Introduction to planar transmission lines and MMICs

04/17/20231 Lecture-I

04/17/2023Lecture-I2

Contents Planar transmission lines- history, need and

usage Types of planar transmission lines

Microstrip Strip line Slot line Coplanar waveguide Fin lines

MMICs Substrates Connectors

Summary

04/17/2023Lecture-I3

Planar transmission lines

HistoryWW II-Integration of antennas into PCB boardsSatellite equipments – light weight, less expensive,

highly reliable components required. Need

Reduction of parasitic effects of components at higher operating frequencies.

Lumped to distributed components migration. Usage

Warfare- Satellite applications, air borne applications like missiles, radio proximity fuse.

Commercial- cell phones, USB dongles, wireless routers.

04/17/2023Lecture-I4

Types of planar transmission lines - Microstrip

This line consists of a single dielectric substrate with ground plane on one side and a strip on the other face.

Compact, simple PCB fabrication techniques can be used.

Only Quasi –TEM modes propagate. Contributions of Harold Wheeler of ITI lab.

W

Top View

Bottom View

04/17/2023Lecture-I5

Typical points Power handling capacity is upto few Watts. Used in CPUs as interconnects. Fringing fields exist and hence vulnerable to cross talk. The same can be turned advantageous in coupled lines. Popularly used MMIC line. No series stub connections possible which is a

disadvantage

04/17/2023Lecture-I6

Electrical characteristics Characteristic impedance, Power and the

width of the line are related directly. Length of the line and εeff depends on

frequency of operation.

0 50 100 150 200 250 3000

2

4

6

8

10

12

14

16

18

20

X: 50.03Y: 1.925

z0 in ohms

w/h

wheelers curve

2.8 3 3.2 3.4 3.6 3.8 4 4.20

2

4

6

8

10

12

14

16

18

20

eeff

w/h

Shown are curves for FR-4 with εr=4.4

04/17/2023Lecture-I7

Typical microstrip circuits

04/17/2023Lecture-I8

Strip line Strip line is a three –conductor transmission line

with ground plane on both the top and bottom surface of the substrate.

Supports TEM mode of propagation. Can be imagined as a planar co-axial cable.

Complex fabrication techniques required. Main reference: Strip line circuit design by Harlon

Howe.

Top View

Bottom View

04/17/2023Lecture-I9

Typical points on strip line

Homogeneous transmission line. Used in circuits which require high EMI and

temperature shielding. Power handling in the order of few watts. Series and shunt stub connections possible.

Field distribution in Stripline

04/17/2023Lecture-I10

Electrical characteristics

Equations for characteristic impedance and width are given below:

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

X: 0.455Y: 50.15

z0 in o

hm

s

w/h

strip line curve curve

Strip line impedance curve for FR-4. Since transmission line is homogeneous, no effective dielectric constant unlike microstrip.

04/17/2023Lecture-I11

Slot lines Slot line consists of a separation in a

conducting plane in which power flows. Non-TEM mode of propagation as there exists a

magnetic field component along the direction of propagation.

More dispersive unlike microstrip as impedance depends on frequency too.

w

Top view bottom view

04/17/2023Lecture-I12

Typical points on slot line

Very convenient for mounting of components as signal and ground are in same plane.

Easier fabrication unlike strip line. Since magnetic field component is present

along direction of propagation, this can be exploited while designing ferrite based components.

Waveguide Transitions are easier to perform in slot lines unlike microstrip.

Fields in slotline

04/17/2023Lecture-I13

Electrical characteristics

04/17/2023Lecture-I14

Coplanar waveguides All conductors are in the same plane with

centre strip being the signal and two strips on either side as ground plane.

Low dispersion and at low frequencies, mode of propagation is approximated as quasi TEM.

s

Top view bottom view

side view

w

04/17/2023Lecture-I15

Typical points on CPW Mounting of active or passive components both

series and shunt manner is possible. Simple fabrication technique is sufficient. Low loss and can support wide range of

impedance from as low as 10Ω to 300Ω.

04/17/2023Lecture-I16

Electrical characteristics The characteristic impedance of the CPW is

given as per the relation below:

Effective permittivity in substrate

5.01

)1(2log

5.0

1

)'1(2log

)'(

)(

2

2

withkk

k

withk

k

kkK

kK

04/17/2023Lecture-I17

Impedance curve for CPW

The curves are for GaAs substrate for εr=12.9

Reference: Coplanar waveguide circuits, components and systems, by Rainee Simons

04/17/2023Lecture-I18

Typical circuits using CPW line

04/17/2023Lecture-I19

Fin lines Fin lines can be imagined as planar

transmission lines inserted into waveguide(Shielded slot line).

Good contact between fins and waveguide not necessary.

Impedance, frequency of operation depends on the waveguide cut off frequency.

Introduced by Meier.

Structure of a fin line

04/17/2023Lecture-I20

Types of fin lines

Substrate

Metal

Printed fin line Unilateral Symmetric fin line

Bilateral Symmetric fin line

Bilateral Antipodal fin line

Strip fin line

04/17/2023Lecture-I21

Typical points and electrical characteristics

Produce a normalized shunt capacitance across the centre of the waveguide (explained in detail in ‘Foundations for microwave engineering’ by R.E.Collins).

Under resonance condition,

04/17/2023Lecture-I22

Other planar transmission lines Suspended microstrip line Suspended strip line Coupled strip line Coupled microstrip line Coupled slot line Conductor backed CPW Broad side coupled CPW/strip line Edge coupled CPW/strip line CPW with microshield Substrate integrated waveguides

04/17/2023Lecture-I23

MMIC boards

04/17/2023Lecture-I24

Traditional transmitter board

MMIC board

04/17/2023Lecture-I25

MMIC boards Substrates used: Rogers, textronics, Isola,

Arlon. FR-4 substrate is best avoided in case of

antenna applications. It has the ability to contain the fields within it and best suited for low frequency circuits.

Rogers substrates have low loss and is more rugged to operate at high frequencies under high temperatures and pressure.

04/17/2023Lecture-I26

•Coefficient of Thermal Expansion (CTE),•Thermal Coefficient of Dielectric Constant (TCDk),•Low modulus•Non-wettability •managing heat flow with its high thermal conductivity

Parameters of substrate

04/17/2023Lecture-I27

Criterion for selection of substrates Military: materials meet or are tested in accordance to military

specifications. (Mil-S-13949 now obsolete). Commercial: materials are designed for cost sensitive

applications (military grade materials can be used in commercial applications but pricing is higher).

Multilayer: mechanical properties are such that allow for reliable multilayer (MLB, other than basic stripline) constructions

Hybrid: mechanical properties are such that allow for reliable FR4 hybrid MLB constructions

Broad Band: dielectric constant allows for very high frequency/broadband use

Temp Cycling: electrical and mechanical properties allow for stable use in temperature varying environments

Surface Mount: mechanical properties allow for surface mount technology 

Miniaturize dielectric constant reduces sizes of circuit elements

04/17/2023Lecture-I28

RF connectors

04/17/2023Lecture-I29

Summary Types planar transmission lines – their

Structure Field distribution Z0 vs. w curves and eff vs. w curves power handling capacity and nature of connections

MMIC boards Typical receiver boards Selection of substrate for high frequency circuits

04/17/2023Lecture-I30

References Stripline circuit design by Harlan Howe Microstrip lines and Slotlines by K.C. Gupta, Ramesh

Garg and Prakash Bhartia. Coplanar waveguide circuits, systems and

components by Rainee Simons Analysis, Design and Application of Finlines by

Bharathi Bhatt and S.K. Koul Microwave Engineering by D.M. Pozar. Foundations of microwave Engineering by R. E. collins Microwave solid state circuit design by I. Bahl and P.

Bhartia Rogers corporation website

04/17/2023Lecture-I31

Exercises

1. Generate impedance vs width curves for microstrip and CPW lines for FR-4, RT duroid 5880, 5870 substrates.

2. Comfortable with the structure of all planar lines especially microstrip and CPW