 2002 Microchip Technology Inc.

DS00232A-page 1

AN232

INTRODUCTION

Low frequency magnetic communications (LFMC) is a

viable “wireless” communications alternative to tradi-

tional radio frequency (RF) or Infrared communica-

tions. It is well suited for certain applications when

considering some of the characteristics of the topology.

Some of the main advantages of using low frequency

magnetic communications are:

• Good field penetration capabilities - Can pene-

trate non-magnetic materials such as water,

concrete, plastic, etc. (no line-of-sight required).

• Limited and precise control of range - This may be

a disadvantage if long range is required, but for

certain applications this is a big advantage where

limited or fixed range is required. For example, it

is useful in automotive communications, or invisi-

ble fence control such as required for pets or

water safety around pools.

• Low power designs are possible, especially on

the receiver side. This factor makes LFMC very

attractive for PKE (Passive Keyless Entry), where

the key fob constantly “listens” for a valid car and

the device needs to be powered by small lithium

batteries with years of useful battery life. It is use-

ful in TPM (Tire Pressure Monitoring) where the

sensor is awakened by a low frequency signal to

preserve battery life.

• Low frequency design techniques (i.e., relatively

low frequency compared to RF) allow the

designer to use low frequency analog tools and

building blocks. The designer has the freedom to

use regular Op Amps, comparators, and a gen-

eral-purpose oscilloscope.

• Energy transfer - It is possible to power a receiver

from the magnetic field. A good example is RFID,

or alternatively, a low voltage back-up.

• Low cost - A low cost transceiver can easily be

implemented by adding a resonant tank (LC) to a

microcontroller with a PWM and comparator.

ABOUT THIS APPLICATION NOTE

This Application Note covers some basic aspects to

consider when designing the transmitter portion of a

LFMC link, such as:

• A description of the components that comprise the

LFMC link.

• Explanation of the magnetic basics and assump-

tions made in the Application Note.

• Calculating the generated field strength that is

inversely proportional to the cube of the distance.

• A practical method for generating a magnetic field

is to create a serial resonant tank circuit.

• Data transfer is, in turn, accomplished by ampli-

tude modulation of the field.

• Basic data formats that can be used in this kind of

application.

• A description of a typical drive circuitry to gener-

ate the LFMC field.

LFMC LINK COMPONENTS

A LFMC link (see Figure 1) in its most basic form con-

sists of a field generation source transmitter and a

magnetic sensor that is sensitive to the generated field.

Thus, there needs to be a field propagation path to

“link” the transmitter and receiver. The propagating

medium plays a major role in the performance of the

communications link. It should be stressed that mag-

netic field behavior is not the same for electromagnetic

waves normally associated with RF communications.

Electromagnetic waves propagate for long distances in

free space. The RF electromagnetic wave is, however,

susceptible to scattering and distortion. Magnetic field

lines, however, are less prone to distortion and known

to penetrate water very well. A magnetic field does

attenuate much more rapidly when compared to an

electromagnetic wave.

This document focuses primarily on serial resonant

tanks as a field transmission source. The tank consists

of an air-coiled inductor and capacitor. Signal detection

is typically accomplished with a parallel resonant tank.

Author:

Ruan Lourens

Microchip Technology Inc.

Low Frequency Magnetic Transmitter Design