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Under the guidance of
Prof. Subratra Sanyal & Dr Arijit De by
Bhaskar Ray Karmakar
10EC63R05
DEPARTMENT OF ELECTRONICS & ELECTRICAL COMMUNICATION ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY
KHARAGPUR – 721 302
DESIGN OF NARROWBANDPASS SHARP REJECTION STRIPLINE FILTER FOR SPACE APPLICATION
The bandpass filter is suppose to be designed with following characteristics.
Sharp rejection,narrow bandpass with out of band rejection of 50MHz. Maximum insertion loss of 2dB & return loss better than 15dB Having centre frequency 2GHz & bandwidth of 100MHz. Compact Size.
OBJECTIVE
A microwave filter can be manufactured out of
MOTIVATION
A dielectric resonator filter Waveguide filter. Transmission line filter.
Fig:1Type of microwave filter
Waveguide Filter Microstrip Filter Stripline FilterWaveguide filters are manufactured by placing irish or slits.
It is made by etching one side of a dielectric substrate.
Here a piece of transmission line is inserted in a pool of dielectric substrate.
The mode of propagation is either TE or TM
Here mode of propagation is quasi TEM, since its half lying in dielectric & half in air.
Pure TEM propagation is found in stripline circuit as it is completely inside the dielectric substrate.
Since discontinuities are introduced which aid in working of a waveguide filter, it is difficult to construct.
It is easiest to construct. Some discontinuities are necessary to etch the resonating structure. Discontinuities like bend , open ended & closed end are often seen. Proper treatment like chamfering is essential for the optimum performance of the circuit
Discontinuities of same types are find here, but a better performance is observed due to pure TEM mode of propagation.
It is heaviest of all the three types of available options
It is lightest of three. It is much lighter than waveguide filter but little heavier than microstrip filter due to requirement of two dielectric slabs .
It offer the low loss with highest Q among the three substrate
They offer low Q ,while insertion loss is low & return loss is acceptable for good performance.
Unloaded quality factor is stripline is quite close to that of microstripline, but here the insertion loss and return loss in this case is better than microstrip configuration.
COMPERATIVE STUDY DIFFERENT KIND OFWAVEGUIDE,MICROSTRIP & STRIPLINE FILTER
Depending on the configuration bandpass filter can be designed on the following circuits.
Stepped Impedance. Parallel Coupled Line. End Coupled Line. Interdigital. Comb Line. Hairpin. Irregular line.
DIFFERENT AVAILABLE CIRCUITS FOR CONSIDERATION
HairpinLadder Network
Fig 2: A CAD layout of ladder and hairpin structure
COMPARASION OF AVAILABLE CIRCUITSParallel coupled line
End Coupled Line.
Interdigital Combline Hairpin
Cascade of parallel coupled half wavelength line
It’s a series of half wavelength long strip resonator
Its an array of quarter wave length long line with alternating short & open circuits.
Its same as parallel coupled line except that each resonator is short circuited at one end & grounded at other end through capacitor
The half wavelength resonators are folded into a U shaped.
Small in size and easy to fabricate due to absence of short circuit
It become extremely large at low frequency and length is two times greater than paralled coupled filter.
Dimension of these filters are one quarter of physical wavelength at operating frequency building such filter at low frequency is tedious job.
They are more compact than interdigital filter and particularly suitable for narrowband pass application.
Since it much more compact it finds it application in MIC or MMIC.
No ground connection is required.
No ground connection is required.
They need to be grounded through grounding
They are short circuited at one end and connected to the ground with a capacitor.
Since grounding is not required mass production on single substrate is possible with low cost.
Suitable for wideband application
Narrowband width is hard to achieve since finding a optimum spacing between the resonators is hard to achieve.
With a high unloaded Q a deep skirt can be obtained but sharp rejection is almost impossible task.
It is ideal for narrow band pass application where sharp rejection is not desired.
They will look for quite large resonator spacing if narrowband pass is desired.But as they are more suitable for MIC’s they are not employed for narrowband circuit.
M14
M12
M45 M58Mn-3,n
M23 M67 Mn-2,n-1
M34 M56 M78 Mn-3,n-2 Mn-1,n
CASCADED QUADRIPLET
Fig:3 Typical Coupling structure for CQ filters
A CQ bandpass filter consists of cascade sections of four resonators each with one cross coupling.The cross coupled can be arranged in such a way that improves the selectivity.
ORIENTATION TYPES OF COUPLING
Three different types of coupling are
1. Magnetic Coupling 2. Electric Coupling 3.Mixed Coupling
The coupling in all these structure is proximity coupling.
The coupling co efficient of the mixed coupling decrease with the increase of the spacing.
Electric coupling is dominant for a small spacing and magnetic coupling is dominant for a large spacing.
Fig 4: Different types of coupling
The coupling co efficient ‘k’ is defined as ratio of coupled energy to stored energy.A general expression is given by
Where E & H electric & magnetic field vectors & ‘k’ is coupling coefficient.
For extraction of ‘k’ a more generalized formula is given by
For a synchronously tuned structure the expression reduce to
COUPLING COEFFICIENT
1 2 1 2
2 2 2 2
1 2 1 2
. .
* *
E E dv H Hk
E dv E dv H dv H dv
2 2 2 22 102 01 02 02
2 2 2 201 02 2 1 02 01
1
2p p
p p
f ff f f fk
f f f f f f
2 22 1
2 22 1
p p
p p
f fk
f f
EXTRACTION OF ‘k’
For example in deriving the value of ‘k’ the following structure is considered with spacing between the two resonator is .5mm and length of each arm is 9mm. The relative dielectric constant is 10.8.The arm gap is 1.5 mm and ports are located at equal distance from the end.
From the graph the two peaks are located at 2420MHz and 2950MHz.So the calculated value of ‘k’ is .1954.
Following are the inference from the simulated structure Stronger Coupling will result in separation of two peaks.
The port location with respect to the coupled resonators must be same.
Fig 5: Electric Coupling & response
A numerous types of coupling scheme are available.
They are
TYPES OF COUPLING
Loose Coupling.
Direct CouplingSide Coupling
Enhanced Coupling.
Matched Loose Coupling
Fig 6: Different types of coupling
ALTERNATIVE PORT ARRANGEMENT
Fig:7CAD layout and response due to change of the port distance
S21=4.6dBS11=23dB
0.25mm
S21=4.53dBS11=6dB
0.5mm
S21=4.9dBS11=5.2dB
.35mm
DESIGN EXAMPLE & RESULT
Fig: 8a CAD layout with an increased gap area
Fig 8b:Square resonator with interdigital structure
S21=3.2dBS11=6.68dB
PORT POSITION
The tap point can be calculated from
2
0
( ) 4sin ( )2
QR lZ L
l1
l2
l2
l2
Input
Centred
dCenter
output
Fig:9 Different port location & its response
SRRs WITH INTER PARTICLE
Fig 10:Different orientation of inner particle & its response
EFFECT OF THE ORIENTATIONS & INTER PARTICLE SPACING OF THE SRRs
Fig11a: Multiple inner resonators corresponding response
Fig 11b:Increased Coupling area & associated response
C2
C1
C3
Fig 12c:Equivalent diagram in term of circuit element
Fig 12a:End coupled Structure Fig 12b:Side Coupled Structure
1 '0
ln{coth( . )}2
D Sb
D
2 '0
2ln{coth( . )}
2
D Sb
D
Where b1 & b2 is the suceptance ,S is the spacing between two adjacent strip
CALCULATION OF GAP SPACING FOR END SIDE & BROADSIDE COUPLING
Fig 13a: Endside coupling
The normalized susceptance of (i+1)th gap is given as an end coupled filter with "n" stages is bi,i+1 and can be expressed as
On rearranging first equation we get
Fig 13b:Broadside coupling
S1 Sn
, 1, 1 2
, 1
0,1 , 10 1
1
2 1 0
(1 )
2
, 12
( ) /
i ii i
i i
n n
i i
b
where
g g
i ig g
f f f
'1 01
ln{coth }2
bS
D D
20, 1 , 1
0
, 1 1ei i i i
Zi i
Z
20, 1 , 1
0
, 1 1oi i i i
Zi i
Z
The relation between Z0e & Zoe in terms of W/D,S/D is
0
94.15 /
ln 2 1ln 1 tanh( .
2
reZ
W S
D D
0
94.15 /
ln 2 1ln 1 coth( .
2
roZ
W SD D
2
6
0
1 188.3 (1 )ln tanh( . )
2 (1 )r
S
D Z
2
6
0
1 94.15 (1 )ln coth
(1 )r
S
D Z
02
1ln coth .
12 r
S
D D
CO M PARISI O N BETWEEN TH E TWO TYPES O F CO UPLING
DESIG N ALG O RITH M FO R M ICRO WAVE CIRCUITS
Fig 14: An Algorithm for designing microwave circuits
FABRICATION & RESULTS
The width of dielectric in this case is.635mm while the length of each arm of resonator is 7.9 mm and width is 1.11483mm.The port width is .5 mm.
S21=6.43dBS11=15.06dB
Fig 15: CAD layout & response due to simulation on TMM 10
CHOICE OF CQ STRUCTURE
Fig 16:Different orientation of CQ and corresponding response
CIRCUIT DESIGNS & RESULTS
Fig 17:CAD layout for different structures
COMPERASION TABLEDesign Return Loss
(S11) dBInsertion Loss(S21)dB
Out of band rejection at 60dB (MHz)
Stub Length(mm)
Dimension mm*mm
1 15.87 3.79 25.23 No stub 65.87*46.55
2 36.2692 1.30914 28.21 5 66.05*46.55
3 37.11 1.3843 26.3372 5 & 5 mm away from port end
66.05*46.55
4 47.71 1.2562 29.34 2.5 & .5 mm away from port end
65.33517*60.361
CIRCUIT DESIGN & DIMENSION
Fig 18a:Fabrication on TFG Woven Fig18b: Dimension of the fabricated structure.
MEASUREMENT & RESULTS
Fig19: Measured & Simulated response of the designed circuit
The simulated and measured results quite close.
DISCUSSION
Response Central Frequency
Insertion loss
Return Loss Bandwidth Rejection Bandwidth
Simulated 37.71 1.2562 2 180 29.34
Measured 25.21 1.94 1.97 182.2 29.51
All the enclosures were fabricated in house.The etching technique used for the fabrication is quite old.Etching can be improved by doing the same in LPKF machine.The two dielectric structure was attached using a very crude process.
Fig20: Measurement of Insertion loss and return loss on two occasion
MEASUREMENT IN ISAC BANGALORE
Reduction in bandwidth: Though the designed circuit offered desired response in term of insertion loss & return loss, more work is need to reduce the bandwidth. This can be done selecting a proper dielectric substrate and also properly choosing the orientation of the resonators .
Suppression of leaky modes: It can be achieved by using structure like suspended striplines or using suitable bonding structure.
Design of fractal resonators: It can increase the Q of the structure and alo the size is
reduced.
FUTURE SCOPE
[1] George L.Matthaei, Leo Young,E.M.T. Jones(1980),Microwave Filters, Impedance-Matching Networks, And Coupling Structures,” Artech House,INC .
[2] Jia-shen G.hong and M.J.Lancaster (2001),Microwave filter for RF/Microwave Applications, A Wiley Interscience Publication,
[3] J. S. Hong and M. J. Lancaster, “Couplings of microstrip square open loop resonators for cross-coupled planar microwave filters,” IEEE Trans. Microwave Theory Tech., vol. 44, pp. 2099–2109, Nov. 1996
[4] Richardson, J.K,”Gap spacing for End Coupled & Side Coupled Strip-Line Filter,” IEEE Trans. Microwave Theory Tech, vol-15,no-6,June 1967 .
[5] Richardson, J.K,”Gap Spacing for Narrow Bandwidth End Coupled Symmetric Stripline Filter”, IEEE Trans. Microwave Theory Tech., vol-16,no-8, August 1968 .
[6] Bharathi Bhat,Shiban K Koul (2007),Stripline-like Transmission lines for Microwave Integreated Structure, New Age International Publisher,New Delhi,pp 22-456 .
[7] G.K. Gopalakrishnan and K. Chang,”Novel excitation schemes for the microstrip ring resonator with lower insertion loss,” Electronics Letters ,Vol. 30 No. 2, 20th January 1994.
[8]Kai Chang (1996), Microwave ring circuit and antennas,Wiley Series in Microwave and Optical Engineering, pp 261.
[9] L.Hsieh and Kai Chang,”Compact dual mode elliptic function bandpass filter using a single ring resonator with one coupling gap,” Electron.Letter., Vol.36,No 19,pp.1626-1627,September 2000.
REFERENCE