Vector network analyzers (VNAs) are increasingly being used for
impedance measurements of electronic components and circuits,
especially in R&D environments where the versatility and flexibility of the
VNA-based solution are preferred. This presentation introduces the
impedance measurement solution of the E5061B-3L5 ENA Series LF-RF
network analyzer, and discuss how to get the most of its impedance
measurement capabilities for better measurements.
1
Before starting the discussion on the impedance measurement, let’s
quickly look at the product overview of the E5061B-3L5 LF-RF network
analyzer.
2
The E5061B-3L5 offers versatile network analysis in the broad frequency
range from 5 Hz to 3 GHz. Comprehensive LF network measurement
capabilities including built-in 1 Mohm inputs have been seamlessly
integrated with the high-performance RF network analyzer. This slide
summarizes key features of the E5061B-3L5.
3
The E5061B-3L5 has two kinds of test ports; the S-parameter test port
(Port 1 and 2) and the gain-phase test port. The S-parameter test port
has a 50 ohm built-in S-parameter test set that seamlessly covers a
broad frequency range from 5 Hz to 3 GHz. While providing an excellent
RF performance equivalent to other RF VNAs, the E5061B-3L5’s Sparameter test port offers much wider frequency coverage down to 5 Hz
and better dynamic range in the low to middle frequency range below 10
MHz.
4
The gain-phase test port has reference and test receiver inputs whose
input impedance is switchable to 1 Mohm or 50 ohm. The frequency
range is 5 Hz to 30 MHz. The most typical application is the frequency
response analysis for low-frequency devices and circuits, such as OP
amps and control loop circuits of DC-DC converters.
5
And the E5061B’s function we discuss in this presentation is the E5061B005 impedance analysis function, which is a firmware option dedicated to
the E5061B-3L5 LF-RF network analyzer. This impedance analysis
function fully supports basic functionalities of the impedance analyzer. In
addition, by supporting multiple impedance measurement methods, it
covers a variety of impedance measurement applications.
The E5061B-3L5/005 primarily targets at a simple and convenient VNAbased impedance measurement solution for general R&D use, in contrast
to dedicated impedance analyzers which pursue an ultimate impedance
measurement performance. In many applications, however, the E5061B3L5/005 allows you to perform very accurate impedance measurements
comparable to the dedicated impedance analyzers by properly selecting
the measurement method and optimizing measurement conditions
including the calibration and fixturing. Furthermore, this impedance
analysis solution offers unique capabilities such as a very wide frequency
coverage and a milliohm impedance measurement.
6
Now let’s see more details about the E5061B’s impedance measurement
methods.
7
The first two slides quickly review the measurement methods of
impedance analyzers.
Low-frequency impedance analyzers/LCR meters such as the 4294A
employ the auto balance bridge method. The negative feedback loop
circuit maintains the potential at the low terminal to virtual ground (zero
volt), which eliminates the stray capacitance at this point and enables the
voltmeters V1 and V2 to accurately measure the voltage and current at
the DUT. Also, the auto ranging capability provides a very wide
measurement range with an excellent linearity. Thus, the auto balance
bridge method offers very high accuracy in the very wide impedance
range. The frequency range is up to 110 MHz
Note that the actual auto balance bridge is a more complicated feedback
loop circuit than the very simplified OP-amp model shown here.
8
RF impedance analyzers/LCR meters such as the E4991A employ the
RF I-V method. With the combination of the source/receiver mainframe
and the test head module which senses high-frequency current and
voltage at the very close location to the DUT, the RF I-V method offers
higher measurement accuracy and wider measurement range than the
VNA-based reflection method. However, this method is not applicable to
the low frequency range because the current is sensed with a
transformer.
Next, let’s see the impedance measurement methods supported by the
E5061B-3L5/005 network analyzer.
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The reflection method is traditionally the most common VNA-based
impedance measurement technique mainly used in the middle to high
frequency range. The reflection method derives the impedance from the
S11 measurement data. The impedance measurement range is a little
narrower than that of the RF I-V method. The 10% accuracy range
(Supplemental Performance Data) is about 1 ohm to 2 kohm. The graphs
shown in the slides indicate the impedance range where each of the
VNA-based measurement methods provides a good impedance
measurement sensitivity. In the reflection method, the S11 measurement
value dynamically varies in the magnitude (in case of resistive DUTs) or
the phase (in case of reactive DUTs) when the measured impedance is
about 1 ohm to 2 kohm. This is the impedance range where the reflection
method provides accurate impedance measurements.
The advantage of the reflection method over the RF-IV method is the
lower frequency coverage due to the broadband S-parameter test set of
the E5061B-3L5.
We can use 7 mm type component measurement fixtures with the
16201A terminal adapter which converts the analyzer’s N-type test port to
the 7 mm type.
10
The series-thru method measures the impedance by connecting the DUT
in the transmission series configuration as shown in the block diagrams.
This method is suitable for measuring high impedance. The 10%
accuracy range is about 5 ohm to 20 kohm, which covers 1-decade
higher impedance range than the reflection method.
Both the gain-phase and the S-parameter test ports can be used for the
series-thru method. Especially, the gain-phase test port is convenient
because the 4-terminal-pair type component test fixtures can be directly
connected to the gain-phase test port. The upper frequency limit is 30
MHz.
Also, it is possible to perform the series-thru method up to higher
frequencies by using the S-parameter test port and a user-prepared test
fixture. However, it is difficult to fully eliminate the errors around the
series-thru fixture as the frequency goes higher than hundreds of MHz.
So the practical upper frequency limit is up to around 200 or 300 MHz.
Note that the grounded DUT cannot be measured with the series-thru
method.
11
The shunt-thru method measures the impedance by connecting the DUT
in the transmission shunt configuration as shown in the block diagram.
This method is suitable for measuring very low impedance, and
commonly used for milliohm impedance measurements of power integrity
applications. The 10% accuracy range is 1 mohm to 5 ohm (in the case of
using the gain-phase test port), which covers lower impedance range
than impedance analyzers.
Both the gain phase and S-parameter test ports can be used for the
shunt-thru method. For the low impedance measurement up to high
frequencies over 30 MHz, the shunt-thru method using the S-parameter
test port is the solution. However, for the low impedance measurement at
low frequencies below 100 kHz, it is recommended to use the gain-phase
test port rather than the S-parameter test port. Unlike the E5061B’s Sparameter test port and other existing VNAs, the gain-phase test port
employs a semi-floating receiver architecture which eliminates the
measurement error caused by the ground loop, and we can measure very
low impedance in the low frequency range easily and accurately .
For details about the ground loop error and the semi-floating receivers,
refer to Appendix of this presentation material.
12
This slide shows an effect of the semi-floating receiver of the gain-phase
test port. The DUT in this example is a resistor whose DC resistance is
about 1 mohm.
The screen shot in the upper left is the measurement result using the Sparameter test port whose outer shield is connected to the instrument’s
chassis ground, like other conventional VNAs. The measured result in the
low frequency range is incorrectly higher than the DUT’s true impedance
value due to the ground loop between the source and receiver.
The screen shot in the lower left is the measurement result using the Sparameter test port with a magnetic core attached to the test cable, which
is a traditional way to reduce the ground loop errors. The measurement in
the low frequency range is improved by increasing the outer-shield
inductance with the core. But it is not so easy to fully eliminate the errors
down to the very low frequency range, as shown in this screen shot.
On the other hand, the screen shot on the right side is the measurement
result using the gain-phase test port with no magnetic core attached to
the cable. As you can see, the DUT’s milliohm impedance is correctly
measured down to the very low frequency range, even without using the
core.
13
This graph shows the 10 % accuracy range of three measurement
methods using the S-parameter test port.
14
This graph shows the 10% accuracy range of the series-thru and shuntthru method using the gain-phase test port.
15
This table summarizes how to select the measurement methods
depending on the applications. The basic recommendations are;
• For general component measurements at middle to high frequency
ranges, the reflection method is recommended.
•For general component measurements at low to middle frequency
ranges, select either of the gain-phase series-thru method or the
reflection method depending on the DUT’s impedance value.
• For very low impedance measurements, the shunt-thru method is
recommended.
On the other hand, impedance analyzers such as the 4294A and E4991A
are basically recommended in the following cases;
• Very high accuracy is required.
• Accurately measure very high impedance.
• Accurately measure very high Q (=X/R) or very low D (=R/X).
16
This slide shows recommended migration paths from legacy NA/ZA
combination analyzers to the E5061B-3L5 with the option 005 impedance
analysis function.
17
The next section discusses calibration techniques and other tips for
improving the measurement accuracy in the actual impedance
measurement applications of the E5061B-3L5/005.
18
Firstly, let’s see the basics of the calibration for impedance
measurements.
The series-thru and shunt-thru methods are based on the 2-port
transmission measurement. If the measurement system used for these
methods is the S21 measurement configuration with the 50 ohm system
impedance, the VNA calibration methods such as the response thru and
the 2-port full (SOLT) can be used for impedance measurements.
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On the other hand, if we look at the impedance only, without worrying
about the measurement in the S-parameter domain, we can perform the
calibration by considering the measurement system as a black box and
adjusting this black box with the open, short and load whose complex
impedance values are known. This is the concept of the open/short/load
calibration in the impedance domain. The E5061B-005 impedance
analysis firmware provides this function as an Impedance Calibration
function, and you can perform the open/short/load calibration not only in
the reflection method but also in the series-thru and shunt-thru methods.
Here it should be noted that the measurement system treated as the
black box must satisfy the following conditions;
• Operating in the sufficiently linear region (no gain compression and
distortion).
• Three standards can be clearly distinguished with measured voltages.
• Load device can be measured stably.
Considering these basic points about the calibration, next let discuss
more details about the calibration techniques of each measurement
method.
20
The typical calibration of the reflection method is the open/short/load
calibration at the 7 mm coax plane, the port extension (selecting the
fixture model) to compensate for the phase shift at the fixture’s coax
section, plus the open/short compensation to eliminate the stray
capacitance and the residual inductance around the fixture’s electrodes.
For the open/short/load calibration at the 7 mm connector plane, you can
use either of the Impedance Calibration function or the conventional 1port full calibration function. In the case of the Impedance Calibration
function, you can perform the low-loss-capacitor calibration in addition to
the open, short, and load. The low-loss capacitor is an air capacitor with
approx. 3pF capacitance, and it improves the measurement accuracy of
the phase, Q, or D at high frequencies over 300 MHz by reducing the
phase uncertainty of the 50 ohm load termination. The low-loss capacitor
is included in the 16195B 7 mm calibration kit.
21
The typical calibration of the gain-phase series-thru method is the
open/short/load calibration at the 4-terminal-pair fixture. Leaded and
SMD-type 50 ohm resistors are provided as the E5061B’s optional
accessory.
In the series-thru method up to higher frequencies using the S-parameter
test port and a user-prepared test board, the most practical calibration is
the SOLT calibration at the coaxial cables plus the port extension to
compensate for the transmission lines on the test board. In addition, it is
possible to perform the open compensation to remove the stray
capacitance around the measurement terminal.
22
In the gain-phase shunt-thru method used for low impedance
measurements in the low frequency range, the response thru calibration
is the primarily recommended calibration method. The simple response
thru calibration gives enough accuracy in most of low-frequency milliohm
measurement applications, where only the impedance magnitude |Z| and
the reactance Cs and Ls need to be measured. But if the impedance to
be measured goes up to higher impedance ranges over 1 ohm, or if you
want to measure the phase or ESR at higher frequencies over 10 MHz, it
is recommended to perform the open/short/load calibration because the
measurement errors in these situations cannot be fully eliminated only
with the response thru calibration.
In the shunt-thru method using the S-parameter test port up to hundreds
of MHz or GHz ranges, the recommended calibration is the SOLT
calibration at the coaxial cables plus the port extension to compensate for
the fixture/probes. In case you use an RF probe station (for example, to
characterize IC packages or PCBs), you can perform the SOLT
calibration at the end of the probes by using the calibration standards
provided by the probe vendors.
23
This table summarizes other tips for improving the impedance
measurement accuracy of the E5061B.
24
This slide explains more details about the tips on how to contact to the
DUT in the shunt-thru method.
25
The last section shows actual impedance measurement examples of
typical DUTs using the E5061B-3L5/005.
26
The first three measurement examples are basic inductor and capacitor
measurements. These examples show that the E5061B can accurately
measure impedance by choosing an appropriate measurement method
and taking care of the calibration and other tips discussed in the previous
section.
This is a measurement example of an 100 nH RF inductor using the
reflection method. The test frequency range with the E5016B is from 10
kHz to 3 GHz, which cannot be covered by existing impedance analyzers.
The two graphs shown on the right hand side compare the measurement
results with the E5061B (at 10 kHz to 3 GHz), 4294A (at 10 kHz to 100
MHz), and the E4991A (at 1 MHz to 3 GHz). The E5061B’s measurement
results show very good agreements with two impedance analyzers in
both Ls and Q values, although it is partly due to the DUT’s moderate |Z|
and Q values which are relatively easy to measure.
27
This is a measurement example of a high-impedance device using the
gain-phase series-thru method. The DUT is a 10 nF capacitor and the
test frequency range is 100 Hz to 10 MHz. As shown in the graphs on the
right hand side, the E5061B’s measurement results has good
agreements with the 4294A in both Cp and D.
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This is an example of a very-low impedance measurement using the
shunt-thru methods. The DUT is a low-ESR SMD electrolytic capacitor
with 200 uF capacitance. The test frequency range is 1 kHz to 10 MHz.
The graphs on the right hand side compare the measurement results of
the gain-phase shunt-thru method (with the thru cal, or the
open/short/load cal), the shunt-thru method at the S-parameter test port
(with the SOLT cal), and the 4294A (with the 42941A 7 mm adapter and
the 16092A 7 mm fixture). Both Cs and Rs (=ESR) measurement results
of the E5061B exhibit good agreements with the 4294A, except the Rs
measurement value with the thru calibration in the high frequency range.
Also, it should be noted that the Rs measurement traces of the E5061B’s
shunt-thru methods are more stable than the 4294A’s. This indicates an
excellent low-impedance measurement sensitivity of the VNA-based
shunt-thru method.
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The next example is a resonator measurement, which is also a typical
measurement application for impedance analyzers. The DUT in this
example is a crystal resonator whose Q-factor is much higher than
ceramic resonators.
The measurement shown on the left hand side is performed in the span
fully covering the DUT’s resonant frequency Fr and the anti-resonant
frequency Fa. In this case, the equivalent circuit parameters are correctly
derived by using the E5061B’s equivalent circuit analysis function, but the
CI value (|Z| at 0 degree) read by putting the marker on the 0 degree
point at Fr is incorrectly higher than the actual DUT’s CI value. To
accurately measure the CI value of the high-Q resonator whose response
speed is generally not so fast, we need to make the analyzer’s sweep
speed slow enough and take a lot of measurement points around the Fr
area.
To measure CI more accurately, the measurement shown on the right
hand side is performed with a narrower span just focusing on the Fr area.
In this case, the CI value is correctly measured. But note that the
equivalent circuit parameter C0 can be erroneous because the antiresonant area is not included in the measurement span.
30
To evaluate components under their actual operating conditions, we often
want to measure their impedance by applying the DC bias. This is a
measurement example of a ceramic capacitor which has a strong DC
voltage dependency. The measurement performed in the channel-1 is the
frequency sweep for |Z|, phase, Cs, and Rs. And the measurement
performed in the channel-2 is the DC bias sweep (-10 Vdc to +10 Vdc) for
Cs by using the E5061B’s internal DC source. The DC voltage
dependency of the capacitor is nicely measured with the DC bias sweep.
31
To evaluate the DC current dependency of power inductors and ferrite
beads, we need to apply a large DC current that cannot be provided by
the E5061B’s internal DC source. This slide shows a solution example
using the 16200B DC current bias adapter (1 MHz to 1 GHz, up to 5 Adc)
and an external DC source.
32
Also, we sometimes want to apply large AC signals to high-power
components such as ultrasound resonators, ceramic actuators, and
power inductors. Similarly to the impedance analyzers of the auto
balance bridge method, we can expand the E5061B’s source output level
using an external power amplifier as shown in this block diagram. The
measurement configuration is the series-thru connection, but we set the
input impedance of the receivers to 1 Mohm and connect external homemade voltage divider circuits to prevent excessive AC voltage inputs. The
large current flowing through the DUT is sensed by connecting an
external high-power resistor Rc with the resistance value around 1 or 10
ohm.
33
This is a measurement example of a power inductor by applying a large
AC signal using the power amplifier. The power sweep is performed from
-20 to + 8 dBm, and the source output is amplified with a gain of 50. In
this measurement example, about 1 Arms current is being applied to the
DUT at +7 dBm source output point.
34
The last examples are in-circuit impedance measurements.
This example is an in-circuit impedance measurement with hand probing.
Typical target applications are in-circuit impedance measurements in
MHz ranges, such as printed RFID antennas, negative impedance of
oscillator circuits, and so on. The measurement method is the reflection
method. The probe consists of a semi-flexible SMA cable with a nonmetal
coating and the probe head of the 42941A impedance probe. The screen
shot shows a measurement example of a simple LC parallel resonant
circuit using this probe. The DUT’s resonant response is nicely
measured.
35
The next example is an output impedance measurement of a DC-DC
converter, which is a typical measurement application in the power
integrity field. The gain-phase shunt-thru method is used for measuring
very small impedance in the low frequency range. The receiver input
impedance is 50 ohm in this configuration. But if the DUT’s output voltage
is less than 5 Vdc, we can directly connect the converter without using
DC blocking capacitors. The screen shot shows the measurement result.
The test frequency range is 10 Hz to 10 MHz. As you can see, the output
impedance in the low frequency range is suppressed to milliohm order
due to the converter’s feedback loop operation.
36
The last example is also a power-integrity application. The DUT is a MPU
board populated with bypass capacitors, and its PDN (Power Distribution
Network) impedance is measured by using the shunt-thru method at the
S-parameter test port. In this example, two SMA connectors are soldered
to a thru hole from the top and bottom sides of the PCB to avoid the
additive inductance caused by the inter-probe coupling (see Appendix).
The screen shot shows a measurement result in the wide frequency
range from 100 Hz to 1 GHz. As you can see, the PDN’s low impedance
is measured up to hundreds of MHz without being affected by the additive
inductance error.
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One problem of the shunt-thru method using conventional LF VNAs is the
measurement error associated with the test cable ground loop.
In this block diagram, ideally the source signal flowing through the DUT
should return to the source side through the outer shield of the cable as
shown in the blue dotted line. But actually, in the low freq range below
100 kHz, the source current also flows into the test cable shield of
receiver side, as shown in the red dotted line. The shield current flowing
into the receiver side will cause the error voltage across the cable outer
shield resistance, and this will cause the measurement error. It is not
possible to measure the DUT’s impedance if it is smaller than the test
cable’s shield resistance, which is typically around 10 mohm or more.
40
The conventional solution for this problem is to attach magnetic cores to
the test cables at either of the source side or receiver side. The shield
impedance caused by the magnetic core’s self inductance will reduce the
source current flowing into the shield of receiver side. Also, the core
attached at the source side will force the source current returns through
the shield, back to the source side.
However, it is actually not so easy to implement this configuration,
because we need a magnetic core that has very large inductance, and it
is generally not so easy to find a good core that has sufficiently large
inductance down to very low freq range.
Therefore, a more convenient solution has been desired.
41
Now the E5061B’s gain-phase test port offers a quite different approach
to solve this problem.
The receivers of gain-phase test port have a semi-floating impedance,
Zg, to the chassis ground. This floating impedance is about 30 ohm in the
low freq range below 100 kHz, which is sufficiently larger than cable
shield resistance. So the error voltage across cable shield resistance can
be ignored. As a result, the receiver can correctly monitor the DUT’s
output voltage, Vo. And the DUT’s milliohm impedance can be accurately
measured.
With this unique receiver architecture, we can easily and accurately
measure milliohm impedance of DC-DC converters and passive PDNs
populated with bulk bypass capacitors at low frequencies, even without
using external magnetic cores.
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