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Module-2Module 2RF PCB Design g
Microstrip Discontinuities & Simulation in Momentum
Rashad.M.Ramzan, Ph.D
FAST-NU, Islamabad
Today's Topicsoday s op cs
• Modeling &n EM Simulationsg– Schematic Simulation & Layout Simulation
Momentum Simulation– Momentum Simulation
• Microstrip Discontinuities
• GND path Breaks
• AC resistance and Skin Effect• AC resistance and Skin Effect
• Differential Signaling
• Odd and Even Mode Impedances
D li C it• Decoupling CapacitorsRF PCB Design: Lecture- 2 © Rashad.M.Ramzan 2010-11 2
Design Flowes g ow
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RF PCB …and….MaxwellC …a d…. a we
Maxwell’s Equations
Faraday’s Law: Ampère’s Circuital Law:
q
DJHt∂∂
+=×∇BEt∂∂
−=×∇
G ’ L C tit ti E ti
ρ=⋅∇ D0=⋅∇ B
Gauss’ Laws:
HB μ= ED ε=
Constitutive Equations:
Actual solution complex and for realistic problems require approximations
S l i M ll’ i i i l i i f iSolutions to Maxwell’s equations using numerical approximations of is known as the study of computational electromagnetic (CEM)
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CEM TechniquesC ec ques
• Examples of full-wave computational l i (CEM) h i i l delectromagnetic (CEM) techniques include:– Finite difference time domain (FDTD) Method
M h d f M (M M) M h d– Method of Moments (MoM) Method– Finite Element (FE) Method
T i i Li M t i (TLM) M th d– Transmission Line Matrix (TLM) Method– The Method of Lines (MoL)
The Generalized Multipole Technique (GMT)– The Generalized Multipole Technique (GMT)
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ADS Software OverviewS So twa e Ove v ew
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Design Steps ….ADS & Momentumes g Steps …. S & o e tu
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ADS SchematicS Sc e at c
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ADS MomentumS o e tu• 2.5D (vias) simulation tool for passive circuits.
• Layout driven (accepts arbitrary geometry).
• Method of Moments technique as planar solver.
• Green s functions used to solve integral equations.
• Unlimited substrate database for computation.
• Full microwave or RF mode S-Parameter solutions.
• Layout look-alike components for ADS simulation.
• Co-simulation with ADS and optimization.
• Visualization of current and far-field patterns.
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ADS MomentumS o e tu
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ADS Momentum Current VisualizationS o e tu Cu e t V sua at o
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When to use MomentumW e to use o e tu• When no analytical model exists- co-simulate with ADS.
• Arbitrary geometry on a uniform planar substrate.
• To determine coupling effects-thin layers & close proximity of metal.
• To find narrow resonances not found using analytical models.
• To display radiation patterns plot far-fields (antennas, etc.)
• To display current patterns and density.
• For CPW: coplanar waveguide results with no slot mode.
• When 3D solvers take too long or are not required.
• To optimize (modify geometry) a passive layout to achieve desired results.
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How Momentum Works?ow o e tu Wo s?
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ADS MomentumS o e tu
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SPICE & MomentumS C & o e tu
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ADS EMDSS S
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Transmission LineTransmission Line DiscontinuitiesDiscontinuities&Corrections&Corrections
Transmission Line Discontinuitiesa s ss o e sco t u t es
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Transmission Line Discontinuitiesa s ss o e sco t u t es
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Transmission Line Discontinuitiesa s ss o e sco t u t es• Introduction of
discontinuitiesdiscontinuities will distort the uniform EM!!
• Most of these induced higher order modesorder modes are non-propagating
• The effect of discontinuity is usually reactive (the energy stored in y y ( gythe local fields is returned back to the system
• The effect of reactive system to the voltage and current can be d l d i C i i ( hi h i l )
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modeled using LC circuits (which are reactive elements).
Discontinuities: Through Hole Viasco t u t es: oug o e V a
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Discontinuities: Through Hole Viasco t u t es: oug o e V a
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Discontinuities: Bendssco t u t es: e ds
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Reducing the Effects of Bendseduc g t e ects o e ds• Generally not necessary under 300MHz frequencies
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Discontinuities: Bendssco t u t es: e ds
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Discontinuities: Step in Widthsco t u t es: Step W dt
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Discontinuities: T-Junctionsco t u t es: Ju ct o
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Discontinuities: Stubssco t u t es: Stubs
28
Discontinuities: SMA Launchsco t u t es: S au c
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Discontinuities: SMA Launchsco t u t es: S au c
53
56
59
ms)
47
50
53
nce
(o
hm
41
44
mp
edan
10cm
35
38
60 6 60 9 61 2 61 5 61 8 62 1
I 10cm
60.6 60.9 61.2 61.5 61.8 62.1Time (nsec)
Measured and Design By Rashad
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BW Reduction due to BendsW educt o due to e ds
•Networks can be interpreted as Low-Pass Filters. •The inductor•attenuates the electrical signal at high frequency while the capacitor g g q y pshunts electrical energy at high frequency.•Thus the effect of having too many discontinuities in a high-frequency circuit reduces the overall bandwidth of thefrequency circuit reduces the overall bandwidth of the interconnection.•Another consequence of discontinuities is attenuation due to radiation from the discontinuity
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radiation from the discontinuity.
Discontinuities Modeling and Simulationg
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Manufacturer Tolerances
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AC Current Return PathC Cu e t etu at
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AC Current Return PathC Cu e t etu at
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GND Plane BreakGN a e ea
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Partial GND Breaka t a GN ea
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GND Break increases CouplingGN ea c eases Coup g
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Discontinuities: Through Hole Viasco t u t es: oug o e V a
1 2
3 4
IEEE Circuits & Devices Nov 1997
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IEEE Circuits & Devices Nov. 1997
Skin Depth – AC ResistanceS ept C es sta ceSkin effect confines 63% (e-1) of the current to 1 skin depth – the current
density will decease exponentially into the thickness of the conductory p y
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Skin Depth- AC ResistanceS ept C es sta ce
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GND Current AC ResistanceGN Cu e t C es sta ce
t
2
1)(
IDI O ⋅=
t δH
2)/(1)(
HDHDI
+π(Current Density in plane)
3H
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GND (Current) AC ResistanceGN (Cu e t) C es sta ce• The current density formulae can be integrated to get the total
current contained within chosen bounds
oo
Ho I
IdD
HDH
I795.0)3(tan
2
)/(1
1 13
2=⋅=
+⋅ −∫ ππ
current contained within chosen bounds
H HDH )/(13 +−∫ ππ
• This shows that 79.5% of the current is contained in a distance +/ 3H (W f 6H) f th d t t+/- 3H (W of 6H) from the conductor center
• Assuming a penetration of 1 skin depth, the ground return resistance can be approximated as followsresistance can be approximated as follows
lengthH
F
HAR groundac /
66_ Ω==≈ρπμ
δρρ
HHAground 66 δ
groundsignaltotalAC RRR +=
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groundsignaltotalAC _
Surface RoughnessSu ace oug ess• The formulae presented assumes a perfectly smooth surface
• The copper must be rough so it will adhere to the laminate
• Surface roughness can increase the calculated resistance 10-50% as well as frequency dependence proportions
• Increase the effective path length and decreases the area
Trace
Skin-Depth
Tooth structure(4-7 microns)
Plane
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Copper OptionsCoppe Opt o s
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Mutual Coupling and Crosstalkutua Coup g a d C ossta
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Mutual Coupling and Crosstalkutua Coup g a d C ossta
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Differential Pairse e t a a s• Lower Cross-talk, Lower Radiation
• Common mode noise rejection
• Reduces ground reference problems
• High dynamic range analog applications– Log Amplifiers
• High Resolution ADC/DAC– Low noise (small signal) analog applications
T d• Transducers– Critical High Speed Digital Applications
L lit d l k• Low amplitude clocks
• Low jitter requirements
Al t ALL RF li ti• Almost ALL RF applications
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Differential Pairse e t a a s
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Differential Impedance Definitionse e t a peda ce e t o s
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Differential Impedance Examplese e t a peda ce a p es
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Impedance of Transmission Linespeda ce o a s ss o es
Simulations
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Simulations
Decoupling Capacitorsecoup g Capac to s
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ESR and ESL of CapacitorsS a d S o Capac to s
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Decoupling Caps Packagesecoup g Caps ac ages
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Decoupling Connection Geometryecoup g Co ect o Geo et y
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SummarySummary• Modeling &n EM Simulationsg
– Schematic Simulation & Layout Simulation
Momentum Simulation– Momentum Simulation
• Microstrip Discontinuities
• GND path Breaks
• AC resistance and Skin Effect• AC resistance and Skin Effect
• Differential Signaling
• Odd and Even Mode Impedances
D li C it• Decoupling Capacitors RF PCB Design: Lecture- 2 © Rashad.M.Ramzan 2010-11 57