Physica C: Superconductivity and its applications 519 (2015) 153–158
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Physica C: Superconductivity and its applications
journal homepage: www.elsevier.com/locate/physc
The research of parallel-coupled linear-phase superconducting filter
Tianliang Zhang, Liguo Zhou, Kai Yang
∗
, Chao Luo, Mingyan Jiang, Wei Dang, Xiangyang Ren
School of Astronautics and Aeronautics, University of Electronic Science and Technology of China, Chengdu 611731, China
article info
Article history:
Received 12 June 2015
Revised 15 September 2015
Accepted 16 October 2015
Availableonline28October2015
Keywords:
Filter
HTS
Linear phase
Parallel coupling structure
Sub-network
abstract
This paper presents a research on the mechanism of a linear phase filter constructed with parallel-connected
sub-networks, considering that linear phase characteristic of a filter can be achieved when the group delays of
sub-networks compensate each other. This paper also gives several coupling and routing diagrams of linear
phase filters with different parallel-connected networks, and then the coupling matrixes of three 8-order
filters and one 10-order filter are synthesized. One of the coupling matrixes is utilized to design a 8-order
parallel-connected network high temperature superconducting (HTS) linear phase filter with two pairs of
transmission zeros, so as to verify the correctness of theory data and the feasibility of the circuit design for
the proposed 8-order and higher order parallel-connected network linear phase filter. The HTS linear phase
filter is designed on YBCO/LaAlO
3
/YBCO superconducting substrate, at 77 K, the measured center frequency
is 2000 MHz with a bandwidth of 30 MHz, the insertion loss is less than 0.3 dB and the reflection is better
than −12.5 dB in passband. The group delay is less than ±5 ns over the 60% passband, which shows that the
filter has a good linear phase characteristic.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Nowadays, HTS filter has been widely used in mobile communica-
tion system, satellite space communication system and radar receiver
system, etc. [1–6]. One important reason is that the HTS filter has
very low insertion loss and sharp out-of-band rejection, which can
improve the sensitivity of receiver front-end and suppress the out-
of-band interference signal effectively. In order to ensure the signal
to be transmitted efficiently without distortion, the communication
network should have a good linear phase (flat group delay) character-
istic in passband. As for the communication network, the group delay
characteristics of the filter in passband play a decisive role on the bit
error rate of the system and the communication quality of the en-
tire system. Especially, as the selectivity of HTS filter is much better
than that of the conventional metal filter, the group delay fluctua-
tion of HTS filter is obviously larger in the passband; therefore, when
designing a HTS filter, it not only needs to satisfy the amplitude–
frequency characteristic, but also needs to consider the requirements
of flat group delay. Up till now, researchers have done a lot of work
on the HTS linear phase filter; there are two main models: external
equalization and self-equalization [7–13].
The paper [14] firstly proposed a self-equalization linear phase
filter model with a parallel-connected two-port network, but with
only one coupling matrix of a 6-order filter. In previous work [15],
∗
Corresponding author. Tel.: +86 2861831211.
E-mail address: kyang@uestc.edu.cn (K. Yang).
our research group studied a planar double-coupling structure and
designed a 6-order parallel-connected network HTS linear phase fil-
ter with the coupling matrix from [14]. Obviously, as for this kind
of parallel-connected network, the signal mainly has two paths from
the input port to the output port, and the linear phase characteristic
of the filter can be achieved through mutual group delay compen-
sation of the two paths. Based on this result, a further analysis on
the mechanism of the parallel-connected network linear phase fil-
ter is presented in this paper. Considering the overall network can
be divided into a number of sub-networks, and linear phase charac-
teristics can be achieved if the group delay of sub-networks compen-
sate for each other in the passband. Furthermore, several parallel cou-
pling and routing diagrams for higher order linear phase filter are also
given, the corresponding coupling matrix is synthesized, and one of
the prototypes coupling and routing diagrams and coupling matrix is
utilized to complete the design of an 8-order HTS parallel-connected
network linear phase filter to verify the correctness of theory data.
Lastly, the feasibility of the actual circuit is proved by the production
and measure of the sample.
2. The mechanism analysis of parallel-connected network linear
phase filter and the synthesis of coupling matrix
In a previous work [14], a coupling matrix was provided for
a six-order linear phase filter. The coupling and routing diagram
for the six-order filter is shown in Fig. 1(a), where S and L repre-
sent external ports, and each node with a number represents a res-
onator. The dashed line between Resonator 1 and Resonator 6 in
http://dx.doi.org/10.1016/j.physc.2015.10.006
0921-4534/© 2015 Elsevier B.V. All rights reserved.