AN4327
Application note
CR95HF RF transceiver board tuning circuit with EMI filter
Introduction
The purpose of this application note is to describe the antenna tuning circuit of the CR95HF
RF transceiver board embedding the EMI filter and delivered with the M24LR-DISCOVERY kit.
It explains how to use the CR95HF EMI FILTER CALCULATION.xlsm tool (STSW-
95HF003), which is a separate Excel sheet available on
www.st.com
.
The different impedance matching calculation steps are presented.
Table 1
lists the tool and software concerned by this application note.
Evaluation Tools
Software
Type
Table 1. Applicable tool and software
Part numbers / Product categories
M24LR-DISCOVERY
STSW-95HF003
August 2013 DocID024977 Rev 1
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www.st.com
2
3
Contents
Contents
1
4
5
AN4327
CR95HF tuning circuit with an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2
Designing a tuning circuit without an EMI filter . . . . . . . . . . . . . . . . . . . . . . 5
1.3
Designing a tuning circuit with an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . 6
Calculation explanations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Calculation tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1
Tuning circuit calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2
Input impedance curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3
Circuit voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4
Magnetic field vs distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Practical tuning circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1
Step by step procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2
Input impedance choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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AN4327
List of figures
List of figures
Figure 1.
CR95HF tuning circuit with an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2.
Impedance matching without an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 3.
Impedance matching with an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 4.
CR95HF + EMI filter equivalent generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 5.
Circuit for tuning calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 6.
Tuning circuit calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 7.
Antenna circuit input impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 8.
Voltage calculation at various locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 9.
Magnetic field (H) versus distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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CR95HF tuning circuit with an EMI filter
1 CR95HF tuning circuit with an EMI filter
AN4327
1.1 Description
To limit the spurious emission at high frequencies, the CR95HF RF transceiver board embeds a second order low-pass filter (so called EMI filter).
The EMI filter is placed between the CR95HF and the antenna tuning circuit, as you can see on
Figure 1
. Its goal is to attenuate the frequencies above 13.56 MHz.
Figure 1. CR95HF tuning circuit with an EMI filter
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1.2
CR95HF tuning circuit with an EMI filter
Designing a tuning circuit without an EMI filter
Designing a tuning circuit without an EMI filter for the CR95HF consists in calculating the
C11, C12 and C2 capacitor values, so that the input impedance (Zin1) of the circuit, seen from TX1 and TX2, matches the complex conjugate of the CR95HF output impedance (see
AN3394).
Figure 2. Impedance matching without an EMI filter
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CR95HF tuning circuit with an EMI filter
1.3 Designing a tuning circuit with an EMI filter
AN4327
Designing a tuning circuit with an EMI filter for the CR95HF follows the same procedure except that C11, C12 and C2 are calculated so that the tuning circuit input impedance (Zin2) matches the complex conjugate output impedance of the new RF generator, which is made of the CR95HF and its EMI filter (see
Figure 3
).
Figure 3. Impedance matching with an EMI filter
When the matching criteria is satisfied, the input impedance (Zin) of the circuit seen from
TX1-TX2 also satisfies the condition Zin = Rout, and a maximum power transfer occurs between the CR95HF and the antenna.
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AN4327 Calculation explanations
The CR95HF + EMI filter equivalent circuit comes after some simple transformation (see
Figure 4
).
Figure 4. CR95HF + EMI filter equivalent generator
The cut-off frequency of the EMI filter defined by
14 MHz.
fc
=
2 ( ) is chosen above
It is recommended to use an inductance wired on ferrite cores. However, to maximize the
EMI performance of the circuit on the CR95HF RF transceiver board, the wired inductance has been replaced by a Wurth Elektronik ferrite bead EMI suppressor (ref. 742792042). Its impedance at 13.56 MHz is 653 nH in series with 2
Ω. Above 200 MHz, it behaves as a pure resistor to suppress spurious emission.
Among the available standard SMD ceramic capacitor values, C01 and C02 have been chosen to 180 pF.
The EMI filter cut-off frequency is: fc
=
2 ( )
=
2 (
1
=
14.6MHz
) ×
2
The CR95HF receiving path RX1-RX2 input impedance ZRX_CR95HF = 22pF // 80 k
Ω.
After replacing the RX path by its impedance, the circuit becomes as in
Figure 5
.
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Calculation explanations
Figure 5. Circuit for tuning calculation
AN4327
Assuming C11 = C12 = C1, the resulting equation is:
Zin2
=
C1
2
+
(
C2
||
(
+
CR95HF
)
Solving the Impedance matching criteria Zin2 = Zout_EMI* allows you to find the values for
C11 = C12 and C2.
This calculation is done using the CR95HF EMI FILTER CALCULATION.xlsm spreadsheet.
Connect to
www.st.com
to download this Excel calculation tool.
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AN4327 Calculation tool
The CR95HF EMI FILTER CALCULATION.xlsm spreadsheet includes 4 tabs:
1.
Tuning circuit calculation
2. Input impedance curves
4. Magnetic fields vs distance
This tool allows you to:
•
Calculate the ideal tuning capacitance C11, C12 and C2 based on the system components (select the 1 st
tab).
• Calculate the theoretical circuit input impedance according to the system parameters and custom tuning capacitance values (select the 1 st
tab)
– This feature lets you use tuning capacitance values different from the ideal values, and check the impact on the input impedance.
– In combination with the impedance curve given in the 2 nd
tab, this feature lets you adjust the tuning capacitance values on the Printed circuit board (PCB).
•
Trace the theoretical circuit input impedance curve (magnitude and phase) versus the frequency, according to the custom tuning capacitance values defined in the 1 st
tab.
• Trace the voltage amplitude at different points of the circuit according to the custom tuning capacitance value (select the 3 rd
tab).
•
Estimate the magnetic field strength generated by the reader according to the system parameters of the system (select the 4 th
tab).
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Calculation tool
Select the 1 st
tab of CR95HF EMI FILTER CALCULATION.xlsm spreadsheet:
•
Tuning circuit calculation
Various configurations can be calculated, as you can seen on
Figure 6
.
Figure 6. Tuning circuit calculation
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1. CR95HF and User defined system parameters.
2. Ideal tuning capacitance calculation.
3. Input impedance calculation based on user defined tuning capacitances.
DocID024977 Rev 1
AN4327 Calculation tool
EMI filter
It is possible to calculate the tuning circuit without the EMI filter by simply choosing
L01 = L02 = 0 and C01 = C02 = 10
-40
(simulating an open circuit)
Receiving path
The calculation tool allows to use a resistor in series with a capacitor in the receiving path, by choosing:
•
Crx= 10
12
The calculation is made with a resistor only in the receiving path.
•
Rrx= 0
The calculation is made with a series capacitor only in the receiving path.
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Calculation tool
3.2 Input impedance curves
AN4327
Select the 2 nd
tab of CR95HF EMI FILTER CALCULATION.xlsm spreadsheet:
•
Input impedance curves
Based on the tuning capacitance values chosen in
Figure 6
of the calculation tool, the circuit input impedance is calculated over the frequency (F).
The example in
Figure 7
shows the input impedance calculated with the CR95HF RF
transceiver board tuning capacitances.
Figure 7. Antenna circuit input impedance
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AN4327 Calculation tool
Select the 3 rd
tab of CR95HF EMI FILTER CALCULATION.xlsm spreadsheet:
•
Circuit voltages
Based on the tuning capacitance values chosen of the calculation tool, voltages at various
locations in the circuit are calculated (see
Figure 8
).
This feature is useful to estimate the RX path attenuation which is necessary to limit
VRX1-RX2 below 7 V.
Figure 8. Voltage calculation at various locations
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Calculation tool
3.4 Magnetic field vs distance
AN4327
Select the 4 th
tab of CR95HF EMI FILTER CALCULATION.xlsm spreadsheet:
•
Magnetic field vs distance.
Based on the differential antenna voltage Vantenna from the 3 rd
tab and the antenna parameters (dimensions and number of turns) chosen in the 1 st
tab, an estimation of the generated magnetic field is calculated in the 4 th
tab (see
Figure 9
).
Figure 9. Magnetic field (H) versus distance
Note: For information only.
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4 Practical tuning circuit design
4.1
Practical tuning circuit design
Step by step procedure
Four steps are needed:
Step 1
Measure the CR95HF antenna impedance on the PCB.
Step 2
Estimate the C11, C12, C2 and ZRX impedance values using the CR95HF EMI filter calculation tool. Mount the component values from step 2 on the PCB.
Step 3
a) Without powering the board, measure the circuit input impedance between TX1 and TX2 using a network analyzer or an impedance analyzer. Tune the C11, C12 and C2 capacitance values, if necessary.
b) As a design trick, the tuning frequency can be adjusted using C2, and the impedance magnitude can be adjusted using C11/C12. This can be verified using the impedance curve feature of the CR95HF EMI filter calculation tool.
c) Power up the board and activate the RF generation (this can be achieved by sending a Protocol_Select command to the CR95HF).
d) Measure the DC voltage in the ST_R0 pin: adjust the ZRX component value to limit the voltage measured on ST_R0 below 7 V.
e) After powering down the PCB, check the input impedance and adjust it with C11,
C12 and C2, if necessary.
Step 4
Check the RF performance with a tag.
4.2 Input impedance choice
Because of the reader antenna detuning occurring when the tag is very close to the reader, some communication hole can appear at very short distance.
To overcome this, it is appropriate to choose an input impedance higher than the CR95HF output impedance: for instance, to maintain the energy harvesting capability of the C95HF
RF transceiver board, the input impedance of the antenna circuit has been set to 50
Ω.
This value has a minor impact on the CR95HF to power up the tag at a long distance from the reader antenna, maintaining a magnetic field strength level sufficient to guarantee the energy harvesting function of the M24LRXXE-R at a short distance from the reader antenna.
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Revision history
Date
29-Aug-2013
Table 1. Document revision history
Revision Changes
1 Initial release.
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AN4327
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