# Construction of Equivalent Circuit Model for Capacitive

Document Sample

```					 EMC Studio                                                                      Construction of Equivalent Circuit Model for
Capacitive Voltage Probe
Application Note                                                                          Page 1 of 4

Construction of Equivalent Circuit Model
for Capacitive Voltage Probe
Capacitive    Voltage    Probe       (CVP)   is   a   measurement   device   for
electromagnetic disturbances around multi-wired cables. The cable to be
measured is clamped in the probe. Capacitance between the cable and
inner electrode of the probe is used to obtain measurement of the voltage
on the wires in the cable without coming into contact with them. This allows
conducting measurements without stopping signal transmission. CVP detects
presence of unbalanced i.e. common mode voltages. Device is applicable
to variety types and sizes of cables.

Fig. 1.   Capacitive Voltage Probe

Problem Definition
EMCoS
Consulting and Software

In some EMC problems direct full wave Method of Moments (MoM)
calculations of low frequency problems are very ineffective. Alternatively can
be used low frequency circuit theory. According to low frequency circuit
theory, low frequency interactions can be with high accuracy described as

27 Pekin Str., Tbilisi, 0160, Georgia                                 www.emcos.com
Tel.: ++995-32-389091; Fax: ++995-32-389092
EMC Studio                                                                    Construction of Equivalent Circuit Model for
Capacitive Voltage Probe
Application Note                                                                       Page 2 of 4

quasi-static processes, where parameters (capacitances and inductances) of
equivalent circuit can be obtained from the static considerations.

CVP model can be designed in the form of equivalent circuit. Analysis of
obtained calculation results of the constructed model in comparison with full
wave electromagnetic calculations is performed. This comparison helps to
conclude whether constructed equivalent circuit model is adequate for such
kind of problems.

Model for Full Wave Electromagnetic Analysis
Geometry of CVP model used for full wave electromagnetic analysis is
shown in Fig.2. Electromagnetic solver TriD based on MoM is used for
calculations. Surfaces of electrodes and side plates are represented with
triangular meshes. CVP model is located above infinite PEC ground plane.
Grounding of outer electrode and side shields is modeled as direct wire
connection to infinite PEC ground plane.

Fig. 2.   Model for MoM calculations
As follows from operational principle of CVP voltage detected in output in
certain manner is corresponding to the voltage in the measured cable.
Relation between these two voltages is linear. However transformation
coefficient is not known. Probably value of this coefficient can be derived
from theoretical considerations but there is easier way to determine it via
EMCoS
Consulting and Software

virtual calibration process. Suppose we have a cable transmitting known
voltage Vcable. CVP will detect some voltage Vprobe, which helps to determine
calibration constant Coef=Vcable/Vprobe. Now multiplying this constant on

27 Pekin Str., Tbilisi, 0160, Georgia                                 www.emcos.com
Tel.: ++995-32-389091; Fax: ++995-32-389092
EMC Studio                                                                                                Construction of Equivalent Circuit Model for
Capacitive Voltage Probe
Application Note                                                                                                                        Page 3 of 4

measured voltage in probe one can be sure that CVP is detecting correct
voltages in the cable VCVP.

Equivalent Circuit Model
Fig.3 illustrates equivalent circuit for simulation of CVP operation.
C6

20p
R2
Inner                                                      Outer
electrode                 V+
100k
V-
electrode
C5                        C2

Cable                       0.13843E-11
C3
0.16925E-10
C1                R1
L2                          L1
1              2              1              2              0.35564E-13             0.12687E-10       0.1
V14.0589e-007                 4.0589e-007
1Vac
0Vdc                            C4
R4     0                       0                 0
0.38906E-11
R3                                          250

50

0

Fig. 3.        Equivalent circuit model
Wire is represented with one transmission line segment (T scheme is used);
LC parameters of wire calculated against zero potential are used.
Capacitive connections of inner and outer electrodes with ground are
represented with theirs self-C parameters. Capacitive connection between
electrodes and voltage detection circuit are modeled with parallel
connection of high impedance and mutual C of electrodes. Terminations of
wire are represented directly with resistive elements and voltage source.
Grounding of outer electrode is represented with low resistance.

For evaluation of equivalent circuit model first matrix of LC parameters is
calculated using Static3D solver and than circuit is solved by circuit
analyzing tool.

Numerical Results
In Fig. 4 and Fig. 5 voltages detected by CVP calculated with equivalent
circuit model (Fig. 3; R3) and with MoM model (Fig. 2) are compared for
two cases of cable termination loads.
EMCoS
Consulting and Software

27 Pekin Str., Tbilisi, 0160, Georgia                                                                                   www.emcos.com
Tel.: ++995-32-389091; Fax: ++995-32-389092
EMC Studio                                                                                         Construction of Equivalent Circuit Model for
Capacitive Voltage Probe
Application Note                                                                                                                  Page 4 of 4

85.5
Direct MoM solution
85.4                                            Equivalent circuit model
85.3

CVP voltage [dBμV]
85.2

85.1

85

84.9

84.8

84.7

84.6
100KHz   300KHz   1.0MHz               3.0MHz          10MHz               30MHz
Frequency

Fig. 4.    Voltage coupled in CVP base model. Equal termination loads on cable
(50 Ohm and 50 Ohm)

89.9
Direct MoM solution
89.8                                            Equivalent circuit model
89.7
CVP voltage [dBμV]

89.6

89.5

89.4

89.3

89.2

89.1

89
100KHz   300KHz   1.0MHz               3.0MHz          10MHz               30MHz
Frequency

Fig. 5.     Voltage coupled in CVP base model. Differing termination loads on cable
(50 Ohm and 250 Ohm)

Conclusions
From the results obtained based on the listed above investigations the

Determination of calibration constant of CVP allows virtual measurement
of voltages in cables with high accuracy

Equivalent circuit model of CVP setup is constructed and tested with full-
wave electromagnetic approach. Comparison of results shows high
precision of model in considered frequency range
EMCoS
Consulting and Software

27 Pekin Str., Tbilisi, 0160, Georgia                                                                            www.emcos.com
Tel.: ++995-32-389091; Fax: ++995-32-389092

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 63 posted: 4/4/2010 language: English pages: 4
How are you planning on using Docstoc?