Design of hairpin band pass filters for K-Band application

Abstract

This paper presents Chebyshev three and four poles microstrip hairpin filter for radar applications. The filters are operated at K-band frequency segment of 20-20.3 GHz. The filters were designed using Genesys software and implemented on Roger 5870 substrate. The results from simulation and measurement show that both filters were operating at the desired specification. Based on the experimental analysis, it was observed that the filter with four elements better than the three which was quite agree to other researchers.

Full text

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2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE

2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE
2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE
December 2
December 2 December 2
December 2- - - -4, 2008, Kuala Lumpur, MALAYSIA
4, 2008, Kuala Lumpur, MALAYSIA 4, 2008, Kuala Lumpur, MALAYSIA
4, 2008, Kuala Lumpur, MALAYSIA


R F
M 08
Design of Hairpin Band Pass Filters for K-Band Application


A. A. Sulaiman1, M. F. Ain1, S. I. S. Hassan1, A. Othman1, M. A. Othman1, R. A. Majid1, M. Z.
Saidin1, M. H. A. Hamid1, M. H. Jusoh2, Z. I. Khan2, N. H. Baba2, R. A. Awang2, Z. Awang2, N.
A. Z. Zakaria2, M. K. A. Mahmood2

1School of Electrical & Electronic Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Pulau Pinang, Malaysia.
2Faculty of Electrical Engineering, Universiti Teknologi Mara Malaysia, 40450 Shah Alam, Selangor, Malaysia


Abstract - This paper presents Chebyshev three and
four poles microstrip hairpin filter for radar
applications. The filters are operated at K-Band
frequency segment of 20 – 20.3 GHz. The filters were
designed using Genesys software and implemented on
Roger 5870 substrate. The results from simulation and
measurement show that both filters were operating at
the desired specification. Based on the experimental
analysis, it was observed that the filter with four
elements better than the three which was quite agree to
other researchers.

Keywords – Bandpass, Filter, K-Band, Design, Hairpin, RF,
Simulation.


1. Introduction

Microstrip filters play an important role in many
RF applications. As technologies advances, more
stringent requirements of filters are required. One of
the requirements is the compactness of filters.
Bandpass filters are essential building blocks for
communication systems. They can reduce the harmonic
and spurious emissions for transmitters, and may
improve the rejection of interferences for receivers. To
ensure an easy integration between band pass filters
and other active devices, many previous works on
designing planar filter were reported [1-2]. Among
various types of planar filters, the parallel-coupled
band pass filter based on half-wavelength, λ/2
resonators exhibits the merits of simple synthesis
procedure, good repetition, and a wider range of
realizable fraction in bandwidth.
However, the conventional microstrip parallel-
coupled band pass filter suffers from the spurious
response at twice the center frequency, 2f0, making the
rejection of the upper stop band worse than the lower
stop band. This was due to unequal even and odd mode
propagation constants for microstrip coupled-line
sections.
Many works have been reported to tackle this issue
by equalizing the effective dielectric constants or
electrical lengths of the even and odd modes. However,
these techniques either rely on complex
design/optimization procedures or limit the filter to
specific circuit configurations [3-6]. Some works on
improving the selectivity of the conventional parallel-
coupled band pass filters were also reported in [1], the
shunt-connected together with quarter-wavelength,
(λ/4) open stubs were introduced to create transmission
zeros at stop band. In [2], the capacitive coupled gap
and the λ/4 open stubs were adopted in the circuit to
create two transmission zeros, such that the selectivity
of parallel-coupled band pass filter can be improved.
On the other hand, microstrip band pass filters based on
λ/4 resonators may feature more compact circuit size as
well as better stopband rejection with no spurious
response at 2f0.
This paper will focus on the design of hairpin band
pass filter for K-band application. Two hairpin filters
with different number of elements were designed and
compared the performance of the devices in the
extreme high frequency.


2. Methodology

Order of the filters can be determined as explained
by [7]. Based on the design specification, 3rd and 4th
orders filter were chosen using the proposed hairpin
structure. The element values for both
taken from normalized values of 0.1 dB equal ripple
low-pass prototype and the method of calculation were
such as in [8]. The value of components can be
calculated using equation (1) and (2). Where gn was
indicated the number of elements, while n was an
integer numbers 1, 2, 3…, n+1. High number of n
contributes small transition band and sharp cut-off.


=
g
2
γ

()


=
+
g
i
γ
sin

orders were





n
sin
2
1
π
(1)
()
π
()
π




−
+







−
2




−
2
n
i
n
i
n
i
g
i
π
1
32
sin
12
sin4
1
22
1
(2)

The element values that have been obtained for the
3rd and 4th orders of the filters were shown in Table 1.

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Table 1: Low-Pass Prototype Elements Values

Order g0 g1
3rd 1 1.0315
4th 1 1.1088
g2 g3 g4
1.1474
1.3061
1.0315
1.7703 0.8180


The values from the table then were converted to
band pass filter prototype at the specific cut off
frequency. The characteristic impedance and dimension
of the coupled lines can be obtained from the values. In
this paper, a three pole and four pole microstrip hairpin
filters will be presented. The proposed filters were
operated in the K-Band and covered from 20 GHz to
20.3 GHz. The design specifications of the filters
including the pass band ripple of 0.1 dB and the
minimum attenuation of -30 dB at 20.15 GHz. The
specifications of the filters were shown in Table 2.


Table 2: Bandpass Filter Design Specifications

Centre frequency
Upper cut-off frequency
Lower cut-off frequency
Bandwidth
Passband ripple


One of the most popular microstrip filter
configurations is the hairpin resonator. In term of
manufacturing, it can be considered an ease structure to
be manufactured because of open-circuited ends with a
ground plane. The filter was derived from an edge-
coupled resonator by folding back the ends of the
resonators into a “U” shape. This approach reduces the
length and improves the ratio of the microstrip size
significantly.





(a) 3 elements



a)
Four pole hairpin filter


(b) 4 elements

Fig. 1: The layout of Hairpin filters with different
element

Fig. 1 shows two layouts of the hairpin filter
bended by half wavelength resonator in a U-shape. By
bending structure on the resonators, the overall
dimensions of the filter were greatly reduced as
compared to conventional parallel-coupled and edge-
coupled filters.

20.15 GHz
20.3 GHz
20 GHz
0.3 GHz
0.1 dB

Table 3: Substrate Properties

Dielectric Constant
Loss Tangent
Resistivity
Metal Thickness
Metal Roughness
Substrate Height
2.33
0.0012
1
0.035 mm
2.4e-3 mm
0.508 mm


The circuit was constructed on Roger 5870
substrate with dielectric constant of 2.33, loss tangent
of 0.0012 and substrate height of 0.508 mm. Table 3
indicates the detail properties of the substrate.


3. Results

The circuit then was simulated using Genesys and
optimized using Electronic Design Automation (EDA).
This software using an iteration algorithm which
adjusts space of short couple section cascades with two
adjacent lines to match the characteristics of the
original quarter-wave [9].







-15
Magnitude(dB)












Fig. 2: Responses of filter with three elements

















Fig. 3: Responses of filter with four elements
-35
-30
-25
-20
-10
-5
0
19775199752017520375
Frequency (MHz)
Return Loss
Insertion Loss
-70
-60
-50
-40
-30
-20
-10
0
19775199752017520375
Frequency (MHz)
Magnitude (dB)
Return
Loss
Insertion
Loss
267

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Fig. 2 and Fig. 3 show responses of each filter with
different number of elements. The responses from four
elements were better compared to the three. In term of
return loss and insertion loss, both results were almost
the same. However the four elements return loss able to
swing to up to a value that more than -60 dB. These
two parameters were crucial to analyze to obtain a good
performance of filters. A good filter will be high return
loss and small insertion loss ripple in passband.


4. Discussion


The simulated circuits then were fabricated on
Roger’s microstrip as shown in Fig. 4. The filters then
measured using a Vector Network Analyzer (VNA).
The results obtained from the measurement then were
used to analyze the performance of the designs. There
were slightly different between the measured and the
simulated values.



-




(a) 3 elements








(b) 4 elements

Fig. 4: Fabricated hardware with difference
elements
















Fig.5: Comparison of return loss for circuit with three
elements

Fig. 5 and Fig. 6 indicate comparisons between
return and insertion losses from the design between
simulated and measured results. Both graphs show the
measured values compose of higher losses compared to
the simulated. The measured return loss bandwidth was
smaller compared to the simulated. High return loss at
stop band indicates that the filter was not operate well
at the design frequency since the filter could not
differentiate between the pass and stop bands.
The different between measured and simulated
was about 30 dB for insertion loss in Fig. 6. This may
due to the limitation of the substrate. The substrate may
be unstable to operate well at frequency above 10 GHz.
The dielectric loss of the substrate was high at that
extreme frequency. The other factors may due to
interference that affect the circuit in an open air
environment since the circuits were not measure in an
anechoic chamber.
















Fig. 6: Comparison of insertion loss for three elements


5. Conclusion

Two designs of hairpin filter at K-band frequency
were presented in this paper. Both filters have been
successfully designed at the desired specifications. A
data analysis has been done. The filter with four
elements gives better responses compared to the three.
Even both filters have been successfully fabricated; the
high substrate loss cause the insertion loss drops in the
pass band.


Acknowledgement

Authors would like to thank Universiti Teknologi Mara
Malaysia, Universiti Sains Malaysia, The Ministry of
Science Technology and Inovation (MOSTI) and the
Malaysian Communications and Multimedia Commission
(MCMC) for supporting and funding the project.




-45
-40
-35
-30
-25
-20
-15
-10
-5
0
19775 19875 19975 20075 20175 20275 20375 20475
Frequency (MHz)
Insertion Loss (dB)
Simulation
Measurement
-35
-30
-25
-20
-15
-10
-5
0
19775199752017520375
Frequency (MHz)
Return Loss (dB)
Simulation
Measurement
268

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