15. M.A.S. Silva, T.S.M. Fernandes, and A.S.B. Sombra, An alternative method for the measurement of the microwave temperature coeffi- cient of resonant frequency (sf), J Appl Phys 112 (2012), 074106. 16. Luck, K.M. and Leung, K.W., Dielectric resonator antennas, Research Studies Press LTD, Ingland, 2003. 17. Petosa, A. Dielectric resonator antenna handbook, Artech House, Norwood, MA, 2007. 18. D. Kajfez and P. Guillon (Eds.), Dielectric resonators, Artech House, Norwood, MA, 1986. 19. Balanis, C.A. Antenna theory: Analysis and design, Harper and Row, 1982. 20. S. Kucheiko, J.W. Choi, H.J. Kim, and H.J. Jung, Microwave dielec- tric properties of CaTi03-Ca(Al1/2Tal/2)O3 ceramics, J Am Ceram Soc 79 (1996), 2739–2743. 21. P. Liu, et al. Low temperature sintering and microwave dielectric properties of Ca(Li 1/3 Ta 2/3 )O 3-d -CaTiO 3 , J Eur Ceram Soc 23 (2003), 2417–2421. 22. Y.B. Chen, New dielectric material system of vla(mg 1/2 ti 1/2 )O 3 -(l-v)srtio 3 in the microwave frequency range, J Alloys Compd 491 (2010), 330–334. 23. A. William, Imbriale, Spaceborne Antennas for Planetary Exploration, Jet Propulsion. Laboratory California Institute of Technology, Pasa- dena, CA, 2006. 24. S.M. Wentworth, Fundamentos de Eletromagnetismo com Aplicac ¸~ oes em engenharia, LTC, 2006. 25. R.K. Mongia and P. Bhartia, Dielectric resonator antennas—A review and general design relations for resonant frequency and bandwidth, Int J Microwave Millimeter-Wave Computer-Aided Eng 4 (2007), 230–247. 26. K. Chang, RF and microwave wireless systems, A Wiley- Interscience Publication, John Wiley, Hoboken, NJ (2000). V C 2016 Wiley Periodicals, Inc. A K-BAND PUSH–PUSH OSCILLATOR EMPLOYING DIFFERENTIAL TRANSMISSION LINE LOADED WITH SIW CAVITY OPERATED IN TM 110 MODE Zhipeng Li, Yongzhi Liu, and Jingfu Bao School of Electrical Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China; Corresponding author: lizpmemory@hotmail.com Received 27 August 2015 ABSTRACT: In this letter, a novel differential transmission-line (TL) loaded with circular substrate integrated waveguide (SIW) cavity is pre- sented for realizing K-band push-push oscillator. The SIW cavity is working at TM 110 mode to enforce the 1808 phase difference between two sub-oscillators, and the tuning metal vias perturbation is introduced in the SIW resonator to maximize the distance between desired odd TM 110 mode and adjacent even modes to stabilize the oscillation mode. Unlike the conventional SIW resonator structure, this differential TL possesses a reflective-type property at the resonance frequency. A 19.76 GHz push–push oscillator based on this differential TL is designed and implemented. The measured suppression of fundamental frequency component is 225.62 dBc, and the phase noise is 2118.1 dBc Hz 21 at 1 MHz offset frequency, which corresponds to a figure-of-merit (FOM) of 2191 dBc Hz 21 . V C 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1217–1221, 2016; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.29774 Key words: oscillator; push–push; substrate integrated waveguide; dif- ferential transmission line 1. INTRODUCTION High performance, low cost signal sources are the key building blocks in modern communication and radar systems. Push-push oscillators consist of two sub-oscillators operating at one half of the output frequency where higher Q-factor can be easily obtained. The advantages of push-push oscillator include the extension of frequency generation capability of active device and the reduction of phase noise compared to conventional fre- quency doublers [1]. Therefore, push-push oscillators have been considered as promising method for generating high quality microwave and millimetre-wave signal. Recently, substrate integrated waveguide (SIW) technology has been widely used in the microwave and millimeter-wave device and sub-systems due to its low loss, low cost, high power capability and compatibility with standard printed circuit board (PCB) process. In particular, a lot of researches concentrated on the employment of SIW resonant cavity in oscillators. Conven- tional SIW resonators used in series-feedback oscillator are absorptive cavities [2–5] where the signal is absorbed by the cavity at the resonance frequency. To satisfy the oscillation con- dition at the desired resonance frequency, the resonator with reflective-type property is more preferable for oscillator applica- tion [6]. In [7], the SIW resonator technology has also been employed in push-push oscillator. Because the fundamental mode of SIW cavity is an even mode, in order to ensure the anti-phase operation in SIW push-push oscillator, an asymmetric anti-phase Wilkinson combiner is adopted in [8]. A double-side technology has been adopted in SIW push–push oscillator to support 1808 phase difference at the TE 110 mode [9], which is arranged on the two sides of a multilayer substrate. In addition, the out-of-phase signal can be stabilised with the higher order mode TE 210 of the rectangular SIW cavity in balanced oscillator [10]. However, the SIW resonator structures adopted in push- push oscillators also exhibit an absorptive property at the reso- nance frequency. In this letter, a differential transmission-line (TL) loaded with circular SIW cavity is proposed and utilized to implement low phase noise push-push oscillator. The differential TL exhib- its a reflective-type characteristic at the TM 110 odd mode of the SIW resonator, which acts as a phase coupling network to sup- port the anti-phase operation of two sub-circuits. Based on this differential TL, A K-band push–push oscillator is designed, implemented and measured, which exhibits good suppression of fundamental frequency and low phase noise characteristics. 2. DESIGN OF DIFFERENTIAL TL LOADED WITH CIRCULAR SIW CAVITY The proposed differential TL loaded with a circular SIW cavity is depicted in Figure 1(a). Two parallel host microstrip line ter- minated with matched load are connected to each other through a circular SIW cavity. The SIW cavity is magnetically coupled to the main TL using a current probe structure. The equivalent circuit of the differential TL loaded with SIW resonator is shown in Figure 1(b). The parallel R s -L s -C s tank represents the circular SIW cavity, which operate at the TM 110 mode. The magnetic coupling between resonator cavity and two main microstrip lines is modeled by mutual-inductance L M . This structure is just like the dielectric-resonator (DR) coupled to a microstrip line, and the signal propagation is inhibited around resonance frequency, which results in a reflective characteristic in the differential TL. Only TM modes can be supported in circular SIW cavity, and the cavity can potentially resonate at different mode simul- taneously. Because the field distribution properties of circular SIW are very similar to that of the conventional metallic circu- lar waveguide. The resonance frequency of TM nm0 mode is given by the following expression [11] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 58, No. 5, May 2016 1217