AN912 Datasheet, PDF (9/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

SYSTEM MODIFICATIONS
The system can be modified to better suit the user's
requirements. The first aspect is to change the Bessel
filter for a different LF Talkback TE. The rule of thumb is
to set the filter's 3 dB cutoff frequency to Fc = 1/(2*TE).
The new values for the Bessel filter, with a 400 µs TE,
is given in Table 1.
TABLE 1:
R6 = 3.57k R7 = 15.0k
R8 = 71.5k
R9 = 4.42k
R10 = 5.62k C9 = C12 = 22 nF C11 = 330 pF
In addition to changing the filter cutoff frequency for a
TE of 400 µs, it is possible to increase the gain up to
18 dB and still maintain the carrier rejection chosen. It
is also possible to increase C6 to a 4.7 nF capacitor,
but please note that this will increase the transient
response period. Increasing C6 will not have a
dramatic influence on the overall system performance
and it is not recommended.
LONGER TRANSIENT STABILIZING
PERIOD
The example system was designed with the require-
ment that LF Talkback communications should be able
to start 200 µS after the resonant tank has stabilized.
The tank itself takes 100 µS to 400 µs to stabilize
sufficiently, depending on the drive mechanism. The
example circuit should be able to start LF Talkback
communications with 2 mV of data modulation after
350 µs to 450 µs, from when the tank is turned. The
exact period depends on the residual charge in the
peak detector from previous transmissions.
The system can be simplified and improved if the
system allows for a longer transient stabilizing period
before LF Talkback communications are initiated. The
peak detector capacitor, C5, can be increased propor-
tionally to the longer stabilizing period, but not by more
than a factor of about 3, otherwise it can influence data
modulation sensitivity. R1 and R2 tank resistors should
be reduced if C5 is increased, but not proportionally, it
will effect sensitivity. The combined value for R1 and
R2 should be no less then 4 MΩ.
Capacitor C6 can also be increased, but it will not have
a dramatic performance increase. The biggest advan-
tage of a longer transient stabilizing period is that bias
resistor R3 can be increased. Increasing the value of
R3 will result in a slower change in the signal at point
“A”, which means the tank can be controlled more
accurately during the transient period.
AN912
INCREASED DATA SENSITIVITY
Increasing the system’s sensitivity to the modulated
data signal can increase the LF Talkback range. A solu-
tion has been partly described in the previous section;
increase TE from 200 µs to 400 µs and then increase
the gain by up to 18 dB. The component values for a
system with a 400 µs TE, or a center frequency of
1.25 kHz, and a gain of 100, or a 14 dB increase, is
described in Table 2 below. This approach decreases
the dynamic range that may or may not be used
depending on how well the transponder loads the
resonant tank.
TABLE 2:
R6 = 2.26k
R10 = 5.62k
R7 = 10.5k R8 = 226k R9 = 4.42k
C9 = 33 nF C11 = 100 pF C12 = 22 nF
Another solution is to remove resistor R11 to get the
maximum sensitivity from the comparator, but this will
also increase noise in the data. Another quick solution
is to increase the gain of the decoupler buffer by up to
10 dB and lower the decoupler cutoff frequency by
about half the gain increase ratio.
The existing design makes use of a 3d order Bessel
filter. For improved noise reduction, increase the order
of the filter and add more gain per stage. This would
typically be done if a TE of 200 µs or 100 µs, is desired
with more sensitivity than can be reliably obtained with
the example system.
DRIVE SYSTEM
The example circuit uses a half-bridge driver based on
the TC range of FET drivers from Microchip. To
increase the transient response period of the tank, start
the tank in Full-bridge mode until the desired tank
amplitude is reached and the tank oscillation is main-
tained in Half-bridge mode. This method is described in
AN232 Low Frequency Magnetic Transmitter Design.
CONCLUSION
This LF Talkback Design application note can be used
to implement a cost-effective system to be used in
RFID, passive keyless entry and other bidirectional
transponder based technologies. The example circuit
can be used as a basis for further hardware and firm-
ware development to suit the user's requirements.
 2004 Microchip Technology Inc.
DS00912A-page 9