C.3 Q Factor - Texas Instruments 2000 Series Reference Manual

Q Factor
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:
π
2 fL
Q =
R
Where: f = 134200 Hz (Frequency = 134.2 kHz)
L = Inductance (H)
R = Series resistance ( )
Example:
Inductance (L) = 47 µH
Resistance (R) = 2.2 Ω
π
2 ×
Q =
39.636
=
2.2
= 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:
2
C = 1 / (4π L
tot Ant
If the antenna inductance is less than 46.1 µH you can add an extra capacitor (externally) to the antenna
between pin 16 (ANT1) and pin 17 (ANTCAP). The formula to work out the value of this capacitor is:
C = C – 30 nF
ext tot
If the antenna inductance is more than 47.9 µH you can add an extra capacitor in series with the antenna
between pin 19 (ANT2) and the antenna. The formula to work out the value of this capacitor is:
1 1 1
= –
C C 30 nF
ext tot
46 Antenna Design
Ω
× 134200 × 0.000047
2.2
2
f )
www.ti.com
SCBU027 – May 2000
Submit Documentation Feedback