Design of a Compact Ka-band Balanced Mixer Based on a Novel Wide-band Equivalent Circuit of the Schottky Diode
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Design of a Compact Ka-band Balanced Mixer Based on a Novel Wide-band Equivalent Circuit of the Schottky Diode Jinchao Mou & Weihua Yu & Yong Yuan & Xin Lv
Received: 15 November 2010 / Accepted: 10 March 2011 / Published online: 24 March 2011 # Springer Science+Business Media, LLC 2011
Abstract A compact Ka-band balanced mixer based on a novel Schottky diode model is presented in this paper. According to its physical structure, a novel 3D electromagnetic model of the Schottky diode is proposed. Meanwhile, a wide-band equivalent circuit is built, which takes all high frequency effects existing in the diode into account. All the parasitic reactances are extracted from the electromagnetic model-based S-parameters up to 110 GHz. Based on the proposed equivalent circuit, a Ka-band balanced mixer is designed and optimized, where bandpass filters with open stubs and a lowpass filter based on defected ground structure cells are used. The measured results show that the conversion loss is below 9 dB from 30 GHz to 40 GHz with the minimum of 6.7 dB at 35 GHz. The normalized size of the mixer is only 3.3λg ×3.3λg, whereλgis the wavelength at 35 GHz. Keywords Schottky diode . Balanced mixer . Wide-band equivalent circuit . Defected ground structure
1 Introduction Millimeter-wave mixers are commonly used in receivers and radiometers for imaging systems [1]. Due to high response speed and non-cryogenic condition requirement, GaAs Schottky diodes are fascinating and widely used in millimeter-wave mixers [2, 3]. As the operation frequency increases, the conventional equivalent circuit of the Schottky diode can’t describe its characteristics well because all parasitic reactance elements except the junction capacitance are simply represented by a shunt capacitance. One solution is to study high frequency effects existing in the diode and build an improved equivalent circuit in which all the parasitic elements are taken into account. Instead of testing, a 3D electromagnetic (EM) model is built and analyzed utilizing finite element method (FEM). A wide-band equivalent circuit up to 110 GHz is determined. J. Mou : W. Yu (*) : Y. Yuan : X. Lv Laboratory of Microwave Communication and Electronic Systems, Beijing Institute of Technology, Beijing 100081, People’s Republic of China e-mail: [email protected]
J Infrared Milli Terahz Waves (2011) 32:466–481
467
Based on the proposed diode’s wide-band equivalent circuit, a Ka-band balanced mixer is designed and optimized. Parallel coupled bandpass filters (BPFs) with open stubs and a lowpass filter (LPF) based on defected ground structure (DGS) cells are adopted. The mixer’s size is reduced to 28 mm×28 mm. The mixer shows the conversion loss of 6.7 dB∼9 dB over the bandwidth of 30 GHz∼40 GHz.
2 Schottky diode and its model Figure 1 shows the photograph of the planar Schottky diode studied in this paper, whose structure has been widely used in millimeter-wave and submillimeter-wave applications [4– 6]. As shown in Fig. 2, the epitaxial GaAs structure upon the semi-insulating (S.I.) GaAs sub
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