Design of MF RC500 Matching Circuits and Antennas

ANTENNA TUNING
A correct tuning of the antenna is necessary to achieve an optimum performance. It is recommended to do the final antenna tuning in the complete terminal. This includes, that all parts of the terminal should be connected to the power supply as well as the adjustment of the antenna and the reader PCB to their final position.
The explanation of the tuning procedure is divided into two parts. The first part shows the tuning to achieve an optimum operating distance based on the energy transmission. The second part shows how to check the quality factor of the antenna to ensure proper data transmission.
The antenna tuning procedure is divided into a tuning method for first test during development and secondly during production test. It is recommended to do the first tuning as well as the measurements of the coils equivalent parameters with an impedance analyser. If this device is not available production tuning procedures may also be applied. However in this case a more complex iteration procedure will be necessary.
The most important thing for a successful tuning is the knowledge of the antenna’s equivalent electrical parameters to find the best starting values for the tuning itself. The easiest way to measure the antenna’s equivalent parameters is an impedance analyser.

Tuning Methods for an Optimum Operating Distance
TUNING OF DIRECTLY MATCHED ANTENNAS
The tuning of directly matched antennas should follow the iteration process shown in Figure 7-1. Starting values for the tuning capacitors depend on the coil’s inductance as shown in Table 4

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Figure 7-1. Tuning of direct matched antennas

TUNING OF 50 Ω MATCHED ANTENNAS
50 Ω matched antennas can be tuned using several procedures and different measurement equipment.
–Tuning with an impedance analyser.
This method is the most precise and easiest one, It requires an impedance analyser which is relatively expensive.
–Tuning with an oscilloscope

Tuning with Impedance Analyser
The most precise and easiest way to tune an antenna is using an impedance analyser e.g. a HP 4195 with a 50W-signal divider. Connect the antenna directly to the calibrated analyser and tune the antenna using the
iteration process described in Figure 7-2.
Note: It is important that the analyser is warmed up, calibrated (range e.g. 1-30 MHz) and correctly compensated with the measurement cable for precise measurements!

Iteration Process:
Since the values for CS and CP can only be calculated with an error of 20% the final circuitry has to be determined by the following iteration process:

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