The presentation discusses the importance of housing design for RF/MW sub-systems and the effects of cavity resonances. It describes how 3D electromagnetic simulation software like CST and HFSS are used to analyze the electric and magnetic field distributions and identify resonant modes within enclosures. Various techniques for mitigating unwanted oscillations caused by cavity resonances are presented, including modifying the package geometry and material properties during the design phase or adding absorbers to existing packages.

1. PRESENTATION ON
HOUSING EFFECT
ON
RF/MW SUB-SYSTEMS
BY,
RAMLAL BHARATH. J
R&D (RF&MW)- AEROSPACE SYSTEMS

2.  The package design for RF/MW Sub-systems require adequate
knowledge of EM field distribution inside the housing.
 RF/MW Sub-systems consists of MMICs, Active/Passive Devices
having different package styles.
 Housing design is very important for all Sub-systems as it
provides Environmental protection, Structural support and also
suitable path for heat transfer.
 Cavity electrical size increases at higher frequencies as a result no
of undesirable parasitic effects degrades the performance of the
circuit.

3.  The conductive surface of the enclosure supports several resonant
modes and produces oscillating charges, which in turn produces
surface currents on conducting walls of the package.
 In an assembled package, the effect of various parts like Feed
through, Packaged devices, Mounting screws are taken in account.
 The cavity resonance of the Sub-system degrades the electrical
performance and also causes unwanted oscillations which results in
failure of the Sub-system.
 Enclosure for the RF/MW Sub-systems are designed using Aluminium.

4.  EM design and analysis for the circuit is carried out using 2D EDA
software ADS , but EM analysis for enclosure is not carried out in it.
 3D EDA softwares like CST, HFSS, EMPro based on FEM (Finite Element
Method) is used for Eigen/Driven Mode Study.
 Eigen Mode solver calculates Resonance Frequencies and Fields at the
resonant frequencies.
 Driven Mode solver calculates S-Parameters.
 The following differential equation , known as Helmholtz wave
equation is solved by FEM.
ψ = Electric or Magnetic field.

5.  FEM breaks 3D structure into large number of tetrahedral mesh cells.
 3D EDA softwares computes wave patterns (TE, TM modes) on each
ports of the structure supported by transmission lines.
 TM wave in a cavity resonator is any wave which has no magnetic field
in Z-Direction of cavity. i.e. Ez ≠ 0 ,Hz = 0.
In TM mnp Modes , m,n = 1,2,3…..
p = 0,1,2,3……
 TE wave in a cavity resonator is any wave which has no magnetic field
in Z-Direction of cavity. i.e. Ez = 0, Hz ≠ 0.
In TE mnp Modes , m,n = 0,1,2,3…..
p = 1,2,3……

6.  The field pattern of the travelling wave is calculated by using
Maxwell’s equation.
 The theoretical resonance frequency for rectangular cavity is
calculated by
c= Velocity of light
μr = Relative Permeability
εr = Relative Permittivity
a = Width
b = Height
d = Length

7.  To minimize the effects of cavity resonances , different mitigation
techniques are adopted : 1) Pre-design Techniques
2) Post-design Techniques
 Pre-design techniques are related to Package geometry, Package
material selection and Surface Treatment.
 Post-design techniques are implemented when packages are already
fabricated. They are : 1) Optimization of Package geometry
2) Analysis of Package Material Selection
3) Selection of Absorber