Antenna Resonance Tuning; Tuning Procedure - Texas Instruments 2000 Series Reference Manual

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Note: The transformer of the transmitter power stage is operated at a high magnetic flux. Due to
the high level of magnetic flux change, the transformer may emit an audible tone. This
may also occur with antennas that have ferrite cores (e.g. TI-RFid Standard Stick Antenna
RI-ANT-S02). This tone does not indicate a malfunction.
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 changing the capacitance of the antenna resonator.
To compensate for the tolerances of the antenna coil and the capacitors, six binary weighted tuning
capacitors (C_ATC1 to C_ATC6) have been included. Their values are weighted in steps of 1, 2, 4, 8, 16
and 32, where C_ATC1 has the smallest value corresponding to the factor 1, C_ATC2 has double the
capacity of C_ATC1, so that C_ATC2 corresponds to the factor 2 and so on. Each of the 6 tuning pins has
an adjacent ground pin for antenna tuning, using shorting bridges (jumpers).
Monitoring of the correct antenna resonance tuning can 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 RFM
connector J2 during the tuning procedure, power being supplied from this module.
The following notes refer to antenna resonance tuning in general:
Note: If an antenna has to be installed in an environment where metal is present, the tuning of
the antenna must be done in this environment, since the presence of metal changes the
inductance of the antenna. In addition, the Q factor of the antenna decreases, thereby
decreasing the field strength. The extent of the inductance and quality factor reduction
depends on the kind of metal, the distance of the antenna from it and its size.
When the oscillator signal pulse width, or the supply voltage VSP of a RFM with a
pre-tuned ferrite core antenna (for example: RI-ANT-S02) is changed by a factor of more
than 50%, the ferrite core antenna has to be re-tuned to the new conditions due to the
inductance changing slightly at different field strengths.
Each antenna is tuned individually to the RFM and this results in a unique tuning jumper
arrangement for this combination of antenna and RFM.
If a different antenna or RFM is connected, the new combination must be tuned to
resonance again.
4.3

Tuning Procedure

1. Switch RFM power supply off.
2. Connect the antenna to the RFM by means of the two M3 screw connectors.
3. Install antenna tuning monitoring unit.
4. Switch RFM power supply on.
5. Tune antenna to resonance by changing the tuning capacity.
6. Switch RFM power supply off.
7. Disconnect monitoring unit.
8. Switch RFM power supply on again.
The antenna resonance tuning is now complete.
SCBU022 – November 2002
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Antenna Resonance Tuning
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