AN912 Datasheet, PDF (2/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

AN912
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 kΩ.
The additional parallel impedance that a transponder
represents to induce a 2 mV signal on the tank is in the
order of 500 MΩ. What the LF Talkback system detects
is the result of a 500 MΩ 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
RPARALLEL = 2πLFCQ
L = Tank inductance in H = 162 µH
Fc = Center frequency of tank = 125 kHz
Q = Tank quality factor = 25
THE PEAK DETECTOR
There are a number of aspects to consider in designing
a peak detector for this application:
1. The peak detector has to be able to operate at
the high voltages of the resonant tank.
2. 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.
3. Reduce carrier ripple as far as possible.
4. Maintain the modulation signal.
5. Have a fast large swing dynamic response and
be able to settle quickly after the field is turned
on.
6. 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:
D1
HV-Env
C1
L1
C2
R1
DS00912A-page 2
 2004 Microchip Technology Inc.