AN912 Datasheet, PDF (5/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

AN IMPROVED DECOUPLER
From the previous section, it is clear that a high-pass
filter is needed with either a controllable τ or a nonlinear
τ that is based on the voltage across the output of the
decoupler. Both approaches will be covered and the
latter solution is shown in Figure 5.
FIGURE 5:
HV-Env
C
R
2.5V -2.5V
LP Filter
The addition of the two diodes, shown in Figure 5,
results in a nonlinear τ with respect to voltage because
it effectively lowers the R component of τ whenever the
voltage is either above 3.1V or below -3.1V. In a practi-
cal circuit, the diodes will start conducting when the
tank is turned on and the voltage, across the resistor R,
is around 3 volts, after the tank has stabilized.
Previously, τ was calculated with an initial voltage of
150 VDC, but if the calculation is repeated with an initial
voltage of 3 VDC, then the required τ comes to 25 µs.
The RC time constant is improved by a significant
factor from 16.78 µs to 25 µs with the additional diodes,
however, it is still not in the 64 µs ball park. The diodes
have the additional advantage in that they protect the
low-pass filter from the large positive and negative
voltages that develop across the resistor during tank
transient periods.
The final part to solving the time constant problem is to
add an additional resistor via a switch, as shown in
Figure 6. The switch is closed to reduce τ from 64 µs to
25 µs during transient periods and opened while data is
received via the LF Talkback link.
FIGURE 6:
HV-Env
C
S1
R1 R2
AN912
2.5V -2.5V
LP Filter
The final part of the decoupling stage is to lower the
output impedance by adding an active buffer in the
form of an inverting amplifier that has an input resis-
tance equal to R1. The use of an inverting amplifier has
the additional advantages that it can add gain and a
single order low-pass filter to the decoupler, as shown
in Figure 7. The gain is equal to the ratio of R3/R1, and
the low pass cutoff frequency is set by R3 and C2, as
per Equation 4. The low pass cutoff frequency should
be chosen at least two decades above the main low-
pass filter, otherwise it will have an undesirable effect
on the envelope response. For single ended 5V
designs, the gain should be limited to about 10 dB to
avoid amplifier saturation due to carrier ripple and data
modulation.
FIGURE 7:
HV-Env
C1 S1
R2
C2
R1
- ^R3
Output
+
 2004 Microchip Technology Inc.
DS00912A-page 5