Design Considerations and Tradeoffs for Microstrip ... · Design Considerations and Tradeoffs for...

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Presented by: John Coonrod

Design Considerations and Tradeoffs for Microstrip, Coplanar and Stripline Structures at Millimeter-wave Frequencies

Webinar

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Design Considerations and Tradeoffs for Microstrip, Coplanar

and Stripline Structures at Millimeter-Wave Frequencies

Agenda:

• Elementary transmission line theory• General theory• Millimeter-wave concerns

• Basic structure overview• Microstrip• Coplanar• Stripline

• Structure comparisons across a wide range of frequencies

• PCB variables to consider that affect RF performance (e.g. etching tolerance and plated finish)

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Elementary transmission line theory

General theoryMillimeter-wave concerns

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Elementary transmission line theory, general theory

Waves

Microwave and millimeter-wave engineers frequently refer to “waves” and their properties

Plane wave: wave propagation direction is perpendicular to the forces that create it

Electric field is perpendicular to Magnetic field and also is perpendicular to wave direction

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Wavelength (λ) is the physical length from one point of a wave to the same point on the

next wave

Long wavelength = low frequency

Short wavelength = more waves in the same time frame

Amplitude is the height of the wave and often related to power

High electric field = High magnetic field = High amplitude = High power

Waves


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Transverse ElectroMagnetic (TEM) wave

Electric field varies in z axis

Magnetic field varies in x axis

Wave propagation is in y axis

TEM wave propagation is most common in PCB technology, but there are other waves

Waves


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Waves

Other wave propagation modes are:

TE (transverse-Electric) or H wave; magnetic field travels along with wave

TM (transverse-Magnetic) or E wave; electric field travels along with wave

TEM or quasi TEM waves are typically the intended wave for a transmission line

Some PCB design scenarios will have problems with “modes” or “moding”

Moding issues are when the intended TEM wave is interfered with another wave mode

such as TE or TM modes; this is a spurious parasitic wave or unwanted wave


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Properties

Relative permittivity (εr) or dielectric constant (Dk):The property of material which alters the electric field in the waveVery important property for microwave PCB designMaterials used in PCB technology generally have Dk from 2 to 10The imaginary component of complex permittivity is Df (dissipation factor)Df is the amount of dielectric loss the material imparts on the wave

Relative permeability (μr)The property of material which alters the magnetic field This property is rarely used in microwave PCB applicationsMost PCB materials have μr = 1Some plated finishes used on PCB’s have ferromagnetic properties (μr >> 1)Ferromagnetic issues can cause more conductor loss


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Circuit with low Dk Circuit with high Dk

Dk effect on wavelength


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Conductivity (σ): Copper is typically the conductor for PCB’sMost plating finishes in PCB technology have lower conductivity than copperLower conductivity causes:

more conductor lossdeeper skin depth in the conductor

A copper surface which is rough will cause more conductor losses than smooth

Skin depth is how deep the current density will be in the conductor

At DC (0 Hz) the current will used the entire conductor

At a higher frequency the current will use the “skin” of the conductor

skin depthμ permeabilityσ conductivity


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• There are many kinds of transmission lines

• Wires

• Cables

• Printed circuit boards (PCB)


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• Example of a transmission line with 3 dB insertion loss

• 3 dB is a loss of half of the power

• The load receives half of the power that the generator sent


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2-port system, transmission line

Signal Flow Diagram

Port 1 Port 2

S parameters are an easy way to analyze loss

S21 is insertion lossS21 is the energy at port 2 that came from port 1

S11 is return loss or loss due to reflections at port 1S11 is the energy received at port 1 that came from port 1


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Example of wideband insertion loss curve

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αT is total insertion lossαC is conductor lossαD is dielectric lossαR is radiation lossαL is leakage loss

Dielectric loss (αD) is mostly due to the substrate, prepreg or soldermask

Conductor loss (αC) is due to several issues related to the conductor of the circuit

Radiation loss (αR) is due to many issues related to energy radiating off of the circuit

Leakage loss (αL) is mostly due to electrical leakage, through the dielectric and between conductor layers


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• The dominate loss component can be different, for the different circuit thicknesses

• Three circuit sets made from same material but different thicknesses

• Circuit material used was RO4350B™ laminate


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frequency radiation loss

• There are many variables regarding radiation loss

• Radiation loss is:

• Frequency dependent

• Circuit thickness dependent

• Dielectric constant (Dk) dependent

• Radiation loss can vary magnitude due to:

• Circuit configuration (microstrip, coplanar, stripline)

• Signal launch

• Spurious wave mode propagation

• Impedance transitions and discontinuities

thickness radiation loss

Dk radiation loss

Elementary transmission line theory, millimeter-wave concerns

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A thinner laminate is necessary for minimizing spurious modes at mmWave frequencies

Thin laminates are better for reducing radiation losses

Radiation losses at mmWave (millimeter-wave) frequencies can be a major concern

However a drawback of the thinner laminate is more conductor losses

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Resonances can set up between the signal conductor and the ground plane

Resonances can set up between opposite edges of the signal conductor

• When W is ½ or ¼ wavelength, resonances will occur and a resonance can generate its’ own EM wave

• The resonance generated EM wave will be a spurious wave that can interfere with the intended quasi-TEM wave; a good design limit is no feature > 1/8 wavelength

W

W

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Thickness comparisons, microstrip insertion loss; thicker laminate has more issues at higher frequencies

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Thickness can be a concern, but typically conductor width is more significant

This example: conductor width is 0.036”

1/4 wavelength (λ) is 0.036” at 46.5 GHz

1/8λ is 0.036” at 23.8 GHz

The insertion loss curves after 1/8λ has increasing amounts of noise

1/8λ

1/4λ

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Signal launch

• Signal launch is extremely critical to get the energy from one interconnect (coaxial cable) to another interconnect (transmission line PCB)

• Example using a microstrip transmission line

• Microstrip is a 2 copper layer PCB having a signal layer and ground layer

• The transition from coax to microstrip can be plagued with mismatch issues

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• Signal launch for high frequency RF PCB is the transition from one electric field orientation to another

• Electric fields in the coaxial domain of the connector are cylindrical• Electric fields in the PCB are planar

• Signal launch is also a wave propagation mode transition

• The coaxial connector is a pure TEM mode• A microstrip or grounded coplanar waveguide PCB will have a quasi-TEM mode

• Additionally, there is a change in field distribution and field line length in the signal launch area

• The Effective-Dk of the PCB is typically much different than the Dk of the coax

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• The coaxial medium uses a TEM wave propagation mode where the phase velocity doesn’t change with frequency (no transmission line dispersion)

• The microstrip transmission line circuit uses a quasi-TEM mode which does change phase velocity with frequency

• Thinner circuit has less dispersion; using 10mil RO4350B laminate it would be 0.97 @ 50 GHz

• Using high Dk materials and the phase velocity curve shown here will be much more dramatic; using 25mil RT/duroid® 6010.2LM material, normalized Vp would be 0.87 @ 50 GHz

• Example of 2.4mm coaxial cable and a microstrip transmission line circuit using 30mil RO4350B™ 5E laminate

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Basic structure overview

MicrostripCoplanarStripline

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Basic structure overview, Microstrip

Most common transmission line used in the microwave PCB industry

It is simple, cheap to construct, good reliability and easy for assembly

Wave propagation: Quasi - TEM mode (dominate wave propagation mode)

Due to the wave using air and substrate there is an “effective Dk”

The effective Dk is the Dk which the wave experiences (air+substrate)

Since the wave will have different phase velocity in air than the substrate:

the wave is not a pure TEM wave, but a quasi-TEM wave

there will be some dispersion (wave property changes with frequency)

microstrip SubstrateSignal layerGround layer

Cross-sectional view

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Dielectric loss:

mostly due to dissipation factor of the substrate

soldermask will typically increase dielectric loss

Conductor loss and is due to several issues:

skin effect (frequency dependent), surface roughness[1], plated finish

surface roughness will also affect the wave propagation constant[1]

The copper surface roughness; rougher copper will cause more conductor loss and

slows the phase velocity. A slower phase velocity is perceived as a higher Dk

RO4003CTM

Exaggerated surface roughness example

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Radiation loss:

energy radiated and lost from the circuit

microstrip is more susceptible to radiation loss than other RF structures

frequency dependent (higher frequency has more radiation loss)

substrate thickness dependent (thinner has less radiation loss)

Dk dependent (higher Dk has less radiation loss)

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Basic structure overview, GCPW

There are several types of coplanar circuits

Mostly used at microwave frequencies is the grounded coplanar waveguide (GCPW) or the conductor back coplanar waveguide (CBCPW)

GCPW circuits need PTH (plated through hole) via’s to connect the ground planes on the top layer (coplanar layer) to the bottom ground plane

Via hole placement is critical for obtaining the desired impedance and loss performance

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GCPW circuits have an effective Dk like microstrip

Dispersion is much less for GCPW than microstrip

Radiation losses are significantly better with GCPW than microstrip

Less dispersion and radiation loss: capable of higher frequency ranges

Dominate wave mode propagation is quasi-TEM

Suppressing spurious wave propagation modes is significantly more efficient with GCPW as compared to microstrip

Signal launch issues are significantly better for GCPW than microstrip

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Losses and fields:

Dielectric loss:Mostly due to dissipation factor of the substrateSoldermask will have more effect on dielectric loss than on microstrip

Conductor loss:Same issues as microstripOverall there are more conductor loss for GCPW than microstripConductor losses due to finish plating are worse for GCPW

Radiation loss:When designed properly these can be extremely small or negligible

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Via location and pitch can be critical to RF performance for GCPW

If the PTH via is far from the edge of the coplanar ground plane (left picture), there is more parasitic parallel plate inductances which increases impedance and conductor loss

If the PTH via has a length axis pitch of ¼ wavelength or larger then resonances can occur on the coplanar plane and spurious wave mode interference can be present

Distance from edge of via to coplanar ground edge

Distance from edge of via to coplanar ground edge

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Basic structure overview, Stripline

Probably the second most common

transmission line used in the PCB industry

More complex to fabricate, moderate in cost, moderate reliability and more difficult for

assembly as compared to microstrip or GCPW

Has the capability for extremely wideband (wide frequency range) applications

If all substrate material has the same Dk:

will have a true TEM wave mode propagation

extremely little or no dispersion

When designed correctly, there will be not radiation loss

Balance stripline

Unbalance stripline

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Losses and fields:

Dielectric loss:Mostly due to dissipation factor (Df) of the substrateCan be more difficult to correct evaluate when using prepreg and cores with different Df

Conductor loss:Skin effects, copper surface roughnessConductor losses due to copper surface roughness is much more difficult to model for

stripline due to 4 copper-substrate interfaces which often have different roughness

Generally a 50ohm stripline can have higher loss than microstrip and some GCPW, mostly due to conductor loss, however there are exceptions.

Magneticfields

Electricfields

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Via placement can be critical for stripline

Via pitch should ideally be a distance less than 1/8 the wavelength of the highest operating frequency (same as GCPW)

Via distance (3X) from the signal shown above is a general rule to avoid coplanar effects

Signal launch is very problematic for striplineThe signal launch via can have increased inductance or capacitance depending on designThe launch via can have a stub which can cause radiation and reflectionsThe design of the signal launch via has stepped impedance changes as the via goes down

through different material layers

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Multilayer PCB Example of signal launch issues

Effects of the signal path from the connector to the signal plane of the circuit:

Transition from connector to circuit has an air-substrate Dk difference

Dk difference causes some reflection

When the signal via diameter is less than the connector pin diameter the via will be seen as inductive

The connector-circuit transition copper design can be coplanar, with a smaller space adding capacitance to offset the inductive signal via

Moving down the signal via: the impedance will increase in the prepreg area, the GCPW effect on layer 2 will decrease impedance, then then impedance will increase in the area between layer 2 and layer 3, etc.

The high / low impedance transitions moving down the via need to be minimized to improve signal launch

Ideally, the signal launch via should be removed (back drilled) from bottom of the signal plane to bottom of the circuit

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Structure comparisons across a wide range of frequencies

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Current density (red) comparisons of microstrip and GCPW circuits

Copper surface roughness of the laminate has more impact on microstrip than GCPW

Changing from a rough copper to a smooth copper on GCPW has less significant improvement in conductor loss as on microstrip

Tightly coupled GCPW is less impacted by copper surface roughness than loosely coupled GCPW

Current density uses less of the roughened copper surface on a the GCPW than the microstrip

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Tightly and loosely coupled GCPW with two different copper surface roughness

Copper surface roughness difference is 2.3 microns RMS and 2.9 microns RMS

The coplanar conductor width (w) is 18mils and 21mils. The coplanar space (s) 12mils and 6 mils

Loosely coupled GCPW is more impacted by copper surface roughness difference

Tightly coupled GCPW is w18s6Loosely coupled GCPW is w21s12

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Microstrip insertion loss comparing very smooth copper (rolled copper) to rough copper (ED)

Rolled copper has surface roughness average of 0.35 microns RMS

This particular ED copper has average surface roughness of 2.0 microns RMS

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Unwrapped phase angle vs. Frequency

This shows the difference between phase angle using smooth copper and ED copper, while using the same lot of substrate There is a 61° phase angle

difference at 77 GHz

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PCB variables to consider that affect RF performance (e.g. etching tolerance and plated finish)

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PCB variables to consider that affect RF performance

Material and circuit fabrication variables which impact impedance

The most significant variables for microstrip impedance differences are:

1. Substrate thickness2. Conductor width3. Copper thickness4. Dk

Thinner circuits are more influenced by conductor differences

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The most significant variables at mmWavefrequencies for phase angle differences are:

1. Copper thickness2. Dk3. Conductor width4. Substrate thickness

• Thinner circuits are typically used at mmWave frequencies

• Phase angle variation can be critical to many mmWave applications

• Radar sensors at 77 GHz are especially sensitive to phase angle differences

Material and circuit fabrication variables which impact phase angle

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• A study was done using the same sheet of material 2 different GCPW circuit designs• Tightly coupled GCPW; small space between coplanar Ground-Signal-Ground• Loosely coupled GCPW; wide space

• Additionally, circuits were purposely made to have significantly different copper thickness• Thin copper plated• Thick copper plated

Tightly coupled GCPW with thin copper Tightly coupled GCPW with thick copper

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• Wider conductor has lower loss• Thick copper increases height of conductor sidewalls and more air is used (lower loss)

All circuits evaluated in this study used 10mil thick RO4350BTM laminate with standard ½ oz. ED copper

Tightly coupled GCPW is w18s6 which means 18mil conductor with 6mil coplanar space

Graph is from article on Microstrip vs. Coplanar[2] performance

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• RF testing procedure for determining Effective Dielectric Constant

• Effective dielectric constant (Eff_εr) is the dielectric constant which the circuit realizes and is the combination of substrate and air

• Tightly and loosely coupled GCPW will have fields which use air used more or less and this will causes differences in constant Eff_εr even when using the same substrate

• Two circuits are measured for phase angle (Φ) and the phase response formula adjusted for two circuits which are identical except for physical length (ΔL)

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• The tightly coupled circuits will have concentrated electric fields in air (air is lowest dielectric constant)

• Thicker copper will cause the conductor walls to be taller and more air is used

• The lowest Eff_εr circuit has thick copper and is tightly coupled

• Graph is from article on Microstrip vs. Coplanar[2]

performance

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• Most final plated finish is less conductive than copper

• Due to this fact, when most final plated finish is added to copper the conductor loss will increase, which increases the insertion loss

• The exception is silver

• Typically the silver used in the PCB industry is applied very thin so the skin effects benefit of silver may not be obvious unless at very high frequencies

Conductivity of pure metals

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Microstrip, final plated finish impacts the conductor loss due to high current density at the edge of the conductor

Grounded coplanar waveguide (GCPW) has fields and current densities using 4 edges of the ground-signal-ground conductor

Since GCPW has 4 edges using the plated finish, it will have more conductor loss (and insertion loss) due to the finish than microstrip

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The increased loss due to the final plated finish, depends on the thickness of the circuit

A thinner circuit will be dominated by conductor loss more than a thick circuit

When final plated finish is added to the copper it adds to the conductor loss

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Plated finishes have impact in frequency region where skin depth of the composite metal is changing quickly with a change in frequency

At low microwave frequencies the impact of plated finish is in order of plated finish composite conductivity

When this same study is done on thicker substrate the Dk differences are reduced

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References

[1] J. W. Reynolds, P. A. LaFrance, J. C. Rautio, A. F. Horn III, “Effect of conductor profile on the insertion loss, propagation constant, and dispersion in thin high frequency transmission lines,” DesignCon 2010.

[2] John Coonrod, “Managing Circuit Materials at mmWave Frequencies”, Microwave Journal, Vol 58. No. 7, July 2015

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