant tank. The circulating energy in the resonant tank typically generates 300V peak-to-peak voltage across the transmitting antenna coil at 125 kHz. The transponder, whether active or passive, is magnetically coupled to the base station’s transmitting coil and the transponder’s magnetic loading has a small effect on the quality factor (Q) of the transmitter resonant tank. Talkback is accomplished by changing or modulating the magnetic loading and can be observed as small voltage changes across the base station's resonant transmitter coil. The difficulty is to detect a few mV of modulation on the 300V peak-to-peak carrier. INTRODUCTION This application note builds on application note AN232 Low Frequency Magnetic Transmitter Design (DS00232). It covers the design process to implement LF Talkback functionality. AN232 covers some of the magnetism basics and design principles to implement the drive circuitry. LF Talkback generally refers to the process in which a transponder can communicate back to a magnetic transmitter base station by loading the generated magnetic field. By measuring the small changes in the transmitter coil's voltage, used to generate the field...