Micore Reader IC Family; Directly Matched Antenna Design
General Checklist for Micore antenna design
Table 8: Checklist | |||
Parameter | Requirement | Comment | Check |
Power | maximum | maximum operating distance | |
ITVDD | ITVDD<150mA DC | detuning has to be considered! | |
Urxpin | 1.5Vpp < Urxpin< 3Vpp | measured at pin 29 | |
Quality factor | Q<30 | check with oscilloscope | |
Temperature influence | operating range | Min & max operating distance | |
Temperature influence | ITVDD <150mA DC | ||
Environmental changes | operating range | Min & max operating distance | |
Environmental changes | ITVDD <150mA DC |
Simple method for impedance measurement
If no impedance analyzer is available, the impedance measurement and tuning could be done with the following simple method.
The test setup consists of:
1.Signal generator (13.56 MHz)
2.Oscilloscope with low impedance probe
3.Measurement circuit as shown in Fig 21
The two probes of the oscilloscope (Cxprobe Cyprobe ) are connected to the function generator output and in parallel to the reference resistor. The oscilloscope displays a Lissajous figure, allowing us 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 x-probe capacitance Cxprobe only reduces the amplitude at the function generator output. This has no influence on the tuning results.
The y-probe capacitance Cyprobe 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.
The tuning procedure has to be done in two steps:
Step 1: Calibration
For the calibration a reference resistor of 500 Ω has to be inserted instead of the antenna.
The calibration procedure is depicted in Fig 21. Th e function generator shall be set to:
Table 9: Settings of function generator for calibration | |
Parameter | Value |
Wave form: | Sinusoidal |
Frequency: | 13.56 MHz |
Amplitude: | 2V – 5V |
The calibration capacitor has to be adjusted until the Lissajous figure is completely closed (phase = 0°). Then the calibration capacitance Ccal is equal to the capacitance Cyprobe. The y-probe voltage is in phase and the amplitude is exactly half of the function generator voltage (x-probe).
Remark: If the scale for the x-probe is chosen twice the scale for the y-probe (e.g. xscale: 2V/DIV and y-scale: 1V/DIV) the Lissajous figure angle shall be 45 degree.
A loop of the ground cable of the probe shall be avoided to minimize inductive coupling from the antenna. The use of a low capacitance, high frequency probe is recommended.
Fig 22. Correct connection of ground cable loop
Step 2: Tuning procedure:
After the calibration, the reference resistor has to be replaced by the antenna circuit (Z) as shown in Fig 23 the matching network shall be tuned by the (variable) capacitors C1 and C2 until the Lissajou figure is completely closed. Now the Lissajou figure angle has to equal to the angle of the calibration step. In this case the impedance of the tuned antenna has Z = 2⋅ Z_{ant }= 500Ω ⋅e^{0}^{0 }.
Fig 23. Simple impedance measurement, tuning procedure
Notes to interpret the Lissajou figures:
- If the figure is not closed the phase between x and y is unequal to zero.
- If the angle ϕ=0°, the Lissajou figure is closed completely.
- If the angle is greater than 45°, Z is greater than 500 Ω.
- If the angle is smaller than 45°, Z is greater than 500 Ω
Remark: This calibration and tuning procedure principally may be done with any required impedance value. In praxis the environmental influences have to be considered. Therefore this method typically is limited to impedance values < 1kΩ.
The impedance curve of an antenna (as shown in Fig 9) ha s two points of resonance, where the phase is 0°. It is only possible to tune the lower one of both these resonance
frequencies (fLOW) to the required Z = 2⋅ Z _{ant}= 500Ω ⋅e ^{00}
To be sure that the tuning is done to the lower resonance frequency, it is recommended to reduce the calculated value for C 1 and C2 by 40% and add tuning capacitors in that range: Start the tuning with the lowest values for the tuning capacitors.