AN912 Datasheet, PDF (1/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

AN912
Designing LF Talkback for a Magnetic Base Station
Author: Ruan Lourens
Microchip Technology Inc.
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:
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 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.
Transponder
Base Station
Peak
Detector
DC
Low Pass
Decouple
Filter
Data
Slicer
VREF
M
Data
Data
 2004 Microchip Technology Inc.
DS00912A-page 1