AN232 Datasheet, PDF (1/12 Pages) Microchip Technology – Low Frequency Magnetic Transmitter Design

AN232
Low Frequency Magnetic Transmitter Design
Author: Ruan Lourens
Microchip Technology Inc.
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.
 2002 Microchip Technology Inc.
DS00232A-page 1