Ferrite circulators play an important role in modern wireless telecommunication systems. Non-reciprocal circulator devices generally require permanent magnets in order to magnetically polarize the ferrite material component used in these devices. Typically, the device consists of a three port microstrip guide configuration separated from electrical ground by a ferrite slab (Fig 3.1).
Figure 3.1 Schematic of a microstrip Y- junction circulator.
In Fig. 3.2, it is illustrated that the emission and reception of RF signal frequency can be done simultaneously. The signal propagates from internal port to the receiver and from the transmitter to the antenna. This non-reciprocal behavior originates from Gyrotropic media and the asymmetrical permeability tensor.
Figure 3.2 Circulator design parameters.
To design the circulator, the most important features as shown in Fig. 3.2 are the ferrite disk radius R and the angle ψ which they are related by equation (3.1)
1
sin W/ 2R (3.1)
In resonant condition, we can calculate the radius of ferrite disk by equation (3.2)
1.84 1.84 , 4 (3.2) 4 2 in in A S in S
eff eff eff
H R H H M H M k
Here k is the wavenumber, ɛeff and µeff is the dielectric constant and effective permeability of the ferrite disk.
The development of magnet-less circulators by applying recent materials science which recalled metamagnetic materials, can lead to considerably smaller and lighter components. For example using Hexaferrite material, the self-biased microstrip devices above the 10 GHz can be feasible. Among the M-type hexaferrites, barium ferrites (BaFe12O19) have greatest technology interest due to its low cost and moderate magnetic properties. A narrow and a wide band biased circulators are designed via ANSYS HFSS software based on the
barium ferrite magnetic properties. In Fig. 3.3 the dimension parameters of the circulator are shown, which are corresponding to the values mentioned in Table 3.1 and 3.2.
Table 3.1 Dimension parameters’ values of narrow band circulator.
As illustrated in Fig. 3.4, the isolation and the insertion loss of narrow band circulator were obtained to be -31 dB and -1 dB respectively at 29 GHz.
Figure 3.4 S-parameters of narrow band circulator at 29 GHz.
Parameters 2R W a Ferrite thickness (h) Copper thickness (t)
Values (mm) 0.9 0.21 2 0.25 2*10 -4 R a W 2*W Copper Silicon
A wide band circulator was designed with the sizes reported in Table 3.2. The circulator frequency happened at 70 GHz with the isolation and the insertion loss equal to -29 dB and -1.5 dB respectively (Fig. 3.5).
Table 3.2 Dimension parameters’ values of wide band circulator.
Figure 3.5 S-parameters of wide band circulator at 70 GHz.
Parameters 2R W a Ferrite thickness (h) Copper thickness (t)
Values (mm) 0.45 0.09 1 0.126 2*10 -4
3
2
1
3.2 Coplanar Waveguide Phase Shifter based on Characterized Nanocomposite Substrate
Based on the magnetic parameters of the Nanocomposite substrate and characterizing YIG NWs, a CPW was designed via HFSS. In Fig. 3.7 (a), the magnetic field amplitude is plotted. As it shown, the CPW transmission line with 50 Ω impedance, is fed with a Lumped port. The insertion loss is less than 0.4 dB and reflection is about 14 dB below and above the FMR frequency (Fig. 3.7 (b)).
Figure 3.7 Magnetic field propagation in designed CPW (a) S parameters of CPW versus frequency (b).
(a)
By applying a small external magnetic field and changing the internal magnetic H field by ±8%, the phase of S21 parameter shifts up to 30̊ degrees near 1.7 GHz as shown in Fig. 3.8. It is possible to increase the phase change by increasing the volume loading of the YIG particles or equivalently increasing the length of the nanowires.
Figure 3.8 Phase shift of CPW around 1.7 GHz.
However, by enlarging the composite substrate dimension, the phase shift increases for the same miniscule changes of the internal magnetic field. As shown in Fig. 3.9 (a), while the external field is applied in direction and opposite direction of the internal field, the phase shift around 70̊ degrees can be found in the range of frequency between 1.5 to 2 GHz. In Fig. 3.9 (b), the scattering parameters is plotted in the absence of an external DC field.
Figure 3.9 Phase shift of CPW (a), S parameters of CPW versus frequency (b).
The low insertion loss and reflection above 10 dB can be found in the wide range of frequency as shown in Fig. 3.10. However, our aim is for applications below 2 GHz in order to take advantage of the self-biased alignment of the magnetization in the nanowires. This means with no or small external magnetic fields, planar microwave devices may be feasible.
(a)
Figure 3.10 S parameters of CPW versus frequency.
Chapter 4
Conclusion
The purpose of this research is to establish the science for magnetism of YIG ferrite Nanowires necessary to design and fabricate the first generation of self-biased ferrite planar devices and circulators operating at wireless communication frequencies, especially below 2 GHz.
A variety of different techniques have been implemented to fabricate YIG NWs inside a highly ordered porous silicon membrane. it has been demonstrated that YIG NWs with a high aspect ratio can be routinely synthesized in the membrane using a sol-gel technique and a vacuum suction method; moreover, the sol-gel process has been introduced as a method leading to a high density of YIG NWs. The net saturation magnetization and the internal field of the composite substrate have been obtained from the VSM measurements and the FMR resonance conditions. The purity improvement of the ferrite material achieved using the proposed sol-gel process has also shown that the characterized magnetic properties of YIG NWs (such as 4πMs) make possible that the ferrite composite substrate
can be utilized in potential applications, such as biomedical sensing and microwave device engineering.
Based on the characterized magnetic parameters of the synthesized YIG NWs, a CPW phase shifter was designed and simulated. By applying a small external magnetic field and increasing the internal field by 8%, a phase shift up to 70 ̊ degrees can be obtained near 1.8 GHz. This implies that a new class of self-biased ferrite devices operating in the wireless communication frequencies below 2 GHz are very feasible. In comparison to the use of a plain YIG slab in which an external field of approximately 2,000 Oe is needed to bias the slab, no external field is required to bias ceramic wires of YIG embedded in a Silicon wafer. Therefore, these special YIG NWs substrates have a great potential in the development of a new class of self-biased ferrite devices below 2 GHz.
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