Design of MF RC500 Matching Circuits and Antennas

Tuning procedure with oscilloscope
Remark: The complete tuning of a 50 Ω antenna using only an oscilloscope is not as precise as the tuning with an impedance analyser. It is recommended to do the first tuning of the antenna with an analyser and not with an oscilloscope. The tuning with a scope can be used for production tuning when only one parallel capacitance CP has to be tuned.  The most critical part in the antenna design and tuning procedure without an impedance analyser is to calculate the value for LANT and RANT. Depending on this value the tuning has to be redone after checking the Q-factor. Based on these values for LANT and RANT the values for the serial and parallel capacitors have to be calculated as described in chapter 3.4.5. To tune the antenna, reduce the calculated values for the capacitors by 40 % and insert them as Cs and Cp in the impedance matching network. Add parallel variable capacitors C’s and C’p allowing to adjust – 20% of the calculated value.

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Figure 7-3. Matching circuitry

The necessary equipment for the final tuning is shown in Figure 7-4. A reference resistor of 50 Ω– 2% (e.g. 50 Ω BNC terminating resistor) is inserted in the ground line between the function generator output and the antenna connector. The two probes of the oscilloscope are connected to the function generator output and in parallel to the reference resistor. The components Cy-probe and Cx-probe present the oscilloscope probe input capacitance. The oscilloscope will display a Lissajous figure, allowing to derive the absolute magnitude and the phase. The magnitude is given by the angle of the Lissajous figure and the area as depicted in the figure below gives the phase.
The tuning procedure has to be done in two steps:
Step 1: Calibration
For the calibration a calibration resistance of 50 Ω has to be inserted instead of the antenna. The calibration procedure is depicted in the next figure. The function generator shall be set to:
Wave form:  Sinusoidal 
Frequency :  13.56 MHz 
Amplitude:  2V – 5V 

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Figure 7-5. Calibration

The x-probe capacitance Cxprobe reduces only the amplitude at the function generator output. This has no influence on the tuning results.
The y-probe capacitance Cy probe affects a phase shift, which changes the area of the Lissajous figure. To compensate this effect, the capacitor Ccal is connected in parallel to the matching network. In the calibration phase the matching network is replaced with a second resistor of 50 Ω (e.g. 50 Ω BNC terminating resistor). The calibration capacitor has to be adjusted until the Lissajous figure is completely closed. Then the calibration capacitance Ccal is equal to the capacitance Cy-probe. The y-probe voltage is in phase and the amplitude is exactly half of the function generator voltage (x-probe).
NOTE: If the scale for the x-probe is chosen twice the scale for the y-probe (e.g. x-scale: 2V/DIV and y-scale: 1V/DIV) the Lissajous figure angle shall be 45 degree.
NOTE: A loop of the ground cable of the probe shall be avoided to minimise inductive coupling from the antenna.

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After the calibration, the calibration resistor has to be replaced by the antenna. The matching network shall be tuned by the variable capacitor C’s and C’P until the Lissajous figure is completely close. The Lissajous figure angle has to be compared to the Lissajous figure angle of the calibration resistor. If the angle is equal to the angle of the calibration resistor, the matching circuit impedance is 50 Ω.

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Notes to interpret the Lissajous figures:.
If the figure is not closed the phase between x and y is unequal to zero.
If the angle j=0°, the Lissajous figure is closed completely.
If the angle is greater than 45°, Z is greater than 50 Ω.
If the angle is smaller than 45°, Z is greater than 50 Ω

The resonance curve of an antenna has two zeros in the phase as shown in Figure 7-7. It is only possible to tune the lower frequency fLOW to Z=50 Ω and j= 0°.
The zero at the higher frequency can not be tuned to Z=50 Ω.

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Figure 7-7. Input impedance and phase of a tuned circuit
To be sure that the tuning is done to the lower frequency, it is recommended to reduce the calculated value for CS and CP by 40% and add tuning capacitors in that range. Start the tuning with the lowest values for the tuning capacitors.
The complete tuning procedure is described as a flow chart in Figure 7-8.

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Figure 7-8 . Tuning of 50 W antennas using an oscilloscope

Checking the Q-Factor
To check the calculated Q-factor for a tuned antenna a very simple measurement set-up can be used. An oscilloscope with a bandwidth of at least 50 MHz has to be used and two probes have to be connected as shown in Figure 7-9.

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Figure 7-9. Setup t o check the Q-factor
The probes have to be connected in the following way:
CH1: Form a loop with the ground line at the probe to enable inductive signal coupling.
Hold the probe loop closely above the antenna.
CH2: Connect probe to the NPAUSE0 signal in your MIFAREâ reader, (it is used for easy triggering ) Trigger source = CH2.
To check the pulse shape it is recommended to compare the plot on the scope to Figure 7-10. The values can be found in Table 8. To check the correct tuning the time t2 is of special interest. This time describes the
time span, in which the signal falls under the 5 % value of the 90% value of the amplitude of the signal. For a correct tuning of antenna especially for a correct value of the external resistance REXT the following has to be fulfilled:
The signal has to fall under the 5% value.
The time t2 should not exceed 1.4μs. If t2 is greater than 1.4μs the Q-factor is greater than 35 and the correct data transmission can not be guaranteed. Increase REXT.

If the time t2 is shorter than 0.7 μs the Q-factor is to high and the operating distance will be dissatisfying. Decrease REXT.

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Figure 7-10. Pulse shape according to ISO 14443.

Table 8. Pulse duration according to ISO14443.

Pulses length t1 [µs] t2 [µs] t3 [µs] t4 [µs]
T1 MAX 3.0 1.4 1.0 0.4
T1 MIN 2.0 0.7 1.0 0.4