An integrated tunable isolator based on NiZn film fabricated by spin-spray plating

Due to the demand of compact, lightweight, ultra-wideband, minimized power consumption, and large power-handling capability, the tunable devices are desired for the modern communication systems, radars, and metrology systems. In virtue of decoupling, impedance matching, and protection of transceiver, tunable isolators play a key role in microwave components and systems.^1,2

A ferrite isolator has three necessary components: a transmission line, a ferrite sample, and a dc magnetic field applied to the ferrite. In addition, a ferrite isolator with these three components can be built based on waveguide and planar transmission line technologies according to at least three rather different principles: resonance absorption, field displacement, and Faraday rotation.^3–10 Unfortunately, both field displacement and Faraday rotation isolators can hardly have tunability via magnetic field bias.

Most recently, due to the advantages of rapid growth rate, thick dense films, and the low temperature processing for SSP process,^11,12 NiZn ferrite films with in-plane anisotropy, which have been fabricated via SSP process, have been applied to fabricate tunable microwave devices.^13–17 Lin et al.¹³ reported a dual H- and E-field tunable bandpass filter with ultra-wideband isolation based on the magnetostatic surface wave with non-reciprocity characteristics, the operating frequency was tuned from 3.78 to 5.27GHz with external in-plane magnetic fields range from 100 to 400Oe, with an insertion loss S₁₂ of 1.73 to 3.42dB and an in-band isolation S₂₁ of more than 13dB. Yang et al.¹⁴ described a tunable ultra-wideband phase shifters with NiCo/Rogers Duroid 6010 multilayer films, which showed a phase shift of 384° at 20GHz. Consequently, the integration of a ferrite on a semiconductor chip or printed circuit board provide the possibility of compatibility with the monolithic microwave integrated circuit (MMIC) and radio frequency integrated circuit (RFIC).

In this work, taking into account the miniaturization and integration factors, we designed and fabricated an integrated tunable isolator with NiZn thick film fabricated by SSP process.

With an external magnetic field of 360Oe, the Ni_0.27Zn_0.06Fe_2.67O₄ films with thickness of 10μm were directly plated on the Rogers TMM 10i printed circuits board (ε=9.8 and tanδ= 0.002) with a thickness of 0.381mm with the SSP process at 90°C.

XRD patterns were identified with the Philips diffractometer. Microstructure was observed using scanning electron microscope JEOL JSM-6490L. Magnetic hysteresis loop was carried out by a Lake Shore 7410 vibrating sample magnetometer at room temperature. ΔH was measured with electron spin resonance spectrometer at X-Band. The s-parameters of the isolator were measured via Agilent PNA E8364A.

Fig. 1(a) shows the schematic diagram of the tunable isolator. A NiZn ferrite thick film was then directly plated on the Rogers TMM 10i printed circuits board to cover the comb-like part, with length 10.5mm, width 4mm and thickness 10μm. The comb-like microstrip line shown in Fig. 1(b) is a novel means to form RHCP and LHCP. With regard to the traditional transmission line, the current would flow along y axis direction at the edge of the transmission line, and magnetic field distribution would be dominated by H_x and H_z components. However, in term of the planar comb-like microstrip transmission line structure, the current flowing has been forced along the new edges, which formed H_y and H_z components of rotating magnetic field.¹⁸ Thus, the right-hand or left-hand polarization direction of the magnetic field is depended on the in-plane dc magnetic bias. Simultaneously, for forward and backward directions of propagation, attenuation constants are affected by the location of NiZn thick films which are placed either above or underneath the planar comb-like microstrip transmission line, therefore, the location of NiZn ferrite thick films should be optimized to achieve the excellent insertion/isolation performance.

In Fig. 2(a), the XRD patterns of NiZn ferrite thick films agree well with the powder diffraction file of JCPDS No.08-0234, and the XRD patterns indicated the formation of single cubic spinel phase. Fig. 2(b) represents the average grain size was about 200nm. The normalized magnetic hysteresis loops is shown in Fig. 2(c). The saturation magnetization 4πM_s is 4800Gs, and the inplane coercivity H_c is 22Oe. In addition, the FMR line width ΔH shown in Fig. 2(d) is about 190Oe.

In order to clear and definite the magnetic field distribution, the Ansoft high frequency structure simulator (HFSS) 14.0 full wave simulator has been applied to analyze the RHCP and LHCP. As for either the NiZn ferrite thick films above the transmission line or underneath, the circular polarization of RHCP and LHCP have been shown in Fig. 3. Corresponding to the NiZn ferrite thick films above the transmission line, RHCP was observed in NiZn ferrite thick films and LHCP in Rogers TMM 10i substrate, while, the opposite phenomenon was obtained for the case of the NiZn ferrite thick films underneath the transmission line. At the FMR frequency, for both forward and backward propagation, compared with the LHCP waves, the coupling of RHCP waves and NiZn ferrite would be strong, which contributes to the non-reciprocal effect and isolation characteristic.

Fig. 4(a) shows the backward and forward propagation in the case of NiZn ferrite thick film was placed above the microstrip transmission line. The insertion loss (S₁₂, LHCP) and isolation (S₂₁, RHCP) are 1.7dB and 4.0dB at 6.7GHz, respectively. In addition, in virtue of the edge effect from the interconnection, a side lobe with an opposite insertion loss of 0.8dB and isolation of 1.2dB characteristics can be obtained at 7.5GHz. Correspondingly, as to the NiZn ferrite thick film was placed underneath the microstrip transmission line in Fig. 4(b), the insertion loss (S₂₁, LHCP) and isolation (S₁₂, RHCP) are 3.0dB and 3.5dB at 6.7GHz, respectively. Similarly, there is a side lobe with an opposite insertion loss of 0.9dB and isolation of 1.3dB characteristics at 7.4GHz. Clearly, both the NiZn ferrite thick film was placed above and underneath the microstrip transmission line can result in non-reciprocal effect, which is consistent with the circular polarized microwave magnetic field distribution, however, compared with the isolation of two cases, the former is more beneficial for improving the isolation performance.

The designed tunable isolator with a planar comb-like microstrip transmission line was then fabricated and measured. The NiZn ferrite thick film, with a dc magnetic bias field of range from 0.5 to 3.5kOe, was placed above the comb-like microstrip transmission line. The simulated and measured insertion loss S₁₂ and isolation S₂₁ are shown in Figs. 5(a) and (b), respectively. With the increase of the dc magnetic bias field, the resonance of s-parameters shift to higher frequency. Meanwhile, the isolation rises with the increasing dc magnetic bias field, however, the insertion loss also increase, which is due to the enhancement of the FMR effect with the increase of magnetic bias field. When the dc magnetic bias field is 3.5kOe, corresponding to the simulated insertion loss S₁₂ of 3.6dB and isolation S₂₁ of 11.0dB, the measured insertion loss S₁₂ and isolation S₂₁ are 6.5dB and 11.6dB at 17.57GHz, respectively. Both the forward and backward return losses of S₁₁ and S₂₂ shown in Fig. 5(c) are greater than 10dB for 3.5kOe dc magnetic bias field, which indicated that the mission energy was dissipated in the NiZn ferrite thick film instead of reflecting back to port 1 and port 2. The resonant frequencies of the tunable isolator with a planar comb-like microstrip transmission line can be tuned by the dc magnetic bias field for the Kittel’s equation.¹⁹ 

In summary, an innovative tunable isolator with a planar comb-like microstrip transmission line has been designed, fabricated, and demonstrated. Microstrip transmission lines with planar comb-like structure was revealed to form LHCP and RHCP circularly polarized microwave magnetic field, the different coupling effect with the NiZn ferrite thick films attributes to the non-reciprocal characteristics. The non-reciprocal ferromagnetic resonance absorption leads to 11.6dB isolation and 5.78dB insertion loss at 17.57GHz with magnetic bias field of 3.5kOe. Furthermore, the central frequency was tuned from 5.63 to 17.57GHz with external in-plane magnetic fields range from 0.5kOe to 3.5kOe. The proposed tunable isolator with a planar comb-like microstrip transmission line prototype can have potential applications in microwave components and systems.

Rongdi Guo is grateful for financial support from the China Scholarship Council (201406070032) and the knowledge guide from Nian Xiang Sun’s Lab in Northeastern University.