Specifications

B-field Exposure From Induction Cooking Appliances 20
4 Evaluation of Measurement System
4.1 Sensor Characterization
Helmholtz coil The uncertainty of the Narda instrument was evaluated using the Helmholtz
coil, designed to test the T-coil compatibility of mobile phones (ANSI63.19). It is comprised
of a pair of identical circular magnetic coils (diameter 284.8 mm) that are coaxially aligned
and separated by a distance equal to the radius of the coil. Each coil carries an equal electrical
current. The cylindrical region extending between the centers of the two coils and approximately
1/5 of their diameters has a magnetic field with high spatial uniformity. The Helmholtz coil is
connected to the waveform generator (Agilent 33120A). The corresponding electronic circuit is
given in Figure 13.
Figure 13: Electronic circuit of the Helmholtz coil connected to the waveform generator (Agilent
33120A).
The current I in the Helmholtz coil and the voltage V
out
at the output of the waveform
generator are given in Equations (2) and (3), respectively.
I =
V
R
2
R
2
=
V
out
R
coil
+ R
2
(2)
V
out
= V
s
R
coil
+ R
2
R
s
+ R
coil
+ R
2
≈
1
2
V
s
(3)
In the frequency domain, the current I(ω) in the Helmholtz coil is given in Equation (4),
where I(ω) = Ie
jωt
and V
out
(ω) = V
out
e
jωt+α
are the current and voltage in the complex
domain, ω = 2πf is the angular velocity and f is the frequency. The cut-off frequency is given
by f
c
= R
2
/2π(L
1
+ L
2
) and is here f
c
≈ 20 kHz for the Helmholtz coil. The frequency response
of the coil is shown in Figure 14 b).
I(ω) =
V
out
(ω)
R
2
+ jω(L
1
+ L
2
)
(4)
An approximation of the B-field at the center point of the Helmholtz coil system is given in
Equation (5), where R is the radius (R
average
= 142.4 mm), N is the number of turns in each
coil (N = 20) and I is the current in the coils [6].