AN912 datasheet(2/16 Pages) MICROCHIP

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AN912

DS00912A-page 2

 2004 Microchip Technology Inc.

A high voltage peak detector is used to extract the

basic envelope of the base station's resonant tank. The

output of the peak detector will be 150 VDC with about

2V peak-to-peak of carrier ripple at 125 kHz and then

about 2 mV of modulated signal. The modulation signal

strength is mostly dependent on the distance between

the transponder and the transmitter coil as the

magnetic coupling decreases to the third power of the

distance between the two devices. The next stage is a

passive high-pass filter to decouple or block the high

DC voltage. The DC extracted voltage is then fed into a

low-pass filter, leaving the required modulating signal.

The last stage is the data slicer that compares the

modulating signal to some reference to extract the

original signal sent by the transponder.

LF Talkback receiver can be thought of as detecting

and decoding an amplitude modulation (AM) signal that

has a very low modulation index on a relatively large

carrier.

SYSTEM ASSUMPTIONS

The LF Talkback system designed in this document is

targeted for a LF base station that has the following

characteristics and is based on the design as per

AN232:

• The LF Talkback signal is amplitude modulated at

200

µs multiples. This is also referred to as the

basic pulse element period or TE.

• The tank is driven by a 12V half-bridge driver.

• The tank inductance is 162

µH and the resonant

capacitor is 10 nF with a resonant frequency of

125 kHz.

• The tank Q is 25. As a result, the tank or carrier

voltage is 300V peak-to-peak or 150V 0-to-peak.

• Transponder induced modulation of 2 mV in

magnitude needs to be detected.

To get an understanding of the impedances involved,

lets consider the following: using Equation 1, the

equivalent parallel resistance of the tank is 3.18 k

Ω.

The additional parallel impedance that a transponder

represents to induce a 2 mV signal on the tank is in the

order of 500 M

Ω. What the LF Talkback system detects

is the result of a 500 M

Ω resistor being switched in and

out in parallel with the tank at the data rate. Therefore,

it is very important that the peak detector have a high-

impedance at the data rate to maintain good sensitivity.

EQUATION 1:

THE EFFECTIVE PARALLEL

IMPEDANCE OF A

RESONANT TANK

THE PEAK DETECTOR

There are a number of aspects to consider in designing

a peak detector for this application:

The peak detector has to be able to operate at

the high voltages of the resonant tank.

Maintain a good tank Q or, in other words, it

should not add unnecessary loading on the main

resonant tank. If it does load the tank, it will

result in a lower modulation voltage induced by

the transponder.

Reduce carrier ripple as far as possible.

Maintain the modulation signal.

Have a fast large swing dynamic response and

be able to settle quickly after the field is turned

on.

Cost of the system.

Some of the peak detector requirements are conflicting

and as a result, the designer has to find an acceptable

compromise with the final system performance in mind.

One can sacrifice a specific parameter and make up for

it in a later stage where optimization of that aspect is

easily accomplished.

As an example to optimize requirement 3, one needs to

increase the size of the capacitor C2 (Figure 2), but

that will negatively affect requirements 2, 4 and 5 if a

passive peak detector is used. An active peak detector

could have solved the conflict, but at the 600V swing,

one has little choice but to use a passive peak detector

while maintaining a low-cost design. A relatively low

capacitance value is chosen for C1 of 1 nF. This

maintains the dynamic response requirement for

settling quickly after the field is applied and does not

load the tank unnecessarily. Capacitor C2 should have

at least a 300 VDC peak rating and a high tolerance

capacity is acceptable to save cost. An ultra fast diode

is required in the peak detector with a 400V or better

rating and low junction capacitance. A UF1005 diode

was chosen, it has a 600V rating and 10 pF of junction

capacitance.

FIGURE 2:

RPARALLEL

= 2

πLFCQ

L = Tank inductance in H = 162

µH

Fc = Center frequency of tank = 125 kHz

Q = Tank quality factor = 25

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