Band-pass limiting filter based on microstrip comb line structure
Yuan.Xin.Li1, Ming.Tuan.Lin1,
Ji.Bin.Liu1, and Shi.Xiong.Deng 1,2
1 College of Electronic Science, National
University of Defense Technology, Changsha, China
2 Department of Microwave Integrated Circuits,
Hebei Semiconductor Research Institute, Shijiazhuang, China
Email: leeyx2020@qq.com.
A
band-pass limiting filter (BPLF) based on a microstrip comb line loaded
with PIN diodes is proposed in this letter. In contrast to conventional
comb line filter designs, the BPLF utilizes PIN diodes to replace
capacitor components. When the power of the input signal is below a
certain level, the proposed BPLF acts as a filter. While for an input
signal whose power is above a certain level, it behaves like a limiter.
According to measurement data, the proposed BPLF has a broad passband
with an insertion loss of about 3.5 dB from 8.2GHz to 9.0GHz, and a low
limiting level of less than 17dBm with a high power input of 50W
(47dBm). In general, the proposed BPLF enables dual selectivity of
frequency and power, and can be applied to the front end of RF channels.
Introduction: Filters and limiters are both important components
in radio frequency (RF) systems, to achieve frequency filtering [1]
and protection performance [2], respectively.
In general, filters are designed
to allow in-band high power signals through with low loss, so that
sensitive devices in the back end may be damaged. On the other hand,
limiters based on PIN diodes are designed to have ultra-wideband
[3], low insertion loss or integrated with amplifier [4], rather
than high selectivity. Therefore, in order to ensure frequency selection
and prevent the damage, the filters and limiters are cascaded together.
However, it is costly and not easy to integrate and reduce insertion
loss if the cascade is adopted directly in front of the RF receiver as
usual.
Here we proposed a basic concept of integrating the design of filters
with limiters based on the basic comb line bandpass filters, which is
abbreviated to BPLF. As a kind of classic band-pass filter structure,
microstrip comb line [5], is easy to design and implement. It is
comprised of an array of coupled resonators in parallel, which are
short-circuited at one end, and with lumped capacitances loaded between
the other end and ground. Due to the presence of lumped capacitances,
the resonators can be shortened thereby reducing area of the structure.
In addition, it is possible to design reconfigurable filters by
controlling the value of the loaded capacitances or adjusting lumped
elements, such as frequency-tunable [6-8] and switchable filters
[9].By replacing lumped capacitors with PIN diodes, filtering
performance under small signals and protection effect under large
signals can be achieved together with a more compact size and a lower
insertion loss compared to the cascade design. It can be used to deal
with intentional or unintentional electromagnetic interference in
complex electromagnetic environments.
Principle and design:Fig.1
shows the principle of the proposed BPLF. The topology in the middle
part was modified from the equivalent circuit of a comb line filter
[10]. The key point of the modification is to replace the shunt
capacitor with a PIN diode (inside the blue box in Fig.1). Under the
input with low power, the diodes are equivalent to capacitances in the
turned-OFF state (inside the green box). Thus, the circuit shows the
characteristics of a bandpass filter. On the contrary, if the power of
input electromagnetic signals exceeds the threshold, diodes work in a
turned-ON state and is equivalent to a resistance approximate to zero
(inside the red box). Therefore, the switching process of PIN diodes can
be used to realize limiting properties. The other parts of the topology
include transformers at input and output, parallel shorted shunts, and J
inverters. and in Fig.1 represent admittance of the terminating line and
parallel stub, respectively, the same as the comb line filter equivalent
circuit.