Tegam 3550 - Equivalent Circuits

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Model 3550 LCR Meter Instruction Manual Operating Instructions
Making Accurate Measurements cont’d:
Equivalent Circuits
The DUT can be described in terms of Impedance (Z) or Admittance (Y). The relationships of the
various characteristics are as follows:
The measurable quantities of the 3550 are L, C, R, Z, Y, D, Q, G, Rp, Rs, X, B, and LJ. The
measured value of these parameters will change in value as the test frequency is increased or
decreased. These measured parameters are said to be dependent upon test frequency.* Two
examples are capacitive reactance, which is inversely proportional to frequency and inductive
reactance, which is inversely proportional to frequency. Because of the direct relationship to test
frequency, related quantities such as Dissipation Factor and Quality Factor is also frequency
dependent.
*In measurement applications, the measured R and G values will change with frequency. True R
and G are not theoretically dependent to frequency but in LCR measurement applications, parallel
leakage and series losses can affect the measured R and G value of the component. Compensation
for these errors is the reason for Series and Parallel Equivalent Circuit Measurement Modes.
In general, parallel leakage becomes significant in highly reactive (small capacitors or large
inductors) measurements and series resistance is more significant in low reactance (large
capacitors and small inductors) measurements. This is the reasoning behind the use of series or
parallel equivalent circuit measurement modes.
If the dissipation factor, D is small then the difference between Series Equivalent and Parallel
Equivalent Circuit Measurement Mode measurement will be small. As the dissipation factor
increases, the difference between parallel and series equivalent measurements will increase. Thus,
precautions must be taken when performing measurements on components with high dissipation
factors.
Z : Impedance
R : Equivalent Resistance
X : Reactance (jX)
Y : Admittance
G : Conductance
B : Susceptance (jB)
D : Loss Coefficient
Q : Quality
ș : Phase Angle
f : Measurement Frequency Hz
Ȧ : Measurement Frequency rad/s
B
G
X
R
Z = R + jX
Y = G - jB =
1
Z
X
L
= Z L = 2 S fL
X
C
= =
1
2
S fC
1
Z C
T = tan
-1
= tan
-1
Q =
1
D
D = =
R
X
G
B