диафрагмированные волноводные фильтры / 2a1f6d27-bb3a-4244-9c79-446ee6a138a3

Balun bandpass filter based on multilayer substrate integrated waveguide power divider

J.N. Hui, W.J. Feng and W.Q. Che

A compact multilayer substrate integrated waveguide (SIW) balun bandpass filter is proposed. A 1808 reverse phase characteristic between the input/output ports can be easily achieved by the middle metal-ground of the SIW power divider. Four inductive posts are added to the multilayer SIW power divider to realise a passband. Good in-band balance performance (amplitude and phase imbalance are less than 0.35 dB and 28) over the passband are achieved for the balun bandpass filter.

Introduction: As one of most important RF front-end functional passive devices, the balun bandpass filter integrating balun and bandpass filter is a necessary component for converting a balanced signal into an unbalanced one for a specified bandpass characteristic in RF/microwave communication systems. In the past few years, several techniques have been proposed to design narrow/dual-band, and wideband balun bandpass filters [1–4]. The characteristics of balun and bandpass filtering can be easily realised in these filter structures. However, the application of these filters is limited because the optimisation of the in-band balun performances is not convenient to realise.

In the authors’ previous works [5, 6], two planar substrate integrated waveguide (SIW) magic-T structures with wide band and simple design based on E-plane power dividers were proposed. Compared with the T/Y-junction planar SIW power divider structures [7], the size reduction of the circuits is almost 50 and 75%, respectively. In this Letter, a compact balun bandpass filter with four inductive posts based on the multilayer SIW power divider [5] is designed. A high-order passband (centre frequency 12.5 GHz, bandwidth 4.8%) with high selectivity and extended stopband for the balun filter can be achieved.

Fig. 1 3D view, and top view of the E-plane SIW power divider; electric field distribution at A-A′ plane; dominant mode coupling of multilayer SIW power divider; simulated magnitudes, phases of E-plane SIW power divider

a 3D view of E-plane SIW power divider b Top view of E-plane SIW power divider c Electric field distribution at A-A′ plane

d Dominant mode coupling of multilayer SIW power divider e Simulated magnitudes of E-plane SIW power divider

f Simulated phases of E-plane SIW power divider

70 mm× 20 mm, 7.2lg × 1.2lg W50 ¼ 2.8 mm, W51 ¼ 1.4 mm, L1 ¼ 11.0 mm, L2 ¼ 37.0 mm, L3 ¼ 6.0 mm, a ¼ 12.0 mm, d ¼ 0.6 mm, w ¼ 1.0 mm, tl1 ¼

5.0 mm, tw1 ¼ 5.12 mm, tl2 ¼ 5.0 mm, tw2 ¼ 5.11 mm, s ¼ 2.54 mm, 1r ¼ 2.65, h ¼ 1.0 mm, tan d ¼ 0.002

Filter design: Figs. 1a and b illustrate the 3D view and the top view of the E-plane SIW power divider, consisting of two substrate layers with a metal ground located in the middle of the structure with two ports (port

2, 3) on top and bottom planes. The electric field at the A-A′ plane and the dominant mode coupling of the SIW power divider are described in Figs. 1c and d. We may note that the E-field orientations of the two ports (port 2, 3) are different from each other, indicating a 1808 phase difference. At the same time, the power from port 1 can be divided equally at ports 2 and 3. Figs. 1e and f show the simulated results of the SIW power divider, the return loss S11 is greater than 15 dB (9.2–17.5 GHz), while the phase difference between S21 and S31 is approximately 1808 within 8–17.5 GHz.

Fig. 2 Top view of proposed SIW balun bandpass filter; inductive post in SIW and equivalent circuit

a Top view of proposed SIW balun bandpass filter b Inductive post in SIW and equivalent circuit

W50 ¼ 2.8 mm, W51 ¼ 1.4 mm, L1 ¼ 11.0 mm, L2 ¼ 37.0 mm, L3 ¼ 5.2 mm, L4 ¼ 8.9 mm, L5 ¼ 10.0 mm, L6 ¼ 4.0 mm, L7 ¼ 6.0 mm, a ¼ 12.0 mm, d ¼ 0.6mm, d1 ¼ 0.86 mm, d2 ¼ 2.2 mm, w ¼ 1.0 mm, tl1 ¼ 5.0 mm, tw1 ¼ 5.12 mm, tl2 ¼ 5.0 mm, tw2 ¼ 5.11 mm, s ¼ 2.54 mm

Fig. 2a shows the top view of the proposed balun bandpass filter. Four inductive posts are added to realise a Chebyshev filter for the balun bandpass filter structure [8, 9]. The equivalent circuit of the inductive post is shown in Fig. 2b, and the equivalent circuit parameters jXa and jXb can be calculated from the S-parameters of the equivalent circuit as follows (Z0 ¼ 50 V):

The distance between each inductive post (Ln) can be given by [9]:

lg0 is the guided wavelength of the centre frequency of the passband. The simulated results of the SIW balun filter are shown in Figs. 3a and b. The simulated 3 dB fractional bandwidth is 4.37% (12.3– 12.85 GHz), the minimum insertion loss is 3.4 dB for each path, a three-order passband with return loss greater than 13.5 dB and over 35 dB upper stopband (13.7–16 GHz) is thus realised. In addition, the amplitude and phase imbalances of the balun bandpass filter are less than 0.15 dB and 1.88, respectively.

Fig. 3 Measured and simulated magnitudes, amplitude and phase imbalances of SIW balun bandpass filter; photograph of proposed SIW balun bandpass filter

a Measured and simulated magnitudes of SIW balun bandpass filter b Measured and simulated amplitude and phase imbalances

c Photograph of proposed SIW balun bandpass filter