AN912 Datasheet, PDF (3/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

The envelope detector with only D1 and C2 has a
greatly different response to increasing and decreasing
voltage amplitudes of the resonant tank. The voltage
designated by the signal HV_Env (Figure 2) rises
quickly with increasing tank amplitudes because D1
has a low-impedance in forward conduction. The tank
voltage decreases slower when the tank amplitude is
lowered because C2 can only discharge through D1,
which has a high-impedance in the reverse direction.
The situation can be remedied to some extent by the
introduction of R1 which helps to discharge C2, but the
value of R1 should be high enough to maintain a good
tank Q as per requirement 2 above. A 10 MΩ value for
R1 works well, but note that R1 needs to be
implemented as a series of two resistors. This is done
to stay within the safe voltage range of 0805 resistors
are used.
The 125 kHz carrier ripple voltage, without R1, is about
2V peak-to-peak and is due to the junction capacitance
and reverse leakage of D1. The addition of R1 has little
effect on the ripple voltage, but does improve the
detectors dynamic performance at the data rate. The
carrier ripple voltage will be filtered out at a later stage
where a more effective solution can be implemented.
THE DC DECOUPLER CONFLICTS
The HV_Env signal (Figure 2) consists of three main
components:
1. A 150V DC signal, as a result of the peak
detector.
2. 2V peak-to-peak ripple voltage at the carrier
frequency.
3. The modulated data signal at a TE of 200 µs and
a 2 mV peak-to-peak amplitude, highest funda-
mental harmonic content is at 2.5 kHz [1/(2*200
uS)], irrespective of the modulation scheme
used (i.e., Manchester, PWM etc.).
The aim of the decoupling stage is to reject the high DC
voltage without adding unnecessary loading to the tank
via the peak detector. It should also have a fast dynamic
response and stabilize quickly after the tank is ener-
gized. The dynamic response of the LF Talkback system
is the major design hurdle to overcome as far as the
decoupling stage is concerned. The problem is aggra-
vated when the transponder needs to communicate on
the LF link soon after the base station communicated
with the transponder.
The base station typically uses On Off Keying (OOK)
modulation to communicate to the transponder. This
means the tank resonance is completely halted and
then started up to transfer data via the magnetic link.
The decoupling stage experiences large “step”
responses as data is transmitted to the transponder.
The tank can ramp up to its full resonant amplitude in
100 µs to 400 µs depending on the drive system used.
FIGURE 3:
HV-Env
C
R
AN912
LP Filter
The system can be simplified as shown in Figure 3.
The output of the peak detector can be simplified as the
step response source with a 150V amplitude that also
has the carrier and data signals superimposed on it as
described earlier. The output response of the
decoupling stage is given by Equation 2. This is also
the input signal to the low-pass filter.
EQUATION 2:
V = 150e^-t/τ
τ = RC
It is useful to think in terms of τ (RC time constant)
because the voltage across the resistor reduces by a
factor of 0.368 as every τ second elapses. The
exponential decay curve, for the voltage across R, is
shown in Figure 4 and indicates that the initial voltage
decays rapidly, but settles out slower as the voltage is
reduced across the resistor. The system must be
allowed to settle for a long enough period so that the
step response voltage has reduced to a voltage that is
smaller than the modulation voltage.
The required value for RC, or τ, can be calculated using
Equation 3, based on the following assumptions:
• The system needs to be able to start LF communi-
cations 200 µs after the resonant tank has
stabilized.
• The decoupler should settle to at least half the
data modulation voltage.
 2004 Microchip Technology Inc.
DS00912A-page 3