Diplexer Circuit Analysis and Design

In this chapter, a novel method of designing a microwave diplexer circuit is presented. This technique involves merging a section of a dual-band bandpass filter (DBF) with a section of two separately designed bandpass filters (BPFs). The chapter covers the step-by-step procedures that informed the successful realization of the diplexer circuit model. The circuit model coupling arrangement, simulation, and results are also covered. The diplexer circuit developed here has been simulated using the Keysight ADS circuit simulator. The results presented show a very good isolation between the transmit and the receive bands of the diplexer circuit.

Transmission Line

A transmission line (TL) is simply a medium that is capable of guiding or propagating electromagnetic energy. The transmission line stores the electric (E) and magnetic (M) energies and distributes them in space by alternating them between the two forms. This means that at any point along a TL, energy is stored in a mixture of E and M forms and, for an alternating signal at any point on the TL, converted from one form to the other as time progresses. Transmission line is usually modelled using lumped elements (i.e., inductors for magnetic energy, capacitors for electric energy, and resistors for modelling losses). The electrical characteristics of a TL such as the propagation constant, the attenuation constant, the characteristic impedance, and the distributed circuit parameters can only be determined from the knowledge of the fields surrounding the transmission line. This chapter gives a brief overview of various transmission lines, with more detailed discussions on the microstrip and the SIW.

Substrate Integrated Waveguide Diplexer Design

This chapter implements the microwave diplexer circuit model established in Chapter 4, using the twenty-first substrate integrated waveguide transmission line technology. No separate junction (resonant or non-resonant) was utilised in achieving the diplexer, as the use of an external junction for energy distribution in a diplexer normally increases design complexity and lead to a bulky device. The design also featured a novel input/output coupling technique at the transmit and the receive sides of the diplexer. The proposed SIW diplexer has been simulated using the full-wave finite element method (FEM), Keysight electromagnetic professional (EMPro) 3D simulator. The design has also been validated experimentally and results presented. Simulated and measured results show good agreement. The measured minimum insertion loss achieved on the transmit and the receive channels of the diplexer are 2.86 dB and 2.91 dB, respectively. The measured band isolation between the two channels is better than 50 dB.

Diplexer Overview

This chapter gives an overview of the microwave diplexer, starting with a background into the radio frequency and the microwave spectrums. The chapter also covers the aims and objectives of the book, the motivation, and the diplexer design method discussed in the book. A detailed literature review into the various diplexer design and implementation approaches is also detailed in this chapter. The chapter also looks at a number of transmission line technologies that have been utilised in the implementation of microwave diplexers including slotline, stripline, coplanar waveguide, microstrip, waveguide, and the substrate integrated waveguide (SIW). The microstrip and the SIW implementations of the diplexer is reviewed in more detail, with numerous existing research examples. The chapter concludes by highlighting the emerging research and the opportunities in diplexer design introduced and established in the book.

Future Opportunities in Diplexer Design

This chapter focuses on the future and present state of art relating to microwave diplexer design. It particularly highlights the future opportunities in making improvements in the diplexer design process in order to achieve better miniaturised diplexer components. The ideas discussed in this chapter could be futher investigated by researchers, including postgraduate students pursuing Master of Science (i.e., M.Sc.) and/or Doctor of Philosophy (i.e., Ph.D.) degrees. The chapter briefly discussed some varients of the SIW transmission line technology including the half-mode substrate integrated waveguide (HMSIW) and the folded substrate integrated waveguide (FSIW).

Microstrip Diplexer Design

In this chapter, the microstrip square open-loop resonator (SOLR) has been utilised in the implementation of the microwave diplexer circuit model established in Chapter 4. The SOLR is very popular and well known resonator type commonly used in the implementation of microwave passive devices including filters and diplexers, due to its compact size. The simulation and measurement results show good agreement, with a band isolation of about 50 dB achieved between the transmit (Tx) and the receive (Rx) bands of the diplexer.

Microwave Filter Analysis and Design

A microwave filter is a two-port component usually employed when there is a need to control the frequency response at any given point in a microwave system. They provide transmission at certain frequencies, which are known as the passband frequencies, and attenuation at other frequencies, which are referred to as the stopband frequencies. The frequencies outside the passband are attenuated or reflected. Microwave filter is often named after the polynomial used to form its transfer function (i.e., Chebyshev, Butterworth [or maximally flat], Elliptical, etc.). The filter can be further sub-divided into four categorises (i.e., lowpass, highpass, bandstop, and bandpass filters) according to its frequency responses. This chapter gives a detailed discussion on filter classification and transfer function. It also covers the analysis, design, and implementation of a test microwave filter using the 21st century SIW transmission line. The simulation and measurement results of the test filter is also presented, compared, and discussed.