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RF & Microwave PCB Design Tips about Line types and Impedance (Zo)

RF & Microwave PCB Design Tips about Line types and Impedance (Zo)

1.Waveguide-

--Uses Air as Transmisssion Medium and Side Walls of Tube as Return Path.

--Won’t Support Energy Propagation Below Cutoff Frequency.

--Works Best at Ultra High Frequencies with Millimeter Wavelengths.

--With an Air Dielectric, Signals Propagate at the Speed of Light.

--Very Low Loss due to Smooth Side Walls and the Air Dielectric.

--Ultra Low Loss with High Density, Ultra Smooth Coating on Walls.

--In Very High Power applications, Uses Solid Dielectric to Prevent Voltage Arcing.

 

2.Signal Traces Longer than Critical Length (1/16 λ in DK) Need Impedance Control to Prevent Return Loss due to Reflections.

 

3.Shorter Circuit Elements Don’t Require Impedance Control, but it Usually does NO Harm.

 

4.Don’t bother to Zo Control Short Lines if it Will create a Problem (ie- DFM).

5.Impedance (L/C)-

--Lower Er Materials Net Higher Impedance Traces and Faster Propagation Times per given Trace Width & Trace-to-Ground Separation.

--As Trace Width Increases, Trace Impedance Decreases (Thickness has Min Effect).

--As Trace Spacing from Ground Increases,Impedance Increases.

6.Transmission Line History -

--Two Coplanar Strips in 1936. Later Rolled Up to create Sealed Line.

--Coax Lines during WWII.

--Flat Stripline Using PCB Techniques right after WWII.

--First use of Microstrip Reported in 1949.







7.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.
 8.Coplanar Waveguide

*b’ should be less than λ/2 for best performance.

*Ground Must extend Greater than 5x‘b’ on either side of Trace ‘a’.

*Lower Loss Tangent than Microstrip (Signals Couple Mostly through Air).

*Higher Skin Effect Losses (Fields Concentrate on Edges of Trace and Grounds).

*May Need to Strap Grounds together on Either Side of 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.

*CPW Allows Variation of Trace Width, or Spacing-to-Ground or Dielectric Thickness to Control Zo.

*Zo of CPW Decreases as Dielectric Thickness Increases.

*CPW Produces Smaller Trace per given Zo than Microstrip.

9.CPW verses Microstrip


10.Coplanar Waveguide w/Ground


*To Avoid Microstrip Mode, h >b and Left & Right Ground Extend Away from ‘a’ by More than ‘b’.

*Zo of CPWG is Increased as Dielectric Thickness Increases. Opposite of CPW.

*If ‘ h’ is Large, CPW and CPWG Behave in Similar Fashion.

11.CPWG verses Microstrip


*Beyond Gaps shown above, CPWG like Microstrip

12.Edge Coupled CPW (CP Differential Pair)

*Gives an Extra Degree of Signal-to-Noise Isolation Over standard CPW. (w/o Plane, Fields are Large)
13.Edge Coupled CPWG (CP Diff Pair w/Grnd)


*Much Better Field Containment than Coupled CPW.Better yet in Edge Couple Stripline.
14.Slotline

*Acts Like Waveguide with Air DK.

(Equations in Wadell- Pages 156-160) 

15.Other Configurations

*3 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, .....

16.Zo Calculations

*Use 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 Zo Calculator (i.e.- POLAR Ltd.)

*Don’t use Equations or Calcs for Dig Layout that Don’t Comp for Coplanar Effects.

17.Tpd, Capacitance and Inductance Calcs


To know more:
Guidelines for RF & microwave PCB Design
The basic knowledge about RF Microwave PCB design  

Recommend reading list for RF/Microwave PCB design


 

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