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6
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.
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Prior to the start of the calibration process,
U
mr
must be calculated.
U
mr
represents the voltage to be measured at the output of the 150
Ω
to 50
Ω
Adapter connected to the EUT port of the CDN, and must not be
confused with the
open circuit test level
(
U
o
) for the calibration (i.e.: 1
Vrms, 3 Vrms, 10 Vrms, etc., as specified in Table 1 of IEC 61000-4-6). The
relationship between
U
mr
and
U
o
for CDN calibration is given in Equation
3:
E
E
q
q
u
u
a
a
t
t
i
i
o
o
n
n
3
3
–
–
U
U
o
o
v
v
s
s
U
U
m
m
r
r
R
R
e
e
l
l
a
a
t
t
i
i
o
o
n
n
s
s
h
h
i
i
p
p
NOTE:
The 6:1 ratio, or factor of 6
(15.6 dB)
is due to the following:
a)
the open circuit voltage is double the matched load
voltage, yielding a
2:1
voltage ratio; and,
b)
the EUT port of the CDN is terminated by 150 ohms (150
Ω
to
50
Ω
Adapter
(
100 ohms) in series with the 50 ohm input
impedance of the measuring instrument). So, the
measurement is made across one-third of the total load
impedance, yielding a
3:1
voltage ratio.
So, for instance, for calibration performed at an
open circuit
test level
(
U
o
)
of
10 Vrms
:
