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Agilent
8 Hints for Successful
Impedance Measurements
Application Note 346-4




Characterizing Electronic Components
to Achieve Designed Circuit Performance

SYS MENU
MEAS
FUNC : Z- RANGE : AUTO DISP
FREQ : 100.000 kHz BIAS : 0.000 V
LEVEL : 1.00 V INTEG : LONG BIN
No.

Z : 150.000 BIN


: -90.000 deg
COUNT


LIST
SWEEP
Vm : 50.00mV Im : 10.74mA
CORR: OPEN, SHORT
Contents Impedance
HINT 1. Impedance Parameters
Measurements for
Engineers
HINT 2. Measurements Depend
Impedance is measured using a vari- The techniques employed by these
on Test Conditions
ety of techniques. A particular tech- instruments are independent of ana-
nique is selected according to the lyzer type, and can be RF-IV, IV or
HINT 3. Choose Appropriate test frequency, the impedance Auto-Balancing Bridge (depending
Instrument Display parameter to be measured and the on frequency).
Parameter preferred display parameters.
Engineers perform impedance meas-
HINT 4. A Measurement The Auto-Balancing Bridge urements for a variety of reasons. In
Technique has technique is exceptionally accurate a typical application, an electronic
Limitations over a broad impedance range (m component used in a new circuit
to the order of 100 M). The fre- design is characterized. Normally,
HINT 5. Perform Calibration quency range this technique can component manufacturers state only
be applied to is from a few Hz to nominal impedance values.
110 MHz.
HINT 6. Perform Compensation Design decisions, as well as deci-
The IV and RF-IV techniques are sions affecting the production of the
HINT 7. Understanding Phase also very accurate over a broad assembled product, depend to some
Shift and Port Extension impedance range (m to M). degree on the impedance values
Effects These techniques can be applied attributed to the product's compo-
from 40 Hz to 3 GHz. nents. The performance and quality
HINT 8. Fixture and Connector of the final product are therefore
The Transmission/Reflection determined in part by the accuracy
Care
technique is applied over the and thoroughness with which its
broadest frequency range (5 Hz to components are characterized.
110 GHz). This technique delivers
exceptional accuracy near 50 or This application note provides help-
75 . ful information for using the Auto-
Balancing Bridge, IV and RF-IV tech-
LCR meters and impedance analyz- niques. Refer to Agilent Application
ers are differentiated primarily by Note 1291-1, 8 Hints for Making
display properties. An LCR meter Better Network Analyzer
displays numeric data, while an Measurements (literature number
impedance analyzer can display data 5965-8166E) for information on the
in either numeric or graphic formats. Transmission/Reflection technique.




Figure 0-1. Accuracy Profile

2
HINT 1.
Impedance
Parameters
Impedance is a parameter used to The Quality Factor (Q) and the
evaluate the characteristics of elec- Dissipation Factor (D) are also
tronic components. Impedance (Z) derived from resistance and reac-
is defined as the total opposition a tance. These parameters serve as
component offers to the flow of an measures of reactance purity. When
alternating current (AC) at a given Q is larger or D is smaller, the quali-
frequency. ty is better. Q is defined as the ratio
of the energy stored in a component
Impedance is represented as a com- to the energy dissipated by the com-
plex, vector quantity. A polar coordi- ponent. D is the inverse of Q. D is
nate system is used to map the also equal to "tan ", where is the
vector, where quadrants one and dielectric loss angle ( is the com-
two correspond respectively to pas- plementary angle to , the phase
sive inductance and passive capaci- angle). Both D and Q are dimension-
tance. Quadrants three and four less quantities.
correspond to negative resistance.
The impedance vector consists of a Figure 1-2 describes the relationship
real part, resistance (R), and an between impedance and these
imaginary part, reactance (X). derived parameters.

Figure 1-1 shows the impedance
vector mapped in quadrant one of
the polar coordinate system.

Capacitance (C) and inductance (L)
are derived from resistance (R) and
reactance (X). The two forms of
reactance are inductive (XL) and
capacitive (XC).




+j
Z
jX L Inductor
R jX L XL
Q=
R
X L = 2fL
= L
R

Capacitor
R - jX C
R
D=
XC
1
jX C XC =
Z 2fC
1
=
-j C

Figure 1-1. Impedance Vector Figure 1-2. Capacitor and Inductor Parameters


3
HINT 2.
Measurements
Depend on Test
Conditions
A manufacturer's stated impedance The AC voltage across the compo-
values represent the performance of nent can be derived from the com-
a component under specific test ponent's impedance, the source
conditions, as well as the tolerance resistance, and the signal source
permitted during manufacture. output (Figure 2-3).
When circuit performance requires
more accurate characterization of a An automatic level control (ALC)
component, it is necessary to verify function maintains a constant volt-
the stated values, or to evaluate age across the DUT (device under
component performance under oper- test). It is possible to write an ALC
ating conditions (usually different program for instruments that have a
than the manufacturer's test condi- level monitor function, but not a
tions). Figure 2-1. Frequency Characteristics of a Capacitor built-in ALC.

Frequency dependency is common Signal level (AC) dependency is Control of measurement integration
to all components because of para- exhibited in the following ways time allows reduction of unwanted
sitic inductance, capacitance and (see Figure 2-2): signals. The averaging function is
resistance. used to reduce the effects of random