Agilent 4285A Data Sheet

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.

2

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

3

figure 2. Contact Electrodes

Extension leads and contact
electrodes which are not a 4­terminal pair configuration cause
measurement errors because of
residual impedance and noise, thus
pay close attention to the following
items. Especially when performing
low inductance measurements, the
following must be closely considered,
because for low impedance
measurements, measurement
repeatability depends on contact
electrode stability rather than the
HP 4285A's inductance
measurement repeatability.

 To shield against noise, the
outer conductors of the extension
leads must be connected to each
other at a point as near as possible
to the test device.

 To reduce residual impedance,
the extension leads must be as
short as possible.

For stable residual impedance, the
layout, positioning, and length of
the extension leads and contact
electrodes must not be changed,
and careful consideration must be
given to eliminating mechanical
vibration.

3. Error Correction

The HP 4285A's powerful error
correction function can be used to
compensate for the residual
impedance of extension leads and
contact electrodes to give more
accurate impedance (L and Q)
measurement values.

First, set the cable length function to
1 m to correct for the error caused
by 1 m test leads ( A in Figure 3).

OPEN/SHORT/LOAD correction
should be performed at the actual
measurement frequencies. Up to
seven correction frequencies can
be set (B in Figure 3) so the six
frequency points specified in MIL
C-15305D can be covered.

As shown in Figure 4, leave the
contact electrodes open, then
perform an OPEN correction
measurement to compensate for
the stray admittance of extension
leads and the contact electrodes.

The SHORT correction function
compensates for the residual
impedance of extension leads and
contact electrodes, and is very
important for L and Q measurements.
Short the electrodes to each other
as directly as possible, then perform
a SHORT correction measurement.

If you want to base your measurement
on a working standard as a reference,
then we recommend performing a
LOAD correction measurement in
addition to an OPEN/SHORT
correction measurement. The LOAD
correction function compensates for
measurement errors of a test device
by using load correction coefficients
obtained from the measured values
and the reference values of a working
standard (LOAD). Input the reference
values of the working standard such
as L and Q values (C and D in Figure

3). Connect the working standard to
the contact electrodes, then perform
the LOAD correction measurement.

using the HP 4285A, the reference
values of the working standard can
be obtained at the actual test
frequency and signal level. Measure
the working standard using the
HP 4285A with a test fixture (such
as the HP 16047D or HP 16034E)
setup under actual measurement
conditions, and input these values
as the reference values. For the Q
reference value for high Q devices,
the measured value using the
HP 4285A with an HP 42851A can
also be used for the reference Q
value. (Check the current monitor
level between the HP 4285A and
with an HP 42851A, and set the
appropriate signal levels.)

Figure 3. Correction Function Setup Figure 4. Performing correction Measurements

4

4. Measurement Conditions and
Comparator Setup

The measurement conditions,
including frequency and signal
level, should be set to appropriate
values.

Figure 5 shows a comparator setup
to sort test devices into BIN 1
through BIN 9 by L value and into
IN/OUT by Q value.

5. Interface Timing Design

The HP 4285A Option 201 Handler
Interface has the capability for
adjusting the control signal timing
between the HP 4285A and a
handler. The interface is triggered
by an (/EXT_TRIG) signal from a
handler, and outputs an end of
measurement signal(/INDEX), an
end of comparison signal(/EOM),
and BIN sort data (/BIN1 through /
BIN9). Each output and input is
isolated using an optocoupler, and
the outputs are open collector.

Figure 6 shows a sample timing
chart for an interface. The total
testing time for a device is as
follows.

Measurement Time (30 ms
@SHORT INTEG TIME) ­Calculation/Comparison time +
Device change/Sort Time

If necessary, a delay time (1 ms ­60 s) can be inserted before the
measurement time (Tm1 in Figure

6) to allow for mechanical contact
bounce within the handler.

6. Memory Card

Setup data including measurement
conditions and comparator limit
values can be stored in a memory
card. Using this function, all setup
data can be recalled with just a
few key strokes, this is especially
useful on the production line.

Figure 5. Comparator Setup

Figure 6. Sample Interface Timing Chart

5

Figure 7. Sorting Result

7. Execution of Measurement
and Sorting

During measurement and sorting,
the BIN COUNT DISPLAY page
shown in Figure 7 displays the
device count for each BIN. The
display time (Tm3 in Figure 6) of
the BIN COUNT DISPLAY page, is
approximately 0.5 ms, shorter than
for the other pages.

8. An Example of a High Accuracy
Q Measurement/Sorting System

For accurate Q testing of high Q
(typically greater than 50) devices
at high frequency (typically above
5 MHz but device and application
dependent) the high accuracy Q
measurement system, shown in
Figure 8, is recommended. This
system uses the HP 4285A with the
HP 42851A Precision Q Adapter.

The HP 42851A's measurement
terminals are binding posts. If
needed, you should extend the test
leads to the contact electrodes as
shown in Figure 9.

Residual impedance and stray
capacitance added by this test lead
extension can be canceled by
using the HP 4285A's OFFSET L/C/
R functions. Since, the HP 4285A
measures the test device, including
extension leads and contact
electrodes, longer extension win
cause measurement error and will
loose measurement accuracy. To
minimize this error, extension leads
must be as short as possible, and
their residual inductance, resistance
and stray capacitance must be lower
than the parameters of the device to
be tested (approximately one-tenth
or less). Perform error correction
referring to the following sample
procedure.

i) To obtain the values for the
OFFSET L/C/R function, directly
measure the residuals of the
extension leads and contact
electrodes. Connect the extension
leads and contact electrodes to the
HP 4285A with a test fixture (such
as HP 16047D), and measure them
using the normal LCR measurement

mode of the HP 4285A. For OFFSET
L and R values, short the contact
electrodes together, and measure
the series inductance and
resistance. For OFFSET C value,
open the contact electrodes, and
measure the parallel capacitance.

ii) Before connecting the extension
leads to the HP 42851A, perform the
HP 4285A's SHORT correction.
(OPEN correction is not necessary
for measurements using the
HP 4285lA.) Connect a shorting bar
to the HIGH-LOW binding posts of
the HP 42851A, and perform SHORT
correction measurement. Then
Connect the extension leads and
contact electrodes to the HP 4285lA's
binding posts.

iii) On the MEAS SETUP page,
enter the L, C and R values which
are obtained in procedure i) to the
OFFSET L, C and R function each
other.

iv) When LOAD correction is
required, connect the working
standard to the contact electrodes,
and perform LOAD correction
measurement.

When the HP 4285A using with the
HP 42851A, the measurement time,
Tm1 in Figure 6 is 75 ms - 1.5 s.

Figure 8. An Example of High Accuracy Q Measurement

Figure 9. Extension Leads

6

Conclusion

This application note explains how
the HP 4285A and the HP 42851A,
with their high accuracy/speed,
can be combined to create high
quality and low cost RF inductor
test system.

Furthermore, data acquisition and
statistical analysis are available
for quality control of RF inductors
by adding an HP-IB controller to
the system. If you need DC current
bias characteristics of inductors, it
can be obtained by using the
HP 42841A Bias Current Source
with the HP 4285A (up to 10 A bias).

The HP 4285A can contribute to
producing higher quality and lower
cost inductors with its high
performance measurement
capabilities.

Appendix A. Ordering
Information

Accurate and Fast L-Q
Measurement System (Figure 1)

 HP 4284A Precision LCR Meter

Select one option among the
following.

 Option 201 Handler Interface

Interface to output signals of
measurement end and comparator
data.

 Test Leads

Select one among the following.

 HP 16048A (Total length is

Approx. 95cm, BNC connectors)

 HP 16048D (Total length is

Approx. 189cm, BNC connectors)

 HP PN 04278-61650 Handler

Interface Cable (Amphenol 36 pin
connectors (m))
Approx. 1 m cable to connect
between the HP 4285A and a
handler. If necessary to extend a
cable, make the following parts.

High Accuracy Q Measurement
System (Figure 8)

 HP 4284A Precision LCR Meter

 Option 201 Handler Interface

 Option 002 Accessory Control

Interface

Interface to control the HP 42851A.

 HP 42851A Precision Q Adapter

 HP 10503A BNC Cable (122cm,

(m) connectors): 2 ea.
Cables to connect between the
HP 4285A and HP 42851A. Other
BNC cables up to 2 m can be used.

 HP PN 04278-61650 Handler

Interface Cable (Amphenol 36
pin connectors (m))

 Cable (36 lines)

 Amphenol 36 pin connector (m)

(57FE-30360-20N(D8))

 Amphenol 36 pin connector (f)

(HP PN 1252-2022)

7

H

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applications, services, and for a current
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http://www.hp.com/go/tmdir. You can also
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Copyright © 1998

8

Hewlett-Packard Company
Printed in U.S.A. 9/98
5091-1596E