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Recent Advances in Microwave
Filters
Professor Ian Hunter* CEng FIEEE FIET
Institute of Microwaves and Photonics
School of Electronic and Electrical Engineering
University of Leeds
*[email protected]
Copyright © 2008
1
Outline
A. Frequency Selective Limiters
B. Reconfigurable Filters
C. Filter Techniques Applied to Antenna Design
D. Tunable Bandstop Filters
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A. Frequency Selective Limiter
• Very desirable at the front end of wideband receivers for
electronic warfare systems.
• Benefits are to improve the receiver sensitivity, reduce signal’s
dynamic range and increase probability of interception.
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Nonlinear Bandstop Filters
• Integration of a bandstop resonator and a schottky diode.
• One module works for a particular frequency. They are
cascaded for broadband operation.
• This is a novel technique for a device called “Frequency
Selective Limiter”
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FSLs using Nonlinear Bandstop Filters
• Simple nonlinear bandstop filters have relatively low Q.
• Passive enhancement of resonator Q using reflection mode circuit.
1
S11  (1  2 )
2
j
S12  (1  2 )
2
1
1/ 2
 /4
2
Sub-network
 /4
1
1
1
LC
R
1/ 2
3-dB Quadrature
Hybrid
2
LC
R
At resonance with R=1:
Г 1 = Г 2= Г
S11= 0 (all ω)
S12= 0 (at ω0)
S12= jГ (other ω)
Sub-network
• Input and output ports are matched.
• Bandstop response at ω0 when R=1 (matching condition).
• Allpass response at other ω and when R≠ l (mismatch).
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First order Nonlinear Bandstop Filters (1)
• A first order sub-network consists of one resonator and one
schottky diode.
• An impedance inverter (K) is used for impedance matching.
• Lumped to microstrip circuit transformation using reactance
slope parameter .
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Multi-resonator Nonlinear Bandstop Filters (1)
• The lowpass prototype will be subjected to frequency
transformation and lumped to microstrip transformation.
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Multi-resonator Nonlinear Bandstop Filters (2)
Transfer response
Reflection response
Rise Time
Fall Time
Intermodulation
Third order prototype
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Multi-channel Nonlinear Bandstop Filters (2)
CH 1
CH 2
CH 3
Prototype of a cascaded
nonlinear bandstop filter
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B. Reconfigurable Filters
Switched Delay Line Resonator
SDL Tunable Filter
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Resonant Tuning Methodology
V  e j1  e j1
Vp 

2
2
where    2  1



 Vp 
 
cos  
2
2
V
2

 
S21  j   cos 2   and let  
o
2
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Quasi-elliptic Filter Response
Cascade of 2 resonator sections gives,
  2 cos  
S 21  j   cos 2 
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2
13
Bandwidth and Stopband Rejection Control Element
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Experimental Results
Microstrip Prototype
Cascaded 3-section
Wilkinson Power
Divider
State-1 filter
response:
- D1 (Forward Bias)
State-2 filter
response:
- D2 (Forward Bias)
Scaled impedance
U.E Z=4
 Rogers Duroid 6010
 Switch element : Infineon BAR50-02V in SC79
package.
Copyright © 2008
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Experimental Results
S parameter Measurements


2-state tuning from 1.06GHz to 1.51GHz ~35% tuning
bandwidth.
Maximum passband loss of 1.7dB.
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Experimental Results
IP3 Measurements
Pin=0dBm (SDL_Filter1)

Pin=5dBm (SDL_Filter1)
Δf, kHz
3rd order intermodulation
power, dBm
-80
-75

-70
-65
-60
Input power sweep - 0,
5 dBm
Frequency separation
- 50kHz to 1000kHz
-55
-50 0
200
400
600
800
1000
-45
Pin=0dBm (SDL_Filter1)

IP3>32dBm
Pin=5dBm (SDL_Filter1)
Δf, kHz
3rd order intermodulation
power, dBm
-80
State 1 centered at 1.06GHz
-75
-70
State 2 centered at 1.51GHz
-65
-60
-55
0
200
400
600
800
1000
-50
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Narrowband Quasi-Elliptic Filter
Parallel-coupled SDL Filter
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Narrowband Quasi-Elliptic Filter
Parallel-coupled SDL Filter
The overall transfer function of the SDL network may be derived as,


S 21   K cos2  cos2 2 
2
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Experimental Results
Tunable Filter with Constant Bandwidth
Cascaded 3-section
Wilkinson Power Divider
Impedance scaling
 Rogers Duroid 6010
 Switch element : Infineon BAR50-02V in SC79 package.
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Copyright © 2008
Experimental Results
S parameter Measurements

Three switched states with center
frequency at 1, 1.3 and 1.6 GHz

Passband loss = 2.5- 4 dB

45% tuning bandwidth

Constant bandwidth (50 MHz)
tuning across the entire tuning
range.

Immune from power saturation
effect.
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IP3 Measurements

Input power sweep 0, 5 dBm
 Frequency separation
- 50kHz to 1000kHz

IP3>20dBm
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Copyright © 2008
C. Filter Techniques Applied to
Antenna Design
Copyright © 2008
Microstrip Antennas
• A microstrip device is formed of two parallel conductors
separated by a thin dielectric substrate (thickness<< λ) and
the lower conductor acting as a ground plane.
• Radiation relatively is broad beam broadside to the plane
of the substrates, this type of antenna can be fabricated
using photolithographic technique.
Copyright © 2008
Microstrip Antennas


Advantages
–
Easy and low costs fabrications.
–
Strong mechanical structure and less weight.
–
Patch antennas are probably the most likely option to be integrated in
microwave systems for their planner structure.
Disadvantages
–
Low efficiency.
–
Poor polarisation purity.
–
Narrow bandwidth.
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Microstrip Antennas
–
Dominant Mode is the TM110
–
Microstrip Antennas can be modelled as a high quality cavity
resonator, that treat the side walls as Perfectly Magnetic
Conductors while the top and bottom walls of the cavity are
Perfectly Conducting Electric Walls.
–
Impedance is maximum at the edges and zero at the centre.
–
The radiation is the result of leakage from the cylindrical cavity, via
the cavity laterals.
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Filter Techniques Applied to Antenna Design

Dualmode Antenna;
–
Patch antennas, both square and circular, may support two
orthogonal resonant modes.
–
Each of the resonant modes radiates and can be represented by a
resonant circuit terminated by a resistor.
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Filter Techniques Applied to Antenna Design

Yin
Theory
–
The fundamental resonant mode of this structure is the dual
degenerate TM110 mode, where each of the resonant modes
radiates and can be represented by a resonant circuit terminated
in a resistor.
–
The equivalent circuit of dual mode patch antenna is a second
ordered ladder network where each resonant circuit is
terminated in a resistor R, representing its own internal losses
and radiation.
J01
1
1
J12
1
1
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Filter Techniques Applied to Antenna Design

Theory
–
The return loss of the equivalent circuit of dual mode patch antenna is
p 2  (2  J 01 ) p  1  J12  J 01
2
2
2
p 2  2  J 01 p  1  J12  J 01
2
S11(p) =

2
2

–
For 6dB return loss bandwidth, it can be shown using circuits theory that
the bandwidth can be optimium if J01=(14/3)1/2 and J12=(13)1/2.
–
The equivalent circuit aids in reaching a successful design for the
antenna.
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Filter Techniques Applied to Antenna Design

Circuit, EM Simulation and Measured Results [2]
0
dB(S(1,1))
dB(S(2,2))
Return &
Insertion
Loss dB
-2
C
J01
-4
R
Optimised single mode
(J 01=(5/3)1/2)
Dual Mode (J01=(14/3)1/2 and
J12=(13)1/2).
R=C=1
-6
-8
C
J01
-10
R
J12
C
R
-12
-14
-10
1.88
0
-8
1.9
-6
-4 ω, rad.s
-2 -1 0
1.92
2
1.94
4
6
1.96
8
10
1.98
2
w
Freq (GHz)
Feed point
-2
Notch
S11 (dBs)
-4
-6
-8
mm
-10
-12
-14
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Dual mode simulation results
Single mode simulation results
Dual mode measured results
Single mode measured results
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Filter Techniques Applied to Antenna Design

Quadmode patch antenna
–
The approach taken is to broadside couple two dual-mode patch
antennas, resulting in an antenna with four resonances. The equivalent
circuit of the antenna is similar to that of microwave filters, thus filter
design techniques may be employed to synthesize the antenna and
obtain maximum return loss bandwidth.
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Filter Techniques Applied to Antenna Design

Quadmode patch antenna
–
Matrix rotations and filter design techniques can be used to reach the
circuit below and obtain the values for the impedance inverters.
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Filter Techniques Applied to Antenna Design

Theory
–
The return loss expression of the equivalent circuit of dual mode patch
antenna is
–
For 6dB bandwidth, it can be shown using dual mode filter theory that
the bandwidth can be optimum if J01 = 2 , J12 = 5.296, J23 = 3.898 and
J34 = 2.976.
–
The equivalent circuit aids in reaching a successful design for the
antenna.
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Copyright © 2008
Filter Techniques Applied to Antenna Design

Quadmode patch antenna
–
Analytical results
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Filter Techniques Applied to Antenna Design

Quadmode patch antenna
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Filter Techniques Applied to Antenna Design

Quadmode patch antenna [3]
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C. Tunable Bandstop Filter
• Dual-band combline structure consisting of a
wideband bandpass filter with integrated
bandstop filter.
|S21|2
∆A
ω1A
ω1B
ω2B
ω2A
ω
∆B
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Copyright © 2008
C. Tunable Bandstop Filter
An inverter coupled Chebyshev lowpass prototype network
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Copyright © 2008
C. Tunable Bandstop Filter
– The lowpass network may be transformed into a dualband combline structure by applying the
transformation*
•
*This is similar in principle to the dualband bandpass network discussed in G. Macchiarella and S. Tamiazzo, "Design
techniques for dual-passband filters," T-MTT-IEEE 2005
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Copyright © 2008
C. Tunable Bandstop Filter
• This gives rise to the combine circuit consist of a wideband
combline bandpass filter with further bandstop resonators
coupled to each bandpass resonator. Suitable choice of
bandwidth scaling factors α and β gives rise to a frequency
response.
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Copyright © 2008
C. Tunable Bandstop Filter
• The frequency of the bandstop filter may be tuned by altering the
capacitors C1B…CNB. It may be shown that the bandwidth of the bandstop
filter is given by
where θB is the electrical length of the bandstop filter at its center
frequency, the bandwidth is almost constant over a wide tuning
bandwidth.
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Copyright © 2008
C. Tunable Bandstop Filter
3.430
3.440
Frequency (GHz)
3.450
3.460
3.470
3.480
3.490
3.500
0
-5
-10
S11 & S12 (dB)
-15
S11
S12
-20
-25
-30
-35
-40
-45
-50
Frequency (GHz)
3.193
0
3.203
3.213
3.223
3.233
3.243
3.253
3.263
3.675
3.685
3.695
-5
-10
Frequency (GHz)
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
S11 & S12 (dB)
-15
S11
S12
-20
-25
-30
-35
0
-40
-45
-5
-50
Frequency (GHz)
S11 & S12 (dB)
-10
3.625
3.635
3.645
3.655
3.665
0
-5
-15
-10
-15
S11 & S12 (dB)
-20
-25
-20
S11
S12
-25
-30
-35
-30
S12-3.37GH
S12-3.1GHz
S12-3.7GHz
S11-3.37GHz
-40
-45
-50
Copyright © 2008
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Conclusion
New method for the design of Frequency Selective Limiters was presented.
The reconfigurable filter presented addresses one of the most important
issues with the current congested spectrum and solves the nonlinearity
caused by the use of varactors.
New technique for improving microstrip antennas bandwidth using circuit
theory and filter design techniques.
Novel technique for tunable bandstop filter was presented.
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Copyright © 2008
•
• Hvala Vam Na Paznji !
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Copyright © 2008