RF Microwave

1 1RF / Microwave PC BoardDesign and LayoutRick HartleyL-3 Avionics / Microwave Design - Contents1) Recommended Reading List2) Basics3) Line Types and Impedance4) Integral Components5) Layout Techniques / Strategies6) Power Bus7) Board Stack-Up8) Skin Effect and Loss Tangent9) Shields and Shielding10) PCB Materials, Fabrication and Assembly3RF / Microwave - Reading ListPCB Designers Transmission Line Design Handbook Brian C. Wadell(Artech House Publishers) ISBN 0-89006-436-9 HF Filter Design and Computer Simulation Randall (Noble Publishing Corp.) ISBN 1-884932-25-8 Partitioning for RF Design Andy Kowalewski - PrintedCircuit Design Magazine, April, 2000.

2 RF & Microwave Design Techniques for PCBs Lawrence - Proceedings, PCB Design Conference West, / Microwave - Reading ListRF Design Engineers Microstrip Lines and Slotlines Gupta, Garg, Bahl and House Publishers (1996) ISBN 0-89006-766-X RF Circuit Design Chris Bowick. Newnes Publishing (1982) ISBN 0-7506-9946-9 Introduction to Radio Frequency Design Wes Hayward. TheAmerican Radio Relay League Inc. (1994) ISBN 0-87259-492-0 Practical Microwaves Thomas S. Laverghetta. Prentice Hall, Inc.(1996) ISBN 0-13-186875-65RF / Microwave Design - Basics)RF and Microwave Layout encompassesthe Design of Analog Based Circuits in therange of Hundreds of Megahertz (MHz) toMany Gigahertz (GHz).

3 RF actually in the 500 MHz - 2 GHz Band.(Design Above 100 MHz considered RF.)) Microwave above 2 / Microwave Design - Basics)Unlike Digital, Analog Signals can be atany Voltage and Current Level (Betweentheir Min & Max), at any point in Time.)Standard Analog Signals are assumed tobe between DC and a few Hundred MHz.)RF/ Microwave Signals are One Frequencyor a Band of Frequencies imposed on aVery High Frequency / Microwave Design - Basics)RF/ Microwave Circuits are Designed toPass Signals within Band of Interest andFilter Energy outside that Range.)Signal Band can be Narrow or Wide.*Narrow Band Circuits usually have Pass Bandless than 1 MHz.* Broad Band Circuits Pass a Range of Freq-uencies up to 10 s of / Microwave Design - Basics)When Digital and Microwave exist inthe Same Unit, Pass Bands of Micro-wave Circuits usually fall (by design)Outside the Harmonic Range of theDigital / Microwave Design - Basics)RF / Microwave PC Board Layout simplyfollows the Laws of Physics -)When Laws of Physics can t be followed,Know what Compromises are IS NOT BLACK MAGIC!

4 !!10RF / Microwave Design - Basics) Microwave Signals are Very Sensitive toNoise, Ringing and Reflections and Mustbe treated with Great Care.)Need Complete Impedance (Zo) Match-ing (50 ohm out/ 50 ohm line/ 50 ohm in).*Minimizes Return Loss / / Microwave Design - Basics)A Transmission Line is any Pair or Wires orConductors used to Move Energy From pointA to point B, Usually of Controlled Size andin a Controlled Dielectric to create a Con-trolled Impedance (Zo).12RF / Microwave Design - Basics)Inductance (L) is Determined by the LoopFunction of Signal and Return Path.*Small Spacing (Tight Loop) creates HighFlux Cancellation, hence Low Inductance.)Capacitance (C) is Function of Signal spac-ing to the Return Path.

5 *Small Spacing creates High / Microwave Design - Basics)Since Small Spacing (Tight Loop) createsLow L & High C and since Zo = sqrt L/C,Small Spacing creates Low Zo.)Additionally, Zo is function of Signal Con-ductor Width & Thickness and a Functionof the Dielectric Constant ( ) of the Mat-erial surrounding the 14RF / Microwave Design - Basics)Sometimes Dielectric surrounding Trans-mission Line isn t Constant (Outer LayerTrace on PCB).*DK above Trace is Air ( = ).*DK below Trace is FR4 (approx = ).*Effective Relative ( ) is 3 to )Equations given later to Calculate EffectiveRelative ( ).r eff r eff 15RF / Microwave Design - Basics)Signal Return Currents follow the Path ofLeast Impedance (In High Frequency Cir-cuits that = Path of Least Inductance).

6 Whenever we Neglect to provide a LowImpedance Return Path for RF / Micro-wave signals, they WILL find a Path.)It may NOT be what we had in / Microwave Design - Basics)Signal Wavelength -*Wavelength ( ) of a Signal is the Distance itTravels in the Time of One Cycle.)For a Signal Traveling in Free Space -* = c (Speed of Light) / f (frequency).( = /nSec at 1 GHz = ))Signal in a Higher Dielectric -* =rfc /1/ r 17RF / Microwave Design - Basics)Signal Critical Length-*How long a PCB Trace can be before weMUST pay attention to Impedance Control.*Function of Frequency (1/16th Wavelength))At 1 GHz = approx .425 (Microstrip- FR4))At 1 GHz = approx.

7 375 (Stripline - FR4)1611 =effcriticalfcL 18RF / Microwave Design - BasicsSignal Loss / Noise -)Reflections -*Return Loss / VSWR)Skin Effect -*Increased Resistance of PCB Trace due toDecreased Cross Sectional Area.*In Analog Circuits above 100 MHz.*Skin Depth- .000822 @ 10 @ 10 / Microwave Design BasicsSignal Loss / Noise -)Loss Tangent -*Dielectric Loss caused by Molecular Struc-ture of Board Material.*In Analog Circuits above 200 MHz.*PTFE s Far Better than FR4.)Energy Coupling-*Cross Talk.*Noise / Microwave Design -Line Types and Impedance (Zo))Waveguide-*Uses Air as Trans-misssion Mediumand Side Walls of Tube as Return Path.*Won t Support Energy Propagation BelowCutoff Frequency.

8 *Works Best at Ultra High Frequencies withMillimeter / Microwave Design -Line Types and Impedance (Zo))Waveguide -*With an Air Dielectric, Signals Propagate atthe Speed of Light.*Very Low Loss due to Smooth Side Walls andthe Air Dielectric.*Ultra Low Loss with High Density, UltraSmooth Coating on Walls.*In Very High Power applications, Uses SolidDielectric to Prevent Voltage / Microwave Design -Line Types and Impedance (Zo))Signal Traces Longer than Critical Length(1/16 in DK) Need Impedance Control toPrevent Return Loss due to Reflections.)Shorter Circuit Elements Don t RequireImpedance Control, but it Usually does NOHarm.)Don t bother to Zo Control Short Lines if itWill create a Problem (ie- DFM).

9 23RF / Microwave Design -Line Types and Impedance (Zo))Impedance (L/C)-*Lower Er Materials Net Higher ImpedanceTraces and Faster Propagation Times per givenTrace Width & Trace-to-Ground Separation.*As Trace Width Increases, Trace ImpedanceDecreases (Thickness has Min Effect).*As Trace Spacing from Ground Increases,Impedance / Microwave Design -Line Types and Impedance (Zo)Transmission Line History -)Two Coplanar Strips in 1936. Later RolledUp to create Sealed Line.)Coax Lines during WWII.)Flat Stripline Using PCB Techniques rightafter WWII.)First use of Microstrip Reported in / Microwave Design -Line Types and Impedance (Zo)Microstrip26RF / Microwave Design -Line Types and Impedance (Zo)Microstrip(Replace Erwith Eeff))( ' ' ' ++ ++ +++= rrrrwhwhwhZwhere.

10 ''www += + = 'rww () ++= 27RF / Microwave Design -Line Types and Impedance (Zo)Microstripifotherwise + =hwwhZeff48ln600 1<hw + ++ = 28RF / Microwave Design -Line Types and Impedance (Zo)Microstrip ifotherwise1<hw + ++=whrreff12112121 ++ ++= 29RF / Microwave Design -Line Types and Impedance (Zo)Embedded MicrostripMultiply Zo (from Microstrip) by -Can use w/Soldermask over Microstrip (Often NOT Needed)()()[] + 30RF / Microwave Design -Line Types and Impedance (Zo)Centered Stripline31RF / Microwave Design -Line Types and Impedance (Zo)Centered Striplinewhere:() + + += ' ')( ')( thb+= +='() ++ + = 32RF / Microwave Design -Line Types and Impedance (Zo)Off-Center Stripline(Equations in Wadell- Pages 130-133)33RF / Microwave Design -Line Types and Impedance (Zo)Microstrip verses Stripline)Microstrip has Lower Loss Tan Problem.