User's Manual

38
Micro-reader - Reference Guide May ’00
C.3 Q Factor
If the antenna’s Q factor exceeds 20:
1. The output capacitors will be overloaded and long term damage could result.
2. The antenna may still be resonating when the response from the transponder is
received. Without built-in damping the data will not be correctly received.
3. The antenna may be detuned if there is any metal in the area.
The following formula provides an approximate method of calculating the Q factor of
the antenna:
Where: f = 134200 Hz (Frequency = 134.2 kHz)
L = Inductance (henry)
R = Series resistance (ohm)
Example:
The inductance (L) = 47µH.
The resistance (R) = 2.2 Ohm.
=18
C.4 Adapting the Inductance Range
If your antenna is outside of the required inductance range of 46 to 48 µH, you can
adapt it to work with the micro-reader by adding an external capacitor to it, either in
series or in parallel. You can use this external capacitor to change the inductance
range by ± 5 µH.
You can work out the total resonance capacity using the following formula:
If the antenna inductance is less than 46.1 µH you can add an extra capacitor (exter-
nally) to the antenna between pin 16 (ANT1) and pin 17 (ANTCAP). The formula to
work out the value of this capacitor is:
If the antenna inductance is more than 47.9 µH you can add an extra capacitor in se-
ries with the antenna between pin 19 (ANT2) and the antenna. The formula to work
out the value of this capacitor is:
Q
2πfL
R
------------=
Q
2
π×
134200
×
0.000047
×
2.2
-----------------------------------------------------------------=
39.636
2.2
------------------=
C
tot
14
π
2
L
Ant
f
2


⁄
=
C
ext
C
tot
30
ηF
–=
1
C
ext
-----------
1
C
tot
-----------
1
30
ηF
--------------
–=