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. 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.

2 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).)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.

3 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!!!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).

4 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.*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).

5 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 .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.

6 *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.*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.

7 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).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:''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.

8 () + + += ' ')( ')( 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.)Microstrip has Faster Propagation Time.)Stripline has Better Immunity to Crosstalk.)Stripline has Better EMI / Microwave Design -Line Types and Impedance (Zo)Coplanar Waveguide) b should be less than /2 for best performance.)Ground Must extend Greater than 5x b on eitherside of Trace a .35RF / Microwave Design -Line Types and Impedance (Zo)Coplanar Waveguide)Lower Loss Tangent than Microstrip (SignalsCouple Mostly through Air).)Higher Skin Effect Losses (Fields Concentrate onEdges of Trace and Grounds).)May Need to Strap Grounds together on Either Sideof Trace, every 1/20th Wavelength.)Only Need One Side of Board to be Accessible.

9 No Plated Holes Needed,)Can Narrow Trace to Match Component / Microwave Design -Line Types and Impedance (Zo)Coplanar Waveguide)CPW Allows Variation of Trace Width, orSpacing-to-Ground or Dielectric Thicknessto Control Zo.)Zo of CPW Decreases as Dielectric Thick-ness Increases.)CPW Produces Smaller Trace per given Zothan / Microwave Design -Line Types and Impedance (Zo)Coplanar Waveguide()()ktKktKZteff' ,0 = ()()() ' ,+ =kKkKtabeffeffteff ()()() ( )'11' += tttbak=bak= 'ttkk = 'kk = = '1kk = ++=tataat + =tatbbt / Microwave Design -Line Types and Impedance (Zo)CPW verses Microstrip = - Zo = 76 ( = (CPW) & (MS)) = - Zo = 94 ( = (CPW) & (MS)) = - Zo = 94 ( = (CPW) & (MS)) = - Zo = 115 ( = (CPW) & (MS))r eff r eff r r eff eff 39RF / Microwave Design -Line Types and Impedance (Zo)Coplanar Waveguide w/Ground)In Reality, Microstrip Transmission Line inthe RF / Microwave Arena is / Microwave Design -Line Types and Impedance (Zo)Coplanar Waveguide w/Ground()()()()'11' + = ()()()()()()()()

10 '11' '11' ++= bak/= 'kk = '1kk = = 41RF / Microwave Design -Line Types and Impedance (Zo)Coplanar Waveguide w/Ground)To Avoid Microstrip Mode, h >b and Left& Right Ground Extend Away from a byMore than b .)Zo of CPWG is Increased as DielectricThickness Increases. Opposite of CPW.)If h is Large, CPW and CPWG Behavein Similar / Microwave Design -Line Types and Impedance (Zo)CPWG verses Microstrip = - Zo = 94 Ohms (At Gap = 30)( = (CPWG) and (MS)) = - Zo = 115 Ohms (At Gap = 27)( = (CPWG) & (MS)))Beyond Gaps shown above, CPWG like eff r r 43RF / Microwave Design -Line Types and Impedance (Zo)Edge Coupled CPW (CP Differential Pair)(Equations in Wadell- Page 194-195))Gives an Extra Degree of Signal-to-Noise IsolationOver standard CPW. (w/o Plane, Fields are Large)44RF / Microwave Design -Line Types and Impedance (Zo)Edge Coupled CPWG (CP Diff Pair w/Grnd)(Equations in Wadell- Page 197-198))Much Better Field Containment than Coupled yet in Edge Couple / Microwave Design -Line Types and Impedance (Zo)Slotline)Acts Like Waveguide with Air DK.