dm00089926

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 (STSW95HF003), 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.
Table 1. Applicable tool and software
Type
August 2013
Part numbers / Product categories
Evaluation Tools
M24LR-DISCOVERY
Software
STSW-95HF003
DocID024977 Rev 1
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www.st.com
Contents
AN4327
Contents
1
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
2
Calculation explanations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
Calculation tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4
5
2/17
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.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
CR95HF tuning circuit with an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Impedance matching without an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Impedance matching with an EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
CR95HF + EMI filter equivalent generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Circuit for tuning calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Tuning circuit calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Antenna circuit input impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Voltage calculation at various locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Magnetic field (H) versus distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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CR95HF tuning circuit with an EMI filter
AN4327
1
CR95HF tuning circuit with an EMI filter
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
AN4327
Designing a tuning circuit with an EMI filter
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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Calculation explanations
Calculation explanations
The CR95HF + EMI filter equivalent circuit comes after some simple transformation (see
Figure 4).
Figure 4. CR95HF + EMI filter equivalent generator
1
The cut-off frequency of the EMI filter defined by fc = ------------------------------------is chosen above
2π
(
L0
× C0 )
14 MHz.
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:
1
1
fc = ------------------------------------ = ------------------------------------------------------------------------- = 14.6MHz
2π ( L0 × C0 )
180pF
2π ( 2 × 653nH ) ×  -----------------
2
The CR95HF receiving path RX1-RX2 input impedance ZRX_CR95HF = 22pF // 80 kΩ.
After replacing the RX path by its impedance, the circuit becomes as in Figure 5.
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Calculation explanations
AN4327
Figure 5. Circuit for tuning calculation
Assuming C11 = C12 = C1, the resulting equation is:
Zin2 = C1
-------- + ( C2 || ( 2Zrx + Zrx CR95HF ) || Za )
2
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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3
Calculation tool
Calculation tool
The CR95HF EMI FILTER CALCULATION.xlsm spreadsheet includes 4 tabs:
1.
Tuning circuit calculation
2.
Input impedance curves
3.
Circuit voltages
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 1st tab).
•
Calculate the theoretical circuit input impedance according to the system parameters
and custom tuning capacitance values (select the 1st 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 2nd 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 1st tab.
•
Trace the voltage amplitude at different points of the circuit according to the custom
tuning capacitance value (select the 3rd tab).
•
Estimate the magnetic field strength generated by the reader according to the system
parameters of the system (select the 4th tab).
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Calculation tool
3.1
AN4327
Tuning circuit calculation
Select the 1st 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
1. CR95HF and User defined system parameters.
2. Ideal tuning capacitance calculation.
3. Input impedance calculation based on user defined tuning capacitances.
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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= 1012
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
AN4327
Input impedance curves
Select the 2nd 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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3.3
Calculation tool
Circuit voltages
Select the 3rd 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
AN4327
Magnetic field vs distance
Select the 4th tab of CR95HF EMI FILTER CALCULATION.xlsm spreadsheet:
•
Magnetic field vs distance.
Based on the differential antenna voltage Vantenna from the 3rd tab and the antenna
parameters (dimensions and number of turns) chosen in the 1st tab, an estimation of the
generated magnetic field is calculated in the 4th tab (see Figure 9).
Figure 9. Magnetic field (H) versus distance
Note:
14/17
For information only.
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Practical tuning circuit design
4
Practical tuning circuit design
4.1
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
5
AN4327
Revision history
Table 1. Document revision history
16/17
Date
Revision
29-Aug-2013
1
Changes
Initial release.
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