rf / microwave pc board design and layout - jefferson · pdf file1 rf / microwave pc board...

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RF / Microwave PC BoardDesign and Layout

Rick HartleyL-3 Avionics Systems

[email protected]

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RF / Microwave Design - Contents

1) 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 Assembly

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RF / Microwave - Reading List

PCB Designers –

• Transmission Line Design Handbook – Brian C. Wadell(Artech House Publishers) – ISBN 0-89006-436-9

• HF Filter Design and Computer Simulation – Randall W.Rhea (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 M.Burns - Proceedings, PCB Design Conference West, 2000.

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RF / Microwave - Reading List

RF Design Engineers –

• Microstrip Lines and Slotlines – Gupta, Garg, Bahl and Bhartia.Artech 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 0-13-186875-6

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RF / 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 GHZ.

6


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 Carrier.

7


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 MHz.

8


When Digital and Microwave exist inthe Same Unit, Pass Bands of Micro-wave Circuits usually fall (by design)Outside the Harmonic Range of theDigital Signals.

9


RF / Microwave PC Board Layout simplyfollows the “Laws of Physics”-When Laws of Physics can’t be followed,Know what Compromises are available.

THIS IS NOT BLACK MAGIC!!!

10


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 / VSWR.

11


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).

12


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 Capacitance.

13


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 Lines.

rε

14


Sometimes Dielectric surrounding Trans-mission Line isn’t Constant (Outer LayerTrace on PCB).

DK above Trace is Air ( = 1.0008).DK below Trace is FR4 (approx = 4.1).Effective Relative ( ) is 3 to 3.25.

Equations given later to Calculate EffectiveRelative ( ).

rε effε

rε effε

15


Signal Return Currents follow the Path ofLeast Impedance (In High Frequency Cir-cuits that = Path of Least Inductance).

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 mind.

16


Signal Wavelength -Wavelength (λ) of a Signal is the Distance itTravels in the Time of One Cycle.

For a Signal Traveling in Free Space -λ = c (Speed of Light) / f (frequency).(λ = 11.78”/nSec at 1GHz = 11.78”)

Signal in a Higher Dielectric -λ = rfc ε/1/ •

rε

17


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

••=eff

critical fcL

ε

18


Signal Loss / Noise -Reflections -

Return Loss / VSWRSkin Effect -

Increased Resistance of PCB Trace due toDecreased Cross Sectional Area.In Analog Circuits above 100 MHz.Skin Depth- .000822” @ 10 MHz.

.000026” @ 10 GHz.

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RF / Microwave Design Basics

Signal 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 Induction.

20

RF / 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 Wavelengths.

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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 Arcing.

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Signal Traces Longer than Critical Length(1/16 λ 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).

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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 Increases.

24


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 1949.

25


Microstrip

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Microstrip

(Replace Erwith Eeff)

)(0.2

/0.10.1'

0.40.11

/0.80.14

'0.4

0.11/0.80.14

'0.40.1ln

0.10.20.2120

222

0

Ω

++

+

+

+

++

=

πεε

εεπ

π

rr

r

r

wh

wh

whZ

where: '' www ∆+=

+

∆=∆0.2

/0.10.1' rww ε

( )

++

=∆

22

1.1//1/

4ln0.1

twht

etw

ππ

27


Microstrip

if

otherwise

+•=

hw

whZ

eff 48ln60

0 ε1<

hw

+•++

•=444.1ln677.0393.1

11200

hw

hw

Zeffεπ

28


Microstrip

if

otherwise

1<hw

+

−+

+=

wh

rreff 121

12

12

1 εεε

−+

+

−+

+=

2

104.0121

12

12

1hw

wh

rreff

εεε

29


Embedded Microstrip

Multiply Zo (from Microstrip) by -

Can use w/Soldermask over Microstrip (Often NOT Needed)

( ) ( )[ ]hbr

hbeff

eff

ee /0.2/0.2 0.1 −− −+⋅ εεε

30


Centered Stripline

31


Centered Stripline

where:

( )

+

−

+−−

+= 27.6'

0.8'

)(0.8'

)(0.40.1ln0.2120 2

0 wtb

wtb

wtbZ

r πππεππ

thb += 0.2

ttwww ∆

+='

( )

++

+−

=∆

m

twttb

etw

1.1/25.

1/0.21

ln0.12

ππ

32


Off-Center Stripline

(Equations in Wadell- Pages 130-133)

33


Microstrip verses Stripline

Microstrip has Lower Loss Tan Problem.Microstrip has Faster Propagation Time.Stripline has Better Immunity to Crosstalk.Stripline has Better EMI Characteristics.

34


Coplanar Waveguide

‘b’ should be less than λ/2 for best performance.Ground Must extend Greater than 5x‘b’ on eitherside of Trace ‘a’.

35


Coplanar WaveguideLower 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.No Plated Holes Needed,Can Narrow Trace to Match Component Leads.

36


Coplanar WaveguideCPW 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 Microstrip.

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Coplanar Waveguide( )( )ktKktKZ

teff

'0.30

,0 •=

επ

( ) ( )( ) 0.1

'7.00.2/

0.1,

+•−

−−=

kKkK

tab

effeffteff

εεε

( ) ( )( ) ( )'1

1'0.2

0.10.1kKkKkKkKr

eff •−

+=εε

t

tt b

ak = bak =

20.1' tt kk −= 20.1' kk −=

=

hbh

a

kt

t

0.4sinh

0.4sinh

1π

π

210.1'1 kk −=

++=

tataat

ππ

0.4ln0.125.1

+−=

tatbbt

ππ

0.4ln0.125.1

38


CPW verses Microstrip

= 4.2 - Zo = 76 ( = 2.44 (CPW) & 3.02 (MS)) = 2.5 - Zo = 94 ( = 1.66 (CPW) & 1.96 (MS))

= 4.2 - Zo = 94 ( = 2.45 (CPW) & 2.95 (MS)) = 2.5 - Zo = 115 ( = 1.68 (CPW) & 1.92 (MS))

rε effεrε effε

rεrε effε

effε

39


Coplanar Waveguide w/Ground

In Reality, Microstrip Transmission Line inthe RF / Microwave Arena is CPWG.

40


Coplanar Waveguide w/Ground

( )( )

( )( )'1

1'

0.10.2120

0

kKkK

kKkKZ

eff +•=

επ

( )( )

( )( )

( )( )

( )( )'1

1'0.1

'11'0.1

kKkK

kKkK

kKkK

kKkK

r

eff

+

+=

εε

bak /=

20.1' kk −=

210.1'1 kk −=

=

hbh

a

k

0.4tanh

0.4tanh

1π

π

41


Coplanar Waveguide w/GroundTo 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 Fashion.

42


CPWG verses Microstrip

= 4.2 - Zo = 94 Ohms (At Gap = 30)( = 2.92 (CPWG) and 2.95 (MS))

= 2.5 - Zo = 115 Ohms (At Gap = 27)( = 1.89 (CPWG) & 1.92 (MS))

Beyond Gaps shown above, CPWG like Microstrip.

effε

effε

rε

rε

43


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)

44


Edge Coupled CPWG (CP Diff Pair w/Grnd)

(Equations in Wadell- Page 197-198)Much Better Field Containment than Coupled CPW.Better yet in Edge Couple Stripline.

45


Slotline

Acts Like Waveguide with Air DK.


46


Other Configurations3 Line Coplanar Strip w & w/o Ground.Microstrip w/ Limited Width Plane (and/or) Limited Width Dielectric.Metal Plate or Shield Covered CPW/CPWG.Metal Plate or Shield Covered Slotline.Offset CPW or CPWG.Etc, etc. - See Wadell or Gupta, Garg, .....

47


Zo CalculationsUse Equations Given or Wadell or Gupta.Use H.P. AppCAD (DOS and/or Windows).Use Rogers Corp. MWI (Dr R. Trout).Buy Field Solver (2D or 3D) Based ZoCalculator (i.e.- POLAR Ltd.)

Don’t use Equations or Calcs for Dig Lay-out that Don’t Comp for Coplanar Effects.

48


Tpd, Capacitance and Inductance Calcs(For all previous Configurations)

Tpd = / c(spd of light)C = Tpd / ZoL = Zo2 x C (or Tpd x Zo)

effε

49

RF / Microwave Design -Integral Components

Components can be Designed into the PCBoard Utilizing the Right Configurationof Lines and Shapes to form-

InductorsCapacitorsCouplers (Similar to Transformer)Resistors (Very Small Value)Filters

50


Capacitor formed by 2 Copper Plates separatedby PCB Dielectric (Free Component) -

C = x x (A/h)

Where: - DK of PCB Material - Permittivity of Space

(2.25 x 10-13 ferrads/in.)A - Area of Plate (L x W)h - Dielectric Thickness

rε 0εrε0ε

51


Interdigital Capacitor-

52


Interdigital Capacitor

(pF / in)

(Equation valid for h>w/N)

( )[ ]210.30.12 AANL

wC r +−

+=

ε

2

22820444.050133101.02

−=

XtA

2

15287116.03349057.01

−=

XtA

53


Multilayer Capacitor -

(pF)where: A = area of planes in square inches n = number of conductor layers d = plate spacing

( )d

nAC r 0.1229.0 −=

ε

54


Inductor- Inline Inductor is formed by aVery Thin, High Impedance Trace.

Length Must be Shorter than Critical Lengthto Prevent Reflections. Can Remove Plane(s)to Boost Inductance.L = Zo2 x C or Tpd x Zo (Many Equationsavailable. This is Extremely Accurate.)

55


Spiral Inductors -


56


Other Discrete ExamplesCPW & CPWG Shunt Capacitor -

57


Other Discrete ExamplesCPW & CPWG Series Capacitor -

58


Other Discrete ExamplesCPW & CPWG Shunt Inductor -

59


Other Discrete ExamplesCPW & CPWG Series Inductor -

60


Other Discrete ExamplesGap in Centered Stripline Conductor-

(Equations in Wadell - Pages 360-361)

61


Other Discrete ExamplesRound Hole in Centered Stripline -


62


Other Discrete ExamplesRectanular Hole in Centered Stripline -


63


Filters can be made from the L & CCircuit Elements discussed.

The Following Illustrates VariousFilters that can be Constructed.

64


λ/4 Stub is Series Resonant Circuit at Frequency.Circuit Shorts to Ground at λ/4, 3/4λ, etc.Open Circuit at DC, λ/2, λ, etc.

2W Wide for High Q and to Prevent Reflections.

65


Open Stubs (one just shown) have NarrowFrequency Over which they Short to Ground.Flaring the Stub Increases Frequency Response.


66


λ/4 Stub, Shorted to Ground, is Parallel ResonantFilter at Frequency of Interest.Circuit Shorts to Ground at DC, λ/2, λ, etc.Open Circuit at λ/4, 3/4 λ, etc.

67


Low Pass Filter -

68


Edge Coupled Band Pass Filter -

69


End Coupled Band Pass Filter -

70


Directional Coupler -

Input Output

Strongest WeakestCoupled Pulse Coupled Pulse

71


Directional Coupler can be used as -A Filter at λ/4 Frequencies.Non Loading Method to Transfer Energy toAnother Circuit.A Method to Monitor Power Send to Port 3.Closed Loop Feedback Control.A Non-Loading way to Measure a Signalwith an Oscilloscope.

72


Resistors -Impractical when made from PCB Copper.Requires Extremely Long Lines to achieveResistance of a Few Ohms.

One Exception -When Very Small ‘R’ is needed to Measure aVery Large Current.

73

RF / Microwave Design -Layout Techniques and Strategies

Low Level Analog, RF/Microwave andDigital Sections Must be Separated.Divide RF/Microwave Section into CircuitGroups (VCO, LO, Amps, etc.).Place High Frequency Components First, toMinimize Length of Each RF Route (Orient-ation for Function More Critical than DFM).Place Highest Frequency ComponentsNearest Connectors.

74


Don’t Locate Unrelated Outputs and InputsNear Each Other. Especially Multi-StagesWinding Back on One Another.When Either the Output or Input to Amp-lifiers Must be Long, Choose the Output.

(Where Do Resistors Go?)

75


Remember, Trace Impedance (Zo) is aCritical Factor in the Effort to ControlReflections.Impedance must Match Driver and Load.In Traces Shorter than 1/20th λ Long, ZoMatching is Usually Not Important.

76


When Pull-up Resistors or Inductors areUsed on the Outputs of Open CollectorDevices, Place the Pull-up ComponentRight At the Output Pin it’s Pulling.Also, make Certain to Decouple the Pull-up, in Addition to the Main Power Pins ofthe IC.

77


Inductors have Large Magnetic FieldsAround Them-

They Should Not be Placed Close Together,when In Parallel (Unless Intent is to have TheirMagnetic Fields Couple).Separate Inductors by One(1) Times BodyHeight (Min) -(OR)-Place Perpendicular to One Another.

78


Keep “ALL” Routes Confined to the Stageor Section to which they are Assigned-

Digital Traces in the Digital Section. Period.Low Level Analog in Low Level Analog.RF / Microwave in RF / Microwave Section.

Don’t Route Traces into AdjoiningSections.

79


Short RF Traces should be on ComponentSide of Board, Routed to Eliminate Vias.Next Layer Below RF Traces to be Ground.Minimizing Vias in RF Path MinimizesBreaks in Ground Plane(s)-

Minimizes Inductance.Helps Contain Stray Electric & Magnetic Fields.

Controls Lines can be Long, but Must RouteAway from RF Inputs.

80


RF / Microwave Lines Must be Kept AwayFrom One Another By Min Distances toPrevent Unintended Coupling & Crosstalk.Minimum Spacing is a Function of HowMuch Coupling is Acceptable.Equations in Wadell, Pages 181 - 254.

Good for Crosstalk, Directional Couplers, Oddor Even Mode Coupled (Diff) Lines.

81


82


83


When Circuit MUST Loop Back on Itself andOutputs end up Near Inputs -

Place Ground Copper (20 H Wide) BetweenSections, Most specifically Between Inputs andOutputs.Use 20 H Wall if Copper is Less than 20 H Wide.Attach Ground Copper to Board Planes Every1/20th Wavelength of Principal Frequency.

Use Same Methods when Unrelated Inputs andOutputs Must Be Near One Another.

84


In Multilayer Boards, When Signals MustChange layer, Route in Layer Pairs -

Layer 1 Signals Reference Ground on Layer 2.When Direction Change Needed, Via Signal toLayer 3.

i.e.: First Four(4) Layers of a Board -- - - - - - - - - - - Layer 1 (Signal - X Direction)------------------- Layer 2 (Ground Plane)- - - - - - - - - - - Layer 3 (Signal - Y Direction)------------------- Layer 4 (Ground Plane)

85


Components Connecting to Ground -Flood Component Lead with Surface Ground.(Let Soldermask or Mask Dam Define Pad).Ground Vias as Close to Component Lead asPossible. Preferably ON Component Lead.Multiple Vias (3, 4, etc.) Reduce Inductanceand Help Eliminate Ground Bounce.Direct Connection. No Thermal Vias.Must attempt to permit Proper Solder Reflow.

86


Ground: All Designs, 2 Layer or Multilayer -Unused Areas of Every Layer to be Poured withGround Copper.Ground Copper and All Ground Planes throughBoard to be Connected with Vias Every 1/20thWavelength Apart (Where Possible).Vias Closer than 1/20th λ are Better.Very Critical Circuits - Vias Closer than 1/ 20thλ Help Reduce Noise.Direct Connect Vias. NO Thermal Vias.

87


Ground:‘Copper Pours’ Too Small to have Vias Must beRemoved (Can Act as Antenna).Arrange Poured Ground Around Signals toCompletely Surround Signals -

Ground Vias to Include Picket Fencing at Edge ofBoard. In Very Critical Circuits, Plate Board Edge.

88


Ground:By Maintaining Isolation Between Circuits, DoNot Split Ground Plane.Attach Ground to Case Continuously.One Exception is Ground for Cable Shields.

89


Mismatched Source and Load Impedance:If Line can be λ/4 Long -

(λ/4 is Calculated From Frequency of Sourceand Eeff of the Transmission Line.)

loads ZZZ •=0

6850x=

3400=Ω= 3.58

90


Mismatched Source and Load Impedance:If Line Can NOT be λ/4 Long -

91Mismatched Source and Load Impedance

92http://home3.netcarrier.com/~chan/EM/PROGRAMS/STUBMATCH/

93


Trace Corners -

94


Trace Corners -Others Considered Fair to Good

95


‘T’-Junctions:Ideal is the Wilkinson Splitter-

(What if Split to Load is Less than Critical Length?)

96


‘T’-Junctions (Acceptable):

97


Impedance of Vias-

REFERENCEWang, Taoyun, et al., “Quasi-static Analysis of a Microstrip Via Through a Hole ina Ground Plane,” IEEE Transactions on Microstrip Theory and Techniques, Vol.MTT-36, No. 6, June 1988, pp. 1007-1013.

98


Copper Patches can be placed Next to SignalTraces to Create Attachment Points for Wireor Solder to Create Tuning ‘C’ or ‘L’ -

99


Long Microstrip Traces can be Antennafor Radiation of EMI or Reception ofNoise.Ideal Trace Antenna is 1/4 WavelengthLong.In Designs where Stripline is available,Keep Outer Layer Traces under CriticalLength.

100

RF / Microwave Design -Power Bus

Route Power in 2 Layer Board (Microstrip,CPW or CPWG) (Only Plane is Ground).In Multilayer Boards Power Can be Planeif One(1) Voltage or Split Plane if SeveralVoltages.In Multilayer Board with Many Voltages,Power is Usually Routed on One (1) ormore Layers.

101


When Routed, Make Power Grid if two(2)or more Layers are used. Grid Most CloselyEmulates Behavior of a Plane.Due to Self Resonance of Decoupling Caps,selected to match Frequency of Operation,Wide Routes work as well in Analog Cir-cuits to distribute Power as do Planes.

102


Power is generally ------------------- GroundRouted between -- -- -- -- -- -- PowerGround Planes to ------------------- Groundhelp Lower Noise Coupling in Power Bus.Ground Planes are Always Continuous.(Only Split at Front Panel for Cable Shieldsand Filters).

103


Power Decoupling Consists of Low PassFilter with Several Capacitors to Cover aBroad Range of Frequencies and Currents-

(Notice Order of Capacitors)

104


Select Capacitors in Low Pass Filter soSmallest Value has Self Resonance ofOperating Frequency of Circuit.Largest Value selected to carry MaximumCurrent Drawn by IC.Capacitors Progress Upward in Value inSteps of 10 Times.Place Caps w/ Smallest Value Near IC, thenNext Largest Value, etc.

105


Place Smallest Value Capacitor AT Powerand Ground Pins of IC.Ideally, Capacitors are in Parallel with Power& Ground Pins of IC.When Not Possible, Place Smallest ValueCapacitor at Power Pin of IC and as NearGround Pin as Possible.Attach Caps w/ Wide Traces & Gnd Floods.Many Vias, Large Enough to Carry Current.

106


107

RF / Microwave Design -Board Stack-Up

In 2 Layer Boards, Dielectric Will be Tight-ly Controlled (And Usually Not .062”).Dielectrics of .015-.025” Thick, Common.In ALL Designs, One Ground Plane MIN.In CPW, to create Continuous Ground, StrapAcross Sig Lines From Ground to Ground.In Microstrip / CPWG, Pour Ground on SigSide, w/ Continuous Ground Opposite Side.

108

RF / Microwave Design - Board Stack-Up

In Multilayer Board, have Ground on EveryOther Layer.Signals Located on Either Side of GroundPlane Must Cross at Right Angles.(Planes give 60 dB of Isolation of Currentson Either Side. 60 dB May Not be Enoughin RF / Microwave Circuit, Hence RightAngle Routing.)Remember, Route in Layer Pairs.

109


Typical High Layer Count Board-Layer 1 -- - -- - -- - -- - -- Devices, Short Signals, GroundLayer 2 -------------------- Ground PlaneLayer 3 -- - -- - -- - -- - -- Signals, Ground PourLayer 4 -------------------- Ground PlaneLayer 5 -- - -- - -- - -- - -- Signals, Ground PourLayer 6 -------------------- Ground PlaneLayer 7 -------------------- Power Plane or Power RoutesLayer 8 -------------------- Ground PlaneLayer 9 -- - -- - -- - -- - -- Signals, Ground PourLayer 10 -------------------- Ground PlaneLayer 11 -- - -- - -- - -- - -- Signals, Ground PourLayer 12 -------------------- Ground Plane

110


When RF Stages Located on Opposite Sides of aBoard, Blind Vias May Be Needed in Each Stageto Effectively Create Back-to-Back Boards-Example-

Layer 1 -- - -- - -- - -- - -- Devices, Short Signals, GroundLayer 2 -------------------- Ground PlaneLayer 3 -- - -- - -- - -- - -- Signals, Power, Ground PourLayer 4 -------------------- Ground PlaneLayer 5 -------------------- Ground PlaneLayer 6 -- - -- - -- - -- - -- Signals, Power, Ground PourLayer 7 -------------------- Ground PlaneLayer 8 -- - -- - -- - -- - -- Devices, Short Signals, Ground

111

RF / Microwave Design -Signal Attenuation

Increases or Decreases Pulse Amplitude -1)Reflections (Return Loss / VSWR - Critical).2) Signal Cross Talk (Critical in RF).3)Reference Voltage Accuracy (Critical in RF).4) Power Bus Noise (Minimal- Filtered).5) Ground/Vcc Bounce (Minimal in RF).6) Skin Effect (Resistive Loss in Conductor).7) Loss Tangent (Property of PCB Dielectric).

112

RF / Microwave Design -Skin Effect

Increases Resistive Signal Loss (Adds Heat).

Losses Increase with Increased Frequency.

Amplitude Loss in Analog Circuits.

Most effected by Line Width and Length.

Can be a problem above 10’s of MHz inAnalog circuits.

113


effAREALengthR •

=ρ ρ = 6.787 x 10-7 ohm-in

ρ = 1.724 x 10-5 ohm-mm

( ) SDtwAREAeff •+= 2

fSD 6.2

=

w - Trace Widtht - Trace Thickness

SD - Skin Depth in Inchesf - Frequency in Hertz

fSD 66

=SD - Skin Depth in mmf - Frequency in Hertz

114


“R” from equation is ONLY Accurate for CenteredStripline configuration.“R” of all other Transmission Line configurationsmust be adjusted due to ‘Proximity Effect’.

Microstrip (50 to 75 ohm) - Multiply “R” by 1.70Embedded Microstrip (50-75) - Multiply “R” by 1.85Offset Stripline -

– Adjust “R” based on Factor Determined by Percent of Offsetfrom Center (OR)

– Adjust Percent of Attenuation of Signal based on Percent ofCoupling to Nearest Plane.

115


Attenuation of the Signal is a Function of ‘SkinEffect’ Resistance and Current in the TransmissionLine -

( ) IRvoltsAtten •=

LOADED

DRIVER

ZoVI =Where -

( )LOADEDZo

dBRdBAtten 32 •=

116

RF / Microwave Design -Loss Tangent (tan(δ ))

Loss of Signal into PCB Material (Increases Heat).Function of Molecular Structure of PCB Material.Losses Increase with Increased Frequency.Amplitude Loss in Analog Circuits.Worse in FR4 (Alternative Materials available).Material Selection- Weigh Performance and Price.Can be problem above 10’s of MHz in Analog ckt.

117

RF / Microwave Design - Loss Tangent (tan(δ ))

The amount a signal is attenuated from Loss Tangentcan be determined with the equation -

efff εδα ⋅⋅= )tan(3.2Where : α = Attenuation in dB / Inch.

f = Frequency in GHz.tan(δ) = Loss Tangent of Material.εeff = Effective Relative Er of Material.

(Er for Stripline)

118

- Signal Attenuation -

119

RF / Microwave Design - Shields and Shielding

Use a Metal Can, Grounded Shield when -Circuits are so Close Together that Noise CouplingNaturally Occurs.EMI is Extreme and Cannot be Contained.Circuit is So Sensitive that Normal, Ambient EMILevels affect Performance.

Problems! Shields -Use up Valuable Board Space.Are Expensive.Make Trouble Shooting and Repair Very Difficult.

120


Shield Cost -Least Expensive is Off-the-Shelf.Next Lowest Cost is Photo Etched.

Resolve Trouble Shooting / Repair Issues-Tabs Every 1/20th λ instead of ContinuousConnection to Ground on Circuit Board.

121


Traces Running out of Shielded Area to beRouted on Inner Layers, if possible.When Routing from Shield Area on SameLayer as Shield, have a Minimum Openingin Shield Side Wall.Cable Exiting Shield Must have 360 degreeAttachment of Cable Shield to Metal Can.Avoid Other Openings in Shields.

122


Open Soldermask Under Shield Edgesenough to allow Good Solder Attachmentof Shield Walls to Ground Plane.Alternative to Soldered Shield- Make CaseLid with Fins/Vanes Long enough to ReachPCB Surface and Contact Ground to CreateCompartments inside Enclosure.Equations for Shielding Effectiveness inWadell, Pages 117 -123.

123

Example of Micro-strip PC Board.

RF / Microwave Design -Example PC Boards

124


TypicalMulti-LayerMicro-wave PCBoard.

125Typical Application- Cell Phone.


126

RF / Microwave Design - PC Board Materials

Don’t Use FR4 in High Power Circuits orBroad Band Applications (Loss & Er).Dielectric Loss (Loss Tangent) causes asmuch as 1/2 Signal Loss in 3” Trace Runover Thick Dielectrics in FR4 at 12 GHz.Resistive Loss, Especially Skin Effect, Canbe High at Frequencies Above 500 MHz. InVery Sensitive Circuits, even Small LossesCan Create Big Problems (Wadell- pg 25).

127

RF / Microwave Design -Board Fab and Assembly

Teflon Based Materials used in Many RF/Microwave Circuits Require Fab Houseswhich Specialize in Such Materials.Items Like Tetra-Etch are Essential in theFabrication of Plated Holes.Items such as Large Ground Planes, FloodedGround Pins of Parts and Multiple Vias On orNear Component Pads force Very SpecialAttention to Solder Profiles During Assembly.

128

RF / Microwave Design -Bibliography

Transmission Line Design Handbook. Brian C. Wadell, 1991. (ISBN #0-89006-436-9)RF & Microwave Design Techniques for PCBs.Proceedings, PCB Design Conference West, 2000.Lawrence M. Burns.Partitioning for RF Design. Printed Circuit DesignMagazine, April, 2000. Andy Kowalewski -Phillips Corporation.Microstrip Lines and Slotlines. Gupta, Garg, Bahland Bhartia, 1996. (ISBN # 0-89006-766-X)

rf / microwave pc board design and layout - jefferson · pdf file1 rf / microwave pc board...

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