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Impedance Measurements
Evaluating EMC Components
with DC Bias Superimposed
Application Note
Table of Contents
Introduction................................................................................................ 1
About Ferrite Beads ................................................................................. 2
Impedance Measurements with a Vector Network Analyzer........... 4
E5071C ENA Network Analyzer ............................................................. 5
Sample VBA Program and Measurement Steps ................................. 5
Fixture Compensation .............................................................................. 8
Measurement Results Under Actual Operating Conditions ............ 11
Selecting Bias Tees ................................................................................ 13
Summary................................................................................................... 14
Introduction
Recent developments in semiconductor and communications technologies have
accelerated the expansion of digital technology applications to more and more
fields including home electronics and automotive parts and accessories.
Meanwhile, in order to cope with ever-increasing amounts of data, the demand
for higher communication speeds has been increasing daily. In addition,
developers are faced with stricter electromagnetic compatible (EMC) and noise
control requirements and the increasingly complicated challenge of meeting
today's standards and needs for power saving, low voltage design, smaller
components, etc. EMC/noise control requirements will continue to become
stricter and stricter.
Using ferrite beads as an example, this application note gives an overview on
how to correctly evaluate EMC components in a way that satisfies these strict
requirements. It also introduces various EMC measurement solutions.
About Ferrite Beads Figure 1 shows the equivalent circuit of a ferrite bead. Figure 2 shows the fre-
quency characteristics of two different ferrite beads. Unlike ordinary inductors,
Ferrite beads have high frequency components that are mainly resistive. Since
resistive components absorb noise and turn it into heat, ferrite beads are primar-
ily used to rectify signal waveforms or reduce high frequency noise in power or
signal lines.
Figure 1. Equivalent circuit of a ferrite bead
Impedance ()
1500
1200
900
1600
300
0
1 10 100 1000
Frequency (Hz)
Figure 2. Examples of frequency characteristics of ferrite beads
When using ferrite beads, you should take the following precautions:
1) Clearly identify the differences in impedance characteristics. The two ferrite
beads shown in Figure 2 have different impedance characteristics and
therefore their signal rising edges are also different (as shown in Figure 3).
Since the signal rising edge differs depending on the impedance characteris-
tics, choosing between different ferrite beads requires careful consideration
of not only their noise bandwidth, but also their impedance characteristics.
2) Pay attention to inductance saturation. Generally, core inductors such as
ferrite beads have DC current dependencies as shown in Figure 4; the degree
of inductance saturation differs depending on what material is used for the
core of the component. This causes differences in impedance characteristics.
Figure 5 shows how the impedance of a ferrite bead changes when
superimposed with different DC currents. These impedance changes may
cause a ferrite bead to exhibit a different noise removal effect or signal wave-
form than simulated. Therefore, to validate the effect of a ferrite bead, you
have to identify its characteristics under actual operating conditions.
2
EMC/noise regulations have been expanding to cover ever higher frequencies
including the GHz band. In addition, noise margins are decreasing due to a shift
toward low voltage designs for electronic devices. Given these trends, there is
increasing demand for ferrite beads with high frequency noise control; however,
you cannot achieve effective noise control without identifying the impedance
characteristics under actual operating conditions. The E5071C ENA network
analyzer provides an effective solution for measuring the impedance of EMC
components under actual operating conditions. With the E5071C ENA, which
can measure up to GHz range, and the application of a DC bias current from an
external DC source, measurements of EMC components under actual operating
conditions can be achieved.
Sample A Sample B
Voltage (V) Voltage (V)
Time
(t)
Time
(t)
Figure 3. Different signal waveform rising edges resulting from different impedance
characteristics (rough images)
Delta L
0
The higher the DC current, the higher
the tendency of inductance saturation
DC bias current
Figure 4. Inductance vs. DC bias currents
Impedance ()
0Adc 0.2Adc 0.5Adc 1Adc
250
200
150
100
50
0
0.0E + 00 5.0E + 08 1.0E + 09 1.5E + 09 2.0E + 09 2.5E + 09 3.0E + 09
Frequency (Hz)
Figure 5. Comparison of ferrite bead impedance measurements with different DC bias
currents superimposed
3
Impedance Two common methods used to perform impedance measurements with a vector
network analyzer are the reflection method using one port and the shunt/series
Measurements with method using two ports. Figure 6 shows the relationships between the impedance
a Vector Network and S parameter for each of the two measuring methods. When the impedance
measured (Zx) is in the vicinity of the characteristic impedance (Zo = 50 ), the
Analyzer reflection method provides high-sensitivity measurements because the vector
voltage ratio significantly changes with a small change in the impedance. However,
when the impedance measured is not in the same vicinity as the characteristic
impedance, the reflection method is not suitable due to trace noise.
The two port method allows you measure impedance over a wide impedance range
using shunt/series connections. With the two port method, you can use either S11
or S21 to perform measurements; however, S11 has a more limited noise floor than
S21, and it is highly susceptible to VSWR and other factors that exist between the
fixture connector and substrate. Because of this, generally S21 is preferred. The S21
shunt connection is suitable for 50 or lower impedances while the S21 series
connection is suitable for 50 or higher impedances. Because EMC components
span a wide range of impedances depending on their purposes, their impedances
must be measured with an adequate combination of S21 shunt/series connections.
Areas affected by trace noise
20
Horizontal
0 axis Z[W]
1.E-04 1.E-02 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
-20
S21 shunt
-40
S parameter [dB]
S21 series
-60
-80
S11 shunt
-100 (Reflection)
S11 series Reflection method S11 (shunt)
-120 S21 (shunt)
S11 (series)
-140
S21 (series)
-160 Vertical axis [dB]
Areas affected by noise floor
Figure 6. Relationship between impedance (Z) and S parameter (dB) values
One-port configuration Two-port configuration (shunt/series through)
(reflection) (If Zo at ports 1 and 2 is 50 )
Port1 Port2 Port1 Port2 Port1 Port2
Zx
G Zx S11 Zx S11 S21
Zx - Zo -25 Zx Zx 100
= S11 = S21 = S11 = S21 =
Zx + Zo Zx + 25 Zx + 25 Zx + 100 Zx + 100
Figure 7. Measuring method and configuration of impedance measurements for a vector
network analyzer
4
E5071C ENA Setup for DC bias superimposed impedance measurements under
actual operating conditions
Network Analyzer
Figure 8 shows the basic setup for DC bias superimposed impedance
measurements under actual operating conditions using the E5071C ENA
network analyzer. In this setup, ports 1 and 2 are each connected with a bias
tee. Each bias tee is connected to a DC power source and an electronic load.
Ports used for measurement must be connected with bias tees to block DC
signals from the analyzer. DC signals go from the DC power source to the bias
tee, to the device under test (DUT), to the electronic load. Using an electronic
load ensures that the DC bias applied to the DUT can be stably controlled.
AC signals output from port 1 pass through the bias tee and the DUT, and are
finally received by port 2. When AC signals pass through the DUT, they are
superimposed on a DC bias.
Bias tee Bias tee
AC
DC DC DC
power Electronic
source load
DUT
Figure 8. Measurement solution based on the E5071C ENA network analyzer
Sample VBA Program You can automatically control the system using the sample VBA program for
the E5071C with the specified power source and electronic load. The program
displays impedance measurement results of the different DC biases. The
program works with the E5071C and supports the following power source and
electronic load:
Agilent E3633A/E3634A DC power source
Agilent 3300A/E3301A electronic load
Measurement Steps Before starting the measuring
process, copy the following two files
into the VBA folder on drive D:
1 Z_DCI.VBA (sample VBA program)
2 ZvsDCI.chm (help file)
Then, check the E5071C's
Channel/Trace Setup to make sure
that 9 channels and 9 traces are
3 enabled. Press the [MacroSetup]
button located in the INSTR STATE
block on the front panel, and select
4 Load&Run. Select "Z_DCI" on the
5 menu to open the main screen of the
Figure 9. Main screen of the sample impedance VBA program.
VBA program for the E5071C
5
To perform measurements using the sample VBA program, follow the steps listed
below. The step numbers correspond to the numbers indicated on Figure 9. For
detailed descriptions, click the Help button located on the upper right corner of the
screen.
1. Set the GPIB addresses of the DC power source and electronic load. Be sure to
enter the exact GPIB addresses specified for your DC power source and electronic
load.
2. Specify the measurement method. This sample VBA program supports two-port
shunt-through and series-through measuring modes.
3. Set the frequency and measurement conditions of the E5071C. On the Sweep
Type drop-down list, select whether to sweep the frequency or DC bias current.
If you have selected frequency sweep: