371

Document 371-1
Measuring Sensitivity of Transponder Coils
Introduction
Test materials
The sensitivity of a transponder coil is a measure of its
ability to receive a signal from the transmitting antenna.
Higher sensitivity ratings can equate to longer read distances and the ability to receive weaker signals at the
design frequency.
Function generator
Note: Sensitivity is only one of many factors that affect
read distance. The design of the PCB, specifically
composition, placement and orientation of all the
components, plays a significant role in the actual
read distance that can be achieved.
Different methods of measuring sensitivity produce different results. In order to make a fair comparison of
similar transponder coils from different manufacturers
or from different families of one manufacturer, the method
of measuring sensitivity must be consistent. There are
several accepted methods of measuring the sensitivity
of transponder coils. The following test setup and calculations can be used to produce consistent and repeatable results.
Helmholtz coil. 6.5″ (16.5 cm) diameter coils, 2.75″ (7 cm)
between coils.
HP/Agilent 34401A Multimemert or equivalent to measure
at 125 kHz.
Two 0.2 Ohm resistors
Test Setup
Refer to Figure 1 to set up the test equipment.
1. Solder the transponder coil and two test leads to a
printed circuit board.
2. Orient the transponder coil in the Helmholtz coil as
shown in Figure 1.
Note: Ensure that the test leads of the transponder coil
go through the coils of the Helmholtz. Routing
the leads parallel to the coils can adversely
affect the accuracy of the measurements.
3. Connect all the test equipment.
0.2 ⏲
Function
generator
10 Vpp
0.2 ⏲
Device
under
test
Helmholtz
coil
HP/Agilent 34401A
Multimeter
Figure 1. Test Setup for Measuring Transponder Coil Sensitivity
Specifications subject to change without notice.
© Coilcraft, Inc. 2005
Document 371-1
Revised 09/14/05
Document 371-2
4. Set the function generator to create a 125 kHz sine
wave at 10 Vpp.
5. Use the multimeter to take the following measurements:
Voltage across one of the resistors (Vres).
Voltage across the transponder coil (Vdut)
Calculating Sensitivity
The sensitivity of a transponder coil is defined by the following equation:
Sensitivity =
Vdut
I×K
(1)
Where:
Vdut = Voltage across the device under test
K = Helmholtz constant (0.0543 µT/mA)
I is defined by the following equation:
I = Vres
R
4308RV-495
R = 0.2 Ohms
Vres = 27.69 mVac
Vdut = 350.2 mVac
K = 0.0543 µT/mA
Solving for I: 27.69 / 0.2 = 138.45 mA
Invoking Equation 1:
350.2 mVac
138.45 mA × 0.0543 µT/mA
=
350.2 mVac
7.52 µT
(2)
= 46.57 mV/µT
5315TC-495
Note: The Helmholtz constant is different for each coil. The
constant is supplied by the manufacturer of the
Helmholtz coil. If the constant is unknown, it can be
calculated using a DUT with a known sensitivity.
Use the following equation to calculate the constant
for a Helmholtz coil:
Vdut
I × sensitivity
The Helmholtz constant used in these calculations is
0.0543 µT/mA. The diameter of each coil is 6.5″ (16.5 cm)
and the distance between the coils is 2.75″ (7 cm).
Sensitivity =
Where:
Vres = Voltage across one of the resistors
R = Resistance of the resistor
K=
Sample Calculations
(3)
R = 0.2 Ohms
Vres = 27.69 mVac
Vdut = 302.5 mVac
K = 0.0543 µT/mA
Solving for I: 27.69 / 0.2 = 138.45 mA
Invoking Equation 1:
302.5 mVac
Sensitivity =
138.45 mA × 0.0543 µT/mA
=
302.5 mVac
7.52 µT
= 40.23 mV/µT
Specifications subject to change without notice.
© Coilcraft, Inc. 2005
Document 371-2
Revised 09/14/05
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