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AN912

更新时间： 2024-09-24 03:18:51

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美国微芯 - MICROCHIP		/
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Designing LF Talkback for a Magnetic Base Station

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AN912

Designing LF Talkback for a Magnetic Base Station

MAIN BUILDING BLOCKS

Author:

Ruan Lourens

Microchip Technology Inc.

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 gener-

ates 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 gener-

ate the field, the communications’ data is extracted. LF

Talkback is commonly used in RFID, automotive

transponders, active transponders, and many other

bidirectional LF communications topologies.

This document will cover the different stages needed to

implement a typical LF Talkback system and explain

the process in choosing the different stage characteris-

tics. It explains the various performance and cost trade-

offs made for the reference design and how it can be

adapted to better suit the readers needs.

FIGURE 1:

Peak

Detector

Decouple

Low Pass

Filter

Data

Slicer

Base Station

Transponder

VREF

Data

 2004 Microchip Technology Inc.

DS00912A-page 1

12 3 4 5 6 7...16

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