AN232

DS00232A-page 4

 2002 Microchip Technology Inc.

FIGURE 4:

FREQUENCY RESPONSE CURVE FOR RESONANT TANK CIRCUIT

MANUFACTURING TOLERANCES

A good rule of thumb is to stay within the -3 dB limits,

giving component tolerances by Equation 5.

EQUATION 5:

ances for capacitance and inductance. For 2% parts, a

Q of 20 works very well. Lower tolerance components

may be used at the expense of sensitivity, and thus

yielding a lower range. The corresponding final design

must accommodate a wider bandwidth and will,

therefore, have a lower response.

Data Formats

In designing a LFMC system, one has the choice of

implementing any one of a large variety of modulation

formats. On-Off Keying (OOK) lends itself well to real-

izing a practical and reliable system. With OOK, the

signal is modulated by simply turning the field

generation source on and off, depending on a chosen

data format.

An example of a half bridge drive systems dynamic

response is shown in Figure 5. Figure 5 shows the

typical response when turning a tank circuit on (i.e.,

applying a drive signal) and, after some time, switching

the tank off again. It can be seen that the oscillations

amplitude increases rapidly upon start-up. It then

increases up to a maximum amplitude as predicted by

Equation 4. There is some finite time required for the

tank to start-up and reach the eventual maximum

amplitude. There is similarly a finite time required for

the tank oscillations to decrease to some desired level.

The rise and fall times will be the predominant factors

in choosing a baud rate. Other factors are more con-

cerned with receiver design and choice of AGC (Auto-

matic Gain Control) topology. LFte is the elemental

period used in LF communication, and a practical value

is 400

µs or longer.

The Manchester, PWM, and PPM data formats are

shown in Figure 6. Manchester has the advantage of

having a constant duty cycle and more efficient data

rate. PWM encoding, on the other hand, simplifies the

receiver decoding and improves the bit error rate.

When designing an LFMC system, one should keep in

mind that it will be exposed to noisy environments. It is

a good idea to incorporate some error correction and

detection scheme upon implementing a communica-

tion protocol.

0

5

10

15

20

25

F (Hz)

Vo

-3dB

Serial

I

M

2

-------

ind

cap

T

T

Q

+

≤

1