Antenna Resonance Tuning - Texas Instruments 2000 Series Reference Manual

February 2002

4.2 Antenna Resonance Tuning

In order to achieve a high charge-up field strength, the antenna resonator
frequency must be tuned to the transmitter frequency of 134.2 kHz. This is done by
adjusting the capacitance of the Antenna Tuning Board. Fine tuning may be
performed by adjusting the setting of the tuning coil on the Antenna Tuning Board.
When tuning the antenna using the tuning capacitors on the Antenna Tuning
Board, the resonance condition must be monitored. This can be done by
monitoring the field strength generated by the antenna, and can be performed by
measuring the induced RF voltage of a pick-up coil placed at a fixed distance from
the antenna. The antenna is tuned to resonance when the voltage at the pick-up
coil has reached its maximum value. To utilize this method, the RA-RFM must be
switched into repetitive transmit mode by operating it from a controller unit.
Measurement may be done by using an oscilloscope or a voltmeter, Figure 10
showing the necessary configurations.
To oscilloscope
Pick-up coil
ca. 1mH
Figure 10: Antenna Tuning Pick-up Coil Configuration
As the RA-RFM has only to be tuned to the maximum voltage obtainable at the
pick-up coil, any type of coil may be used, the inductance being of little importance.
However, if a pick-up coil with a high inductance (high number of windings and
large size) is used, it may be positioned further from the antenna.
Monitoring of the correct antenna resonance tuning can also be performed using
the Antenna Tuning Indicator (ATI) tool RI-ACC-ATI2.
This device allows the transmitter to be operated in pulsed mode, independently of
the Control Module. It indicates by LEDs whether the tuning capacity should be
increased or decreased (marked on the ATI as IN for increase and OUT for
decrease) and when the antenna is tuned to resonance, in which case the green
LED is on or flashing together with the IN or OUT LED. The device is plugged into
the RA-RFM connector J2 during the tuning procedure, power being supplied from
this module.
1N4148
Pick-up coil
ca. 1mH
33
Associated Antenna Systems
• •
To voltmeter
• •
1 uF 4.7 MOhm