User's Guide

antenna coil axes (X, Y and Z-axis).Typically, this calibration is performed during the final test using high precision
equipment at the fob’s manufacturing facility. However, with the help of the CARS PC application, the end-of-line
calibration sequence is approximated by clicking the Calibrate Fob button in the System Configuration window.
The normalization procedure is used to establish a relationship between a known magnetic flux density and a
measured RSSI value. Ideally, an Helmholtz coil is used to perform this task but, considering the constraints of the
kit, this is not feasible. Instead, the RSSI value for each fob’s LF antenna coil axis (X, Y and Z) is measured at a fixed
distance (50 cm) from the transmitting LF antenna. Arbitrarily using the x-axis as the reference, the difference in
measurements of the other two axes are stored in the EEPROM as the Normalization Constants. On the other hand,
the compensation procedure accounts for any non-ideal influences; for example, magnetic flux disturbances due to
adjacent ferrous bodies, temperature, aging effects and so on. The compensation procedure is based on the
following:
• “Ref. RSSI” – Denotes the internal RSSI values measured with reference conditions (end-of-line) [1]
• “Int. RSSI” – Denotes the current internal RSSI measured with no external LF signal present [2]
• “Ext. RSSI” – Denotes the current external RSSI measured with external LF signal present [3]
The actual RSSI amplitude is, then, calculated by adding the error term [1] – [2] to the values measured at [3]. The
normalization and compensation procedures ensure that a constant RSSI is reported, regardless of the fob
orientation, and ensures accuracy over a large set of influences.
5.3.5 Fob Calibration Process
Perform the following calibration steps for each fob separately:
1. If not done already, follow the procedure detailed in 4. CARS Kit PC Evaluation Utility.
2. In the System Configuration window, choose the fob to be calibrated in the “Selected Fob” drop-down list, as
shown in the following figure.
3. Click the Fob ID button followed by the Calibrate Fob button.
ATAN0219
System Operation
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User Guide
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