Passive and active devices

Passive and active devices

1. PASSIVE AND ACTIVE MICROWAVE
DEVICES
Mrs.V.SrirengaNachiyar, AP/ECE
Ramco Institute of Technology
Academic Year: 2017-2018 (Odd)

2. Introduction
•Microwave engineering has a noticeable difference from the
conventional electronics engineering.

•The conventional low-frequency circuit analysis based on
Kirchhoff’s laws and voltage-current concepts

•It can’t apply to microwave circuits, because the propagation time
of signals from one point in a circuit to another point at micro
wave frequencies becomes comparable with time period of the
signals.

3.  Waveguide transmission systems are not always perfectly impedance
matched to their load devices.
 The standing waves that result from a mismatch cause a
 power loss,
 reduction in power-handling capability,
 increase in frequency sensitivity.
 Impedance-changing devices are therefore placed in the waveguide to
match the waveguide to the load.
 These devices are placed near the source of the standing waves.

TERMINATIONS

4. Matched Termination
•Terminators are used in coaxial lines, strip lines and waveguides to
absorb the incident power without appreciable reflection and
radiation

•It is equivalent to terminating the line in its characteristic
impedance

 A termination is a one-port device which absorbs all the
incident power, never radiate and reflect.

Its only purpose is to ABSORB all the incident energy without
causing standing waves.

The basic characteristics of terminations are:
•VSWR , Power handling capability , Frequency range

5. Matched Load
Types of Terminations
Matched Load

Variable Short circuit

 It provides termination and absorbs all the incident power. It is
also equivalent in terminating the line by its characteristic
impedance.

A simple form of matched load in a waveguide is a piece of
resistive card placed in guide parallel to the dielectric field.

The card must be long enough to absorb all the power.

The front end of the card is tapered, so that it presents no sudden
discontinuity to the signal.

6. MATCHED LOAD
 Any Lossy material which is not a conductor can be used as a matched
load.
 Lossy dielectrics
 Powered metal
 Carbon

TYPES OF MATCHED LOAD
 LOSSY WEDGE
 TAPERED RESISTIVE CARD

7. LOSSY WEDGE
LOSSY WEDGE
 In this termination, a slab of lossy material is inserted into the
guide. This lossy material absorbs all the incident power
 The plane of the wedge is placed perpendicular to the magnetic lines
of force.
 When H lines cut through the wedge, current flows in the wedge and
causes a power loss
 The loss is in the form of heat and very little energy is reached at
the waveguide. Hence, the reflection is minimum at the end.

8. LOSSY WEDGE

9. TAPERED RESISTIVE CARD
TAPERED RESISTIVE CARD
 The tapered high resistive card is inserted into the waveguide. By,
tapering the lossy material edge, the reflections are avoided.
It provides variable input VSWR of 1.01 or less

10. VARIABLE SHORT CIRCUIT
 In waveguide form, it consists of short circuit, usually a resistive
card. It reflects all the incident power.

 The sliding load motion controls the high degree of precision
through extremely accurate matching techniques.

Termination must be a length of ƛ/2 at the lowest frequency that
enables phase reversal.

Manually move the screw to lock the termination at any position.

Phase of the reflected wave is varied by changing the position of
short circuit. It is equivalent in changing the reactance of the
termination.

11. VARIABLE SHORT CIRCUIT
 High Precision laboratory measurements
 Field Measurements
 Production setups requiring low VSWR
 APPLICATIONS
 TYPES OF VARIABLE SHORT CIRCUIT TERMINATION
Adjustable short circuit
 Choke-type plunger
 Alternative choke-type plunger

12. ADJUSTBALE SHORT
CIRCUIT
 It is just a sliding block of copper.
 In a rectangular waveguide, the surface currents on the interior wall flow
up and down along the sidewalls and both across in the axial direction on
the broad walls.
The axial current must flow across the gap between the upper and lower
waveguide walls and vertically across the front face of the sliding block.
 The current flow along the sidewalls only in the vertical direction and do
not need to cross the gap between the waveguide walls and front face of the
plunger.
 The position of the sliding plunger block is varied by means of a
micrometer drive.

13. ADJUSTBALE SHORT CIRCUIT
Poor Performance
 Erratic contact between the sliding block and waveguide walls
 Waveguide walls causes short circuit
 Provides some power leakage and reflection
 DISADVANTAGES

14. CHOKE TYPE PLUNGER
 It overcomes the drawback of variable short circuit
 It uses the impedance transformation properties of a quarter-
wave transformer
 In this type, assume load impedance Z
s = 0 and if Z
2 > Z
1, Z
s’ will
approximate a short circuit by a factor better than Z
s. This principle is
used in choke-type plungers.

15. Attenuators
•A device used to control the amount of microwave power transferred from one
point to another point on a microwave transmission is called microwave
attenuator.

•It is a passive 2-port device used to control power levels in a microwave
system by partially absorbing the transmitted signals

•Attenuators can be classified as fixed and variable attenuators

•Resistive films (dielectric glass slab coated with aquadag) are used in the
design of both fixed and variable attenuators

•Attenuators control the power flow in desired direction

16. Fixed attenuator
•It provides only fixed amount of attenuation
•It consists of dissipative element called pad
•Pad is placed in a waveguide
•Pad is placed in such a way that its plane is
parallel to the electric field. For this two thin metal rods are used.
•The pad is tapered, which provides a gradual transition from
waveguide medium to absorbing medium of pad.
•It also reduces reflection
The amount of power that a fixed attenuator can absorb depends on
•Strength of dielectric field
•Location of pad within waveguide
•Frequency of operation
•Area of pad and pad material used for power absorption

17. Variable Attenuator
•It provides continuous attenuation
•The amount of attenuation is controlled by depth of insertion of
absorbing plate inside the waveguide
•For this a knob and gear assembly is used
•Knob can be calibrated suitably
•The maximum attenuation will be offered when the pad extends all
the way across the guide

•Types of variable Attenuators
1. Rotary attenuator
2. Electronically controls
•PIN diode
•FET attenuator

18. Phase Shifter
•It is device which provides variable insertion phase in a
microwave signal path without altering the physical path length.
•Most phase shifters are two–port devices characterised by low
insertion loss and low VSWR.
•The phase of the microwave signal can be changed by inserting a
low loss dielectric material (or) a ferrite material in the way of
propagation path.
•It should be produce zero attenuation.

19. Phase Shifter
•Phase shifters should perfectly matched
to the input & output lines.
•Input & output signals are equal in magnitude
but opposite in phase.
•Phase shifter is a reciprocal device

20. Phase shifters can be
•Analog
•Digital


Analog
•Provides continually
variable phase shift
controlled by a controlled
voltage.

Advantages:
•Low cost
•Low loss

Digital
• Provides the differential phase shift
changed by only a few pre-
determinded
phase shift valued as 45
o
,

90
o
,180
o
&

etc.
Advantages:
•Noise immunity
•Uniform performance
•Easier to attenuate
•Ability to achieve flat phase over
wide
Bandwidth.
•Higher power handling capability &
linearity.

21. •Phase shifters can be controlled by
•Electrically
•Mechanically





Types of Phase shifters:-
–Dielectric phase shifter
–Ferrite phase shifter
Dielectric phase shifters :
•reciprocal phase shifters
•It can be realized by placing a lossless dielectric slab with in a
waveguide parallel to and at the position of maximum electric field.


Electrically:
•Realized with either PIN /
Varactor Diode.
Mechanically:
• It is analog in nature & it is just a
mechanically lengthened transmission
line.

22. Ferrite phase shifters:




•It is known that most of the ferrite materials have relative
dielectric constants in the range 9 to 16, and dielectric loss
tangents are normally small (0.001)

•Phase shift is a ferrite device is generated by magnetizing the
ferrite inside the waveguide by RF current such that the phase
constant in the propagating line alters due to change in
permeability with magnetization.



Ferrites are non metallic materials with resistivity nearly 10
14
times
greater than metals and relative permeability of the order of 1000

23. Applications of phase shifters :
–phased array antenna system
–Transmitter/Receiver module
–Phase discriminators
–Beam forming networks
–Power divides
–Power amplifiers

24. Rotary phase shifter
•It is an analog phase shifter. It consists of :
 One half wave plate (λ/2)
Two quarter wave plate (λ/4)
Rectangular to circular transition
Circular to rectangular transition

•First quarter wave plate converts TE
11 mode
(circularly polarized mode in input side)

•Second quarter wave plate converts circularly
polarized mode (TE
11 mode in output side)

25. Directional Couplers
•Directional coupler is a four port device
•It consists of primary guide 1-2 and a
secondary guide 3-4.
•It is made of two connected waveguides
•It couples power in an auxiliary waveguide
arm in one direction
•The waveguide 1-2 and 3-4 are identical
•Any one of them can be used as a primary and
other acts as auxiliary guide.

26. DIRECTIONAL COUPLERS

27. Contd…







•When all ports are terminated by characteristic impedance, there is
a free transmission of power without reflection between port 1 and
port 2

28. Contd…
•There is no coupling of power between port 1
and port 4 OR port 2 and port 3
•Two types of directional couplers:
 Forward directional coupler
 Backward directional coupler

29. Applications
•Used to:
 measure microwave parameters of microwave
devices
Sample the incoming 0microwave power in
transmission towers
Monitor power
It is used in impedance bridges & Balanced
amplifier circuits

30. TYPES OF DIRECTIONAL
COUPLERS
DIRECTIONAL
COUPLERS
•APERTURE COUPLED DIRECTIONAL COUPLERS

•BETHE-HOLE COUPLER
•TWO-HOLE COUPLER
•MULTI-HOLE COUPLER
•SCHWINGER REVERSED-PHASE COUPLER

•PLANAR TRANSMISSION LINE DIRECTIONAL COUPLERS

•MICROSTRIP COUPLED LINE DIRECTIONAL COUPLER
•BRANCH LINE COUPLER
•LANGE COUPLER
•HYBRID COUPLER

31. Hybrid junctions
•Waveguide tees are 3-port components.
•It is used in microwave technologies when power in a waveguide
need to be splitted (or) combined.

Tee—junction:
•In a microwave circuits a waveguide (or) coaxial wire with 3
independents ports is commonly referred to a as Tee junction.

•They are used to connect a branch (or) section of the waveguide in
serious (or) parallel with the main waveguide transmission line for
providing means of splitting and also of combining power is a
waveguide system.

32. E-Plane Tee
•It is also called series Tee.
•Axis of its side arm is parallel to
the electric field of main guide.
Here
• port 3 – E arm (or) side arm
•Port 1 & 2 –collinear arm
•If power fed into port 3 is equally divided into arms 1 and 2. The
o/p power appearing at Port 1 & Port 2 of collinear arm will be
equal in magnitude and 180
o
out of phase with each other.
•If power fed in port 1 & port 2, will appear at port 3 as difference
between port 1 & port 2. It also known as differences.
•The S-matrix for E—Plane T can be divided as follow

33. H-Plane Tee
•It is also called shunt Tee.
•Axis of its side arm is shunting
the electric field (or) parallel to the
H field of main guide.
Here
• port 3 – E arm (or) side arm
•Port 1 & 2 –collinear arm
•If i/p fed into port 3, the wave will
split equally into port 1 and port 2 in-phase
and equal in magnitude.
•If two input waves are fed into port 1
& port 2, the o/p wave at port 3 will be
In-phase and additive.

34. Magic Tee
•It is a 4 port device
•Combined version of H-plane tee
and E-plane tee junction
•Also called hybrid Tee

•It produces sum and differences of its 2 i/p signals simultaneously
•It consists of 4 arms
•One shunt arm
•One series arm
•Two side arms
•No coupling between shunt and series arm
•All four ports are perfectly matched

35. •If wave fed into port 3, is equally divided into port 1 and port 2
with 180
o
phase shift (E-arm)
•If wave is fed into port 4 is equally divided into port1 and port 2
with in-pahse (H-arm)
•If waves of equal magnitude are fed into port 1 and port 2, the
output at port 3 are subtractive and hence zero and total output
will appear additively at port 4. hence port3 is called difference
arm and port 4 is called sum arm
•All ports are perfectly matched, s
11=s
22=s
33=s
44=0
•No coupling between port 1 and port 2. s
12=s
21=0
•No coupling between port 3 and port 4. s
34=s
43=0

36. Limitations of Magic Tee
•Analysis is too complicated
•Reflection occur due to mismatch
•High power loss
•Reflection produces standing waves
•Low power handling ability

Applications of Magic Tee
•Measurement of impedance
•As duplexer
•As mixer
•As power combiner

37. Rat-Race ring coupler
•It is also called as Hybrid ring coupler
•Four port device
•Gives 180
o
phase shift between its two output
port
•Due its shape it is called as rat-race ring coupler
•Consists of an annular ring of power electrical
length to sustain standing waves to which four
arms are connected at proper intervals by means
of series / parallel junction
•Circumference of ring = 2πr =
3λ??????
2

38. Contd…
•In addition no waves entered into port 3. P1 &
P3 are uncoupled.
•The phase difference between two waves
propagating in clockwise & anticlockwise
direction is 180
o
; due to their path length
difference by
λ??????
2
. Hence signal is absent at
port 3

39. Power Dividers
•It is a passive network component
•It is used for power division and power combining
•It is a lossless network
•It divides an input power into a no. of smaller amount of power.
•It can provide either equal (or) unequal division
•A simplest power divider is a T-Junction  it is 3 port network
•4 port network  directional coupler and hybrids

40. Power divider and combiner/coupler
divider combiner
P
1
P
2= nP
1
P
3=(1-n)P
1
P
1
P
2
P
3=P
1+P
2
Divide into 4 output
Basic

41. S-parameter for power divider/coupler 











333231
232221
131211
SSS
SSS
SSS
S
Generally
For reciprocal and lossless network jiforSS
N
k
kjki 

0
1
* 1
1
*


N
k
ki
ki
SS 1
131211  SSS 1
232221  SSS 1
333231  SSS 0
*
2313
*
2212
*
2111  SSSSSS 0
*
3323
*
3222
*
3121  SSSSSS 0
*
3313
*
3212
*
3111  SSSSSS
Row 1x row 2
Row 2x row 3
Row 1x row 3

42. Continue
If all ports are matched properly , then S
ii= 0 











0
0
0
2313
2312
1312
SS
SS
SS
S
For Reciprocal network

43. Circulators
•It is a multiport non reciprocal device.
•The wave can travel from one port to the next immediate port in
one direction only
•(i.e) the wave can travel from n
th
port to (n+1)
th
port only in one
direction
•All 4 ports are matched and transmission of wave takes place in
cyclic order only
•An ideal circulator is perfectly lossless
•Classification based on 1. direction of power flow
2. no.of ports
1.direction of power flow 2. no.of ports
–clockwise 3-port
–Anticlockwise 4-port

45. Isolator
•It is a 2 port, non reciprocal device
•Isolator produce minimum attenuation in one direction and very
high attenuation in opposite direction. It is also called uniline.
•It is generally used to improve the frequency stability of
microwave generation.
•When isolator is inserted between source and load.
•Almost all the signal power is transmitted to the load and none of
the power reflects from the load is not feedback to the input ports.
•Isolators can be constructed by many ways. They can be made by
terminating port 3 & 4 of a Four-port circulator with matched
load.
•In other way, isolators can be made by inserting a ferrite rod along
the axis of a rectangular waveguide. Here the isolator is called as
Faraday rotation isolator.

46. Faraday Rotation Isolator







•The input resistive card is in y-z plane and output resistive card is
displaced 45
o
with respect to input card.
•DC magnetic field applied longitudinally to the ferrite rod,
rotates the wave plane of polarization by 45
o

•Degree of rotation depends on length and diameter of the rod and
applied DC Magnetic field.

47. Applications
•It is used in:
 matching networks
Tuning networks
When it is used in between a high power
source & load, it prevents possible reflections
from source. Hence, isolator maintains the
frequency STABILITY in the sources.

48. Impedance Matching Devices
•Design of impedance matching network is an important part of
Microwave Engineering.
•Microwave Engineering comprises of transmission lines and
devices each has their own characteristic impedances.
•In order to transfer maximum power to the subsequent circuits,
impedance matching is must, otherwise reflection from load will
occur and power will be wasted, reducing transmission efficiency.
•In a mismatch impedance reactive component should be cancelled
and real part should be transformed to Z
o.
•It is required to maximize the power transfer / minimize signal
reflection from the load.

49. Comparison between PN diode and
Schottky diode:
Sl.No. PN diode Schottky diode
1. No thermionic emission of
majority carriers
Thermionic emission of majority
carriers determines reverse
saturation current
2. Current magnitude: smaller Current magnitude: larger
3. Depletion layer exists Depletion layer does not exists
4. Low switching speed High switching speed
5. High junction capacitance Low
6. Low recovery time Faster
7. Low turn on & turn off time High
8. Low frequency application High frequency applications

50. Mixers
•Mixer is a three port device that uses a
non-linear (or) time varying element to
achieve Frequency conversion
•An ideal mixer produces an o/p consisting
of sum and difference frequencies of its
two i/p signals
•Practical RF and microwave mixers, is usually based on the non-
linearity provided by either a diode (or) transistor
•A nonlinear component can generate a wide variety of harmonics and
other products of i/p frequencies, so ―filtering‖ must be used to select
the ―desired‖ frequency components
•Modern microwave systems typically use several mixers and filters
to perform the functions of frequency up-conversion and down-
conversion between baseband signal frequencies and RF carrier
frequencies

51. References
•Reinhold Ludwig and Gene Bogdanov, “RF
Circuit Design: Theory and Applications”,
Pearson Education Inc., 2011
•Robert E Colin, “Foundations for Microwave
Engineering”, John Wiley & Sons Inc, 2005
•Annapurna Das and Sisir K Das,’ Microwave
Engineering”, Tata Mc Graw Hill Publishing
Company Ltd, New Delhi, 2005