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Measurement
2 Measurement
Determining the Interference Source Impedance Zs
Generally, interference sources have low impedance so that measurements using the R&S ESH2-Z3
produce correct results. Though, the levels measured with an undefined source impedance may be
incorrect. In order to determine the impedance Zs of the interference source, first measure the level
without and then with attenuator 6 at a test frequency complying with CISPR Publication 16-2-1, Section
2.4.6.2 (see Fig. 1). Due to the different loading and voltage division of the probe with the same Zs a
difference in indication A occurs. The interference source impedance Zs can be determined by means
of this difference in indication A shown by curve A , Fig. 2. The difference in indication A is 6 dB with
very low source impedances and approaches 0 dB with very high impedances.
The reduction in indication A can be computed from equation (1), the interference source impedance Zs
from equation (2).
Measuring and Evaluating the Interference Level
Curve B in Fig. 2 gives the measurement error in dB with a defined interference source impedance Zs.
In this case, the measurement carried out with the probe without attenuator 6 is referred to EMF and the
readout on the test receiver is in dB µ V. The corrected interference level (EMF) is obtained by adding
the errors B (from curve B or from equation 3) to the indicated level.
The measurement error B is given by
Example: The interference level measured at the frequency f by means of the probe without attenuator
is 44 dB µ V while that measured with the attenuator is 39.7 dB µ V.
The difference is 44 dB - 39.7 dB = 4.3 dB. The interference source impedance obtained from equation
(2) is Zs = 841.6
and the indication error B given by equation (3) is 3.9 dB. Hence, the corrected
interference level is:
The reduction in indication A = 5 dB specified in CISPR Pubilcation 16-2-1, Section 2.4.6.2,
corresponds to an interference source impedance of 427
2.17 dB.
Zs < 370
in the test frequency range permits continuous measurements to be made with the
maximum error of 2.2 dB.
1156.6124.32
+
Z
1500
s
=
A
20
log
+
Z
3000
s
(
A
/
20
)
3000
10
1500
=
Z
s
(
A
/
20
)
1
10
+
Z
1500
s
=
B
20
log
1500
44 dB µ V + 3.9 dB = 47.9 dB µ V (EMF)
6
in
dB
(
) 1
in
(
) 2
in
dB
(
) 3
and thus results in an indication error of -
R&S ESH2-Z3
E-1
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