High Accuracy and Fast RF
Inductor Testing
Application Note 369-10
H
HP 4285A
Precision LCR Meters
1
Introduction
RF inductors are employed in
electronic equipment such as VCRs
and IF (Intermediate Frequency)
circuits for AM/FM radios, mobile
radios, pagers and codeless
telephone, which require RF
inductors to be smaller, lower cost,
and have higher reliability. This
application note describes solutions
offered by the HP 4285A Precision
LCR Meter for realizing these
requirements. Information for
accurate and fast RF inductor
testing, and for practical simple
test systems are discussed.
Problems and the Solutions
offered by the HP 4285A
1. Measurement Accuracy and
Measurement Speed
Stable L (inductance) and Q
(Quality Factor) low inductance
measurements are difficult to make,
because measuring low inductance
values means measuring low
impedance values. If a higher
measurement frequency (MHz) is used
to increase the measured impedance,
the LCR meter's measurement
accuracy is worse, so accurate
measurement data cannot be obtained.
The HP 4285A can perform reliable
and fast measurements with 0.5%
inductance measurement accuracy
even in the MHz frequency range,
and with a 30 ms measurement speed.
Formerly LCR meters could not
measure accurately enough for
testing high Q inductors, and Q
meters required long measurement
time and high operator skill.
The HP 42851A Precision Q Adapter,
an auto-tuned resonant circuit Q
measurement adapter, is used with
the HP 4285A to automatically
perform accurate Q measurement
and digital displays the results.
The HP 42851A combined with an
HP 4285A achieves an automatic Q
testing system with 5% Q
measurement accuracy. Table 1 lists
the HP 4285A's key specifications.
2. Measurement Condition Setup
There are many types of RF inductors,
so the measurement frequency range
Table 1. HP 4285A Key Specifications
for an inductor is selected form
across a frequency range spanning
kHz through MHz, depending on the
inductance value to be measured and
the application in which it will be used.
The HP 4285A covers a
measurement frequency range of
from 75 kHz to 30 MHz, including
frequencies defined in the MIL C15305D standard (79.6 kHz, 252
kHz, 796 kHz, .. 25.5 MHz).
A constant current test signal gives
the best inductance measurement
results because inductive devices
are current mode devices and L and
Q values depend on current levels
rather than voltage levels. Q meters
do not offer current test signals, and
formerly LCR meters used voltage
test signals, not current test signals.
The HP 4285A offers both voltage
and current test signal level
selections. Furthermore, its ALC
(Auto Level Control) function allows
constant test signal level
measurements. When the HP 42851A
is used, the test signal level is set as
a voltage, and then the current level
is confirmed using the HP 4285A's
monitor function.
HP 4285A (standard) HP 4285A with Opt.002 and HP 42851A
Measurement Method Auto Balancing Bridge Resonant Method (Auto Tuning)
Measurement Frequency 75 kHz to 30 MHz with 100 Hz resolution
Basic Accuracy 10 mH L: ±O.18 %
(Typical) @100 kHz Q: ±5% @Q =30 L: ± 3.5%
1µH L: ±O.5 % Q: ± 5 % @Q =5~100
@10 MHz Q: ±l0% @Q=30
Measurement Speed (Typical) 30 ms / 65 ms / 200 ms 75 ms~1.5 s
Test signal Level V: 5 mVrms~ 2 Vrms V: ≤ 1.0 Vrms, variable
(10mVrms~1 Vrms in constant mode) V / I Monitors are available.
I: 200µArms~20 mArms
(100µArms~20 mArms in constant mode)
V / I Monitors are available.
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3. Functions for a Test System
Formerly an LCR meters' cable
extension capability was limited to
an upper measurement frequency
limit of approximately 15 MHz, and
they could not fully compensate for
errors due to extension cables and
test fixtures. So, LCR meter
performance was limited when used
as a component in a measurement
test system.
The HP 4285A can use 1 m or 2 m
test leads over its full frequency
range, and its OPEN /SHORT /
LOAD correction function can
compensate for measurement
errors due to extension leads. You
can obtain accurate measurement
values by performing error
correction at the actual
measurement frequencies (for
example the frequencies defined in
MIL standards).
An internal comparator and an
optional automatic handler interface
are available for setting up RF
inductor test systems.
Example of an Accurate and
Fast Measurement System
This paragraph describes an
example of L-Q measurement and
sorting system for RF inductor
production line applications, and
gives some techniques for
designing such systems.
Figure 1 shows a simplified model
of an RF inductor test system. In this
system, the HP 4285A measures L
and Q and performs BIN sorting
using its internal comparator.
Equipment used in this system is
as follows (refer to Appendix A.
Ordering Information for details).
HP 4285A Precision LCR Meter
with Option 201 Handler Interface
HP 16048A Test Leads (1 m, BNC
connectors)
Handler Interface Cable (Amphenol
36 pin connectors (male))
Automatic Component Handler
(compatible with HP 4285A
Option 201)
The following are technical hints
for designing a test system, and for
maximizing the HP 4285A's
performance.
1. Cable Connections
Use HP test leads to extend the
measurement terminals of the
HP 4285A to the handler's contact
electrodes.
You can select 1 m or 2 m test
leads, however, 1 m test leads are
recommended to minimize the
error caused by the addition of
extension cables.
2. Extension Leads and
Contact Electrodes
The HP l6048A test leads will
extend the signal path from the
HP 4285A measurement
terminals to the end of cables
while maintaining a 4-terminal
pair configuration.
Use connector plate (HP PN
16032-60001, furnished with the
HP 16048A) to change the
terminal configuration and
extend the leads to the contact
electrodes as shown in Figure 2.
Figure 1. Example of an Accurate and Fast L-Q Measurement
System
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figure 2. Contact Electrodes
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