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Agilent
High Accuracy and
Fast RF Inductor Testing
Application Note 369-10
Agilent 4285A
Precision LCR Meters
Introduction Problems and the solutions offered
by the Agilent 4285A
RF inductors are employed in electronic equipment such 1. Measurement accuracy and measurement speed
as VCRs and intermediate frequency (IF) circuits for
Stable inductance (L) and quality factor (Q) low
AM/FM radios, mobile radios, pagers and code-less
inductance measurements are difficult to make, because
telephone, which require RF inductors to be smaller,
measuring low inductance values means measuring low
lower cost, and have higher reliability. This application
impedance, values. If a higher measurement frequency
note describes solutions offered by the Agilent 4285A
(MHz) is issued to increase the measured impedance,
precision LCR meter for realizing these requirements.
the LCR meter's measurement accuracy is worse,
Information for accurate and fast RF inductor testing,
so accurate measurement data cannot be obtained.
and for practical simple test systems are discussed.
The 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. Table 1 lists the 4285A's key specifications.
2. Measurement condition setup
There are many types of RF inductors, so the measurement
frequency range for an inductor is selected from
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 4285A
covers a measurement frequency range of from 75 kHz
to 30 MHz, including frequencies defined in the MIL
C-15305D 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 former LCR meters
used voltage test signals, not current test signals.
The 4285A offers both voltage and current test signal
level selections. Furthermore, its auto level control (ALC)
function allows constant test signal level measurements.
Table 1. 4285A key specifications
Agilent 4285A (standard)
Measurement method Auto balancing bridge
Measurement frequency 75 kHz to 30 MHz with 100 Hz resolution
Basic accuracy (typical) 10 mH L: