PIERS Proceedings, Moscow, Russia, August 19–23, 2012
906
Design of High Isolation Electronically Switchable Bandpass Filter
Shih-Fong Chao and Ming-Wei Shih
National Kaohsiung Marine University, Taiwan
Abstract—In this paper, a simple method to design a high isolation electronically switchable
bandpass filter is proposed. The circuit integrates a microwave switch and a microwave bandpass
filter into a single circuit component. The uniform impedance resonator is loaded with a pin
diode to change its resonant frequencies under different bias voltages. By loading the pin diode
at different positions of the constituted resonators, the resonant frequencies can be misaligned
over the band of interest to achieve a wideband isolation in the isolated state. In the thru
state, the circuit shows a bandpass response with a measured insertion loss of 3 dB at the center
frequency of 1 GHz, and the 3 dB fractional bandwidth is 8%. In the isolated state, the measured
isolation is 51 dB at the center frequency and is higher than 30 dB from dc to 5 GHz. The paper
proposed a simple and effective method to realized an electronically switchable bandpass filter
with high isolation, and the circuit successfully increases the level of integration of the RF front
end.
1. INTRODUCTION
A microwave switch is a essential component to control RF signal flow in the modern communication
systems. Several literatures using passive FETs or pin diodes have been reported [1, 2]. However,
these switches are wideband designs, implying that they can not provide adequate band selectivity
for a system application. Therefore, a bandpass filter will be needed to cascade with a switch to
reject out-of-band signals.
Recently, several methods were proposed to integrated the building blocks of the RF front into
a multi-function circuit [3–6]. In [3], an SPDT bandpass filter-integrated switch that integrate
an SPDT switch and a bandpass filter was proposed. Switchable dual-band bandpass filters that
achieve independent switching control of band selection was proposed in [4, 5]. In [6], a balunintegrated bandpass switchplexer that integrates an SPDT switch, a diplexer, and a balun was
demonstrated.
In this paper, a new method to design a high-isolation switchable bandpass filter is proposed.
The pin diode loaded uniform resonator is used to realized the circuit instead of using steppedimpedance resonators. By connecting the pin diode at different locations of the uniform resonator,
the resonant frequencies of the resonator can be controlled. Therefore, a high isolation can be
obtained by interlacing the resonant frequencies over the band of interest in the isolated state.
2. CIRCUIT DESIGN
Figure 1 shows the structure of the diode-loaded uniform resonator and its equivalent circuit under
different bias conditions. The θ1 is the electrical length from the loading location to the open end,
and θ2 is the electrical length from the loading location to the other open end. When the diode is
reversed biased (off-state), the resonator is loaded with a small capacitor. If the diode is forward
biased (on-state), the resonator will be connected to ground via a small resistor R. Since the R is
small, the diode could be considered as a short circuit to ground. Thus, the resonant frequencies
are determined by two quarter-wavelength resonator with one end short to ground.
Figure 2 shows the circuit schematic of the proposed switchable bandpass filter. The circuit
is composed by three diode-loaded hairpin resonators. In the thru state, the diodes are reversed
biased, the first resonant frequency of each resonator is 1 GHz. The circuit was designed to have
diode off
θ1
θ1
θ2
θ1
θ2
θ2
diode on
Figure 1: Photograph of the DPDT filter-integrated switch A.
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 907
R =1 k Ω
R= 1 k Ω
R = 1 kΩ
Port 1
Port 2
Figure 2: Circuit schematic of the proposed switchable bandpass filter.
Figure 3: Photograph of the switchable bandpass filter.
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EM simulation
Measurement
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|S21|
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Measurement
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Frequency (GHz)
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1
2
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(a)
4
5
Frequency (GHz)
6
7
8 8.5
(b)
Figure 4: Simulation and measured results of the switchable bandpass filter in thru state. (a) Narrow band
response. (b) Wideband response.
a 3rd-order Butterworth bandpass response at 1 GHz, and 3-dB fractional bandwidth ∆3 dB of 8%.
The coupling coefficients and the external quality factor Qext can be obtained as
M12 =
Qei =
∆3 dB
= M23 = 0.059
√
g1 g2
g0 g1
= Qeo = 10.9
∆3 dB
(1)
where Mij represents the coupling coefficient between resonators i and j, gn is the low-pass prototype parameter, and Qei and Qeo are the external quality factors associated with the input and
output couplings, respectively [7]. In the isolated state, the diodes are forward biased. By loading
the diodes at different locations of the resonators, the resonant frequencies can be scattered over
the band of interest. Thus, a high isolation could be obtained between the input and the output
ports.
3. MEASUREMENT
The circuit is fabricated on the RO4003C substrate with ²r = 3.58, h = 0.8 mm, and tan δ = 0.0015.
Fig. 3 is the photograph of the switchable bandpass filter. Fig. 4 shows the measured results when
PIERS Proceedings, Moscow, Russia, August 19–23, 2012
908
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Measurement
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Measurement
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Frequency (GHz)
(a)
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1
2
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5
6
Frequency (GHz)
7
8 8.5
(b)
Figure 5: Simulation and measured results of the switchable bandpass filter in isolated state. (a) Narrow
band response. (b) Wideband response.
the circuit is at thru state. It can be observed that a bandpass response with a passband insertion
loss of 3.0 dB at center frequency of 1 GHz was measured, and the 3-dB bandwidth is 8.5%. The
measured isolations of the circuit is shown if Fig. 5. The isolation is about 60 dB at the center
frequency of 1 GHz and is better than 32 dB from dc to 5 GHz.
4. CONCLUSION
In this paper, a high isolation electronically switchable bandpass filter is designed and implemented.
By loading the pin diode at different positions of the constituted resonators, the resonant frequencies
can be misaligned to obtain an isolation in the isolated state. In the thru state, the circuit shows
a bandpass response with a measured insertion loss of 3 dB at the center frequency of 1 GHz. In
the isolated state, the measured isolation is 51 dB at the center frequency. The circuit successfully
integrates a microwave switch and a bandpass filter into a single circuit and thus increases the level
of integration of the RF front end.
ACKNOWLEDGMENT
The authors would like to thank Prof. Jui-Han Lu for the support of measurement equipments.
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