Field Strength Adjustment - Texas Instruments 2000 Series Reference Manual

The magnetic field strength generated determines the charge-up distance of the transponder. The higher
the magnetic field strength, the further the transponder charge-up distance. The charge-up distance does
not, however, increase linearly with the field strength.
The reading distance of a transponder is determined, amongst other factors, by the charge-up distance
and the local noise level. Increasing the charge-up field strength does not necessarily increase the reading
distance.
The field strength generated by the RFM depends on the four factors listed below:
1. Q factor of the antenna.
The Q factor is a measure of the efficiency of the antenna and therefore the higher the Q factor of the
antenna coil, the higher the field strength generated by the RFM, assuming that all other parameters
remain unchanged. The Q factor of the antenna itself depends on the cross-sectional area of the wire,
the wire type, the size of the antenna and the type of antenna (gate or ferrite). The larger the
cross-sectional area of the RF braided wire, the higher the Q factor of the antenna. RF braided wire
gives a higher Q factor than solid wire assuming that all other parameters remain unchanged.
2. Size of the antenna.
The larger the antenna, the higher the field strength which is generated by the RFM, since the antenna
covers a larger area and thus generates a higher flux assuming that all other parameters remain
unchanged. Large antennas have less immunity to noise for receive functions than small antennas.
3. Supply voltage of the RFM power stage.
The higher the supply voltage of the RFM transmitter power stage (VSP voltage), the higher the field
strength which is generated by the RFM assuming that all other parameters remain unchanged.
However, the generated field strength does not increase linearly with VSP supply voltage. In addition,
ferrite core antennas show saturation effects (saturation means here that the ferrite core cannot
generate more magnetic field strength, even with a higher input current).
4. The oscillator signal pulse width.
The bigger the selected transmitter oscillator signal pulse width, the higher the magnetic field strength
which is generated by the RFM, since more power is fed into the antenna resonator by the transmitter
power stage assuming that all other parameters remain unchanged.
The generated field strength can be measured in several ways. It may be measured using a calibrated
field strength meter or by measuring the antenna resonance voltage using an oscilloscope and then
calculating the field strength.
In summary: the generated field strength of an antenna can be adjusted with the supply voltage VSP of
the RFM transmitter power stage and by selecting the corresponding oscillator signal pulse width.
In cases where low field strengths should be generated with large antennas (RI-ANT-G01 and
RI-ANT-G04), the antenna resonator can be additionally damped by closing jumper JP3.
Using this optional damping function allows the field strength to be again fine-tuned to meet FCC/R&TTE
regulations with selection of the oscillator signal pulse width in a wide range of both larger and smaller
values.
SCBU022 – November 2002
Submit Documentation Feedback

Field Strength Adjustment

Field Strength Adjustment
Appendix B
SCBU022 – November 2002
37