Chapter 4 Simulation and Measurement of RE for PCB
4.3 Simulation and test results
This part, use the GEMS to simulate the current density on the PCB, the GEMS is based on the FDTD, the simulation distance is 0.3 cm, the simulation area is shown in Fig. 4.4, it use a PIFA antenna to replace PCB, to simulated electric field at 900 MHz and 1800 MHz is shown in Fig. 4.5 and Fig. 4.6. Fig.4.7 is shown the hardware implementation of the antenna, using the probe to scan the surface and recording results, the aperture field for meander line PIFA with simulated and measured at 900 MHz and 1800 MHz are shown in Fig. 4.8 and Fig. 4.9. The results of simulation and measurement are similar.
4.4 Summary
The wireless communications are more popular in nowadays. Most of the size of cellular in communications is compact with smaller size. The RF components on the PCB of cellular will have mutual electromagnetic interference. If the components of receiver are interfered by near field RF components, the total isotropic sensitivity (TIS) of cellular will be degraded.
The antenna is compact and near the PCB of cellular phone. The efficiency of antenna will also be degraded. Not only the total radiation power (TRP) but also the TIS will be reduced. The performance, such as throughput, bit error rate (BER) of cellular communication systems will be degraded due to the low TRP and TIS. The near field EMI of PCB is quite important for compact size communication systems, but how to measure the electromagnetic interference and locate the EMI sources on PCB is not an easy job, in next
surface current density on the PCB.
Fig.4.1. Structure communication system.
Fig.4.2. Simulated electric current at 900 MHz.
Fig.4.3. Simulated electric current.
Fig.4.4. Simulated area.
Fig.4.5. Simulated surface current density at 900 MHz.
Fig.4.6. Simulated surface current density at 1800 MHz.
Fig.4.7. Aperture field scan of antenna.
Fig.4.8. Simulation and measurement of aperture field at 900MHz.
Fig.4.9 Simulation and measurement of aperture field at 1800MHz.
Chapter 5
Build High Resolution Circuit Image
5.1 Introduction
In this chapter, how to solve the EMI of PCB to improve the performance of communication systems? By special arrangement of RF components on PCB will enhance the TIS and TRP of communications. How to measure the electromagnetic interference and locate the EMI sources on PCB is not an easy job. This chapter proposes the aperture field technique to get the high resolution surface current density on the PCB. In order to verify the technique, simulation the PIFA antennas with surface current density and near aperture field on PCB are analyzed by GEMS. Except for the simulation, PIFA antennas are implemented and aperture field are collected by planar near field range. By using aperture field transformation to the surface of PCB, the surface current density on the PCB is obtained.
5.2 Diffraction field by NF aperture
Fig. 5.1 shows in the aperture antenna radiation diagram, while the S is the radiation source region, the position vector of source point
,
on the aperture is:
x ˆ y ˆ
(5.1)
The aperture field intensity on the source point is：
(5.2)
,
A
is the magnitude of field intensity, ,
is the phase of the field intensity, Fˆ is the polarization unit vector of source point.The diffraction field intensity at fixed
R, ,
is：
(5.3)
When the boundary conditions not any restrictions, if the location of the observation point placed in the far field range, because distance faraway, the radiation source to the observation point R and the origin point from the observation point r can be regarded as nearly parallel to line, 𝑛̂ ∙ 𝑟̂ = 𝑛̂ ∙ 𝑅̂ = cos θ , 𝑎𝑛𝑑
1 𝑟
≪ k =2π λ
, then(5.4)
Whether radiation source or the distribution of the radiation field, even integration of the radiation field, use the Cartesian coordinates to expression is most appropriate, in the integral particular solution, there are three most common and convenient way of expression the plane, there are y-z plane, x-z plane, x-y plane, as shown in the Fig.5.2, if the three planes were used to analyze the distribution of the radiation field, the integral will be different.
Nevertheless, even the different integral of the different plane, the calculated result will be the same, the actual distribution of the radiation field. At different plane respectively：
n s d d
(5.5)
(5.6)
(5.7)
Radiation from the source to the different observation points of the path integral ：
(5.8)
Because it inability to know the actual surface current distribution, so using the equivalence principle, by the actual near field electric field transform to equivalent surface current.
Fig. 5.3 shows a simple example of the equivalence principle. In Fig. 5.3 (a) within the region of S is the actual source of the radiation field distribution, it include the flux current J and M, the M is imagine of the virtual field source, exterior is the free space, let S region exterior the radiation field and the medium retain, while the internal is presume no any radiation field, the internal become a free space, as shown in Fig. 5.3 (b). As the S region exterior still presence a radiation field, so the boundary of the S region, it will
The transformation formula structure as shown in Fig. 5.4, using measurement plane induct the magnetic field energy, equivalent to the surface current distribution on the PCB, as shown in equation 5.10.
J⃗=ẑ × (H
x
x̂+Hy
ŷ+Hz
ẑ) (5.10)From equation 5.10, it can equivalent to the surface current.
𝐽
𝑥 =−𝐻 𝑦
(5.11a)𝐽
𝑦 =𝐻 𝑥 (5.11b)
5.3 Simulation and test results
In this part, it will use simulation tools to simulated the near field electric field and surface current distribution in the different distances are 0.3 cm, 0.5 cm 1 cm, as shown in Fig.5.5, the simulated surface current for PIFA at 900 MHz and 1800 MHz is shown in Fig.5.6 and Fig.5.7. It can find the distribution of the surface current at different frequency, the Fig.5.8 and Fig.5.9 are show the simulated magnetic field density with the distance at 0.3 cm in 900 MHz and 1800 MHz, the Fig.5.10 and Fig.5.11 are show the simulated magnetic field density with the distance at 0.5 cm in 900 MHz and 1800 MHz, Fig.5.12 and Fig.5.13 are show the simulated magnetic field density with the distance at 1cm in 900 MHz and 1800 MHz, sorting out the simulation data of the magnetic field density, it can use the results, by the
current density on the PCB.
The surface current density is transformed from the aperture field by the formula. Fig.5.14 as shown in transformation formula structure with different scope, the transformation scope are 3.5 mm*3.5 mm and 6.5 mm*6.5 mm, when the measurement plane and antenna surface to farther, the integration range must to raise, the Fig.5.15 and Fig.5.16 are show the transformed surface current with the distance at 0.3 cm in 900 MHz and 1800 MHz and the integration range is 3.5 mm*3.5 mm, the Fig.5.17 and Fig.5.18 are show the transformed surface current with the distance at 0.3 cm in 900 MHz and 1800 MHz and the integration range is 6.5 mm*6.5 mm, it can find integration range will affect intensity of the surface current, the Fig.5.19 and Fig.5.20 are show the transformed surface current with the distance at 0.5 cm in 900 MHz and 1800 MHz, Fig.5.21 and Fig.5.22 are show the transformed surface current with the distance at 1cm in 900 MHz and 1800 MHz, The results of transformed surface current from transformation and simulation are quite similar.
5.4 Summary
Most of radiation emission (RE) from PCB will cause the system fail to pass the EMI regulation and degradation the sensitivity in communication system. Since the size of cellular communication system is smaller and smaller, the sensitivity of system is degraded by near field EMI. This chapter proposes a technique to predict the high resolution surface current density on PCB by planar aperture field above the PCB and current density on the PCB.
The aperture field of PCB antenna with dual bands are measured by planar near field scanner. The measured data is transformed to the surface of PCB to get the surface current density on the PCB. The results of PCB current density from both measurement and simulation are quite similar.
Fig.5.1 the diffraction field by near field aperture.
Fig.5.2 (a) Plane position and rectangular aperture antenna radiation analysis: x-z plane.
dz
dx
Fig.5.2 (b) Plane position and rectangular aperture antenna radiation analysis: y-z plane.
Fig.5.2 (c) Plane position and rectangular aperture antenna radiation
analysis: x -y plane.
dy
dx dy
dz
Fig.5.3 Equivalence principle.
Fig.5.4 Transformation formula structure.
Fig.5.5 PIFA antenna structure.
Fig.5.6 Surface current at 900 MHz.
Fig. 5.7 Surface current at 1800 MHz.
Fig. 5.8 Magnetic field at distance 0.3 cm in 900 MHz.
Fig. 5.9 Magnetic field at distance 0.3cm in 1800MHz.
Fig. 5.10 Magnetic field at distance 0.5 cm in 900 MHz.
Fig. 5.11 Magnetic field at distance 0.5 cm in 1800 MHz.
Fig. 5.12 Magnetic field at distance 1 cm in 900 MHz.
Fig. 5.13 Magnetic field at distance 1 cm in 1800 MHz.
Fig.5.14 Transformation formula structure with different scope.
Fig. 5.15 Transformed surface current at distance 3mm in 900 MHz.
Fig. 5.16 Transformed surface current at distance 3mm in 1800 MHz.
Fig. 5.17 Transformed surface current at distance 3mm in 900 MHz.
Fig. 5.18 Transformed surface current at distance 3mm in 1800 MHz.
Fig. 5.19 Transformed surface current at distance 5mm in 900 MHz.
Fig. 5.20 Transformed surface current at distance 5mm in 1800 MHz.
Fig. 5.21 Transformed surface current at distance 1cm in 900 MHz.
Fig. 5.22 Transformed surface current at distance 1cm in 1800 MHz.
Chapter 6 Conclusion
The EMI / EMC problem is prompted by the current on the conductor change to generated electromagnetic radiation; similarly, the external electromagnetic field energy can also be changes the circuit current. Most of high-speed and fast rise time signal will produce the EMI / EMC problems, these problems will be connected to the equipment the cable, the typical solution is shield and filter on the input and output signals on power line. This thesis proposes the aperture field technique to get the high resolution surface current density on the PCB, by using aperture field transformation to the surface of PCB, the surface current density on the PCB is obtained. Almost all of EMI interference is produce current in the product of the somewhere, if these currents can be properly controlled, it only contains work required for the harmonic, reduce the high frequency harmonic unnecessary interference, the EMI / EMC problems will be solved. If during the design phase, can consideration the EMC problem, it will be save time and cost.
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