After finishing the design of the 2.4 GHz current driver, the lower band performance at 2.4 GHz for the dual-band current driver is achieved. In the following, the antenna performance in the higher band at 5.2 GHz will be designed through the parameter study of the 5.2 GHz current driver.
Frequency (GHz)
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Return Loss (dB)
0
5
10
15
20
25
30
N2 = 5 N2 = 4 N2 = 3
Fig. 4.8. Simulated return loss for various finger number N2 of the 5.2 GHz current driver.
Fig. 4.8 shows the simulated return loss for different finger numbers N2. As seen in the figure, the center frequency is shifted higher with a frequency shift of about 40 MHz when N2 varies from 5 to 4. However, the impedance matching is deteriorated when N2
changes from 4 to 3. This is due to the fact that the finger number not only determines the capacitance but also has an effect on the coupling strength between the top layer and the bottom layer of the current driver, which can control the impedance matching. Unlike the property that the 2.4 GHz current driver demonstrated before, the 5.2 GHz current driver is more sensitive to the variation of the coupling strength. This leads to the abrupt impedance deterioration when N2 is set to be 3. In this case, the figure number must be larger than 3 to achieve the adequate coupling strength. Moreover, as expected, the performance in the lower band is unrelated to the 5.2 GHz current driver.
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Frequency (GHz)
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Return Loss (dB)
0
5
10
15
20
25
30
Ws2 = 2.1 mm Ws2 = 2.3 mm Ws2 = 2.5 mm
Fig. 4.9. Simulated return loss for various slot width Ws2 of the 5.2 GHz current driver.
The simulated return loss for the 5.2 GHz current driver with various slot width Ws2 is given in Fig. 4.9. As shown in the figure, about 400 MHz frequency shift in the higher band is observed. The center frequency moves lower from 5.29 GHz to 4.9 GHz when the slot width increases from 2.1 mm to 2.5 mm. Since the variation of the slot width has no obvious effect on the coupling strength between the top layer and bottom layer, good impedance matching maintains despite the frequency shift caused by the slot width. Also, the performance in the lower band is not changed with the variation of Ws2.
As mentioned earlier, the slot length of the current driver can affect the center frequency. Fig. 4.10 is used to demonstrate this property, where the simulated return loss is shown as a function of slot width Ls2. The center frequency moves higher from 4.72 GHz to 5.29 GHz when the slot width decreases from 3 mm to 2.5 mm. However, it is observed that the impedance matching becomes worse when the slot width is lowered to 2.3 mm. Similarly, this is related to the coupling reduction as already mentioned since the figure length of the printed capacitor varies with the slot length. Therefore, the slot length cannot be designed too small. Otherwise, the required coupling strength is hard to achieve. This will caused serious impedance deterioration. Moreover, to design the dual-band current driver with a compact size, the slot width and length is chosen as close as
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possible, which are 2.5 mm and 2.1 mm respectively with consideration about the impedance matching and size reduction.
Frequency (GHz)
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Return Loss (dB)
0
5
10
15
20
25
30
Ls2 = 3 mm Ls2 = 2.5 mm Ls2 = 2.3 mm
Fig. 4.10. Simulated return loss for various slot length Ls2 of the 5.2 GHz current driver.
Frequency (GHz)
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Return Loss (dB)
0
5
10
15
20
25
30
w2 = 0.3 mm w2 = 0.5 mm w2 = 1 mm
Fig. 4.11. Simulated return loss for various strip width w2 of the 5.2 GHz current driver.
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Frequency (GHz)
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Return Loss (dB)
0
5
10
15
20
25
30
l2 = 0.9 mm l2 = 1.2 mm l2 = 1.7 mm
Fig. 4.12. Simulated return loss for various arm length l2 of the 5.2 GHz current driver.
The effect of strip width relating to the coupling strength in the structure of the current driver is investigated,. The good impedance matching is obtained by properly designing the coupling strength. Fig. 4.11 presents the simulation results of return loss for different strip width w2 of the 5.2 GHz current driver. As expected, the strip width has the effect on the impedance matching. As shown in the figure, the impedance matching varies slowly with the decreasing strip width w2 from 1 mm to 0.5 mm, but it deteriorates rapidly when the slot width decreased to 0.3 mm. The strip width is chosen to be 0.5 mm, which shows the best impedance matching. Also, the strip width w2 has little effect on the performance at 2.4 GHz.
In addition to the strip width w2, the arm length l2 also affects the coupling strength of the top layer and the bottom layer for the 5.2 GHz current driver. The more is the arm length l2, the stronger is the coupling strength. Fig. 4.12 shows the simulated return loss with arm length varies from 0.9 mm to 1.7 mm. The impedance matching varies rapidly with various arm length. To achieve good impedance matching, the coupling arm length must be carefully designed for adequate coupling strength, which is 1.2 mm in this case.
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From all the discussion shown above, it indicates that the 5.2 GHz current driver can be easily designed to meet the higher band operation of the dual-band current driver.
Basically, the property of the 5.2 GHz current driver is similar to the one of the 2.4 GHz current driver except that it is more sensitive to the coupling strength between the top layer and bottom layer. As long as the required coupling for the 5.2 GHz is met, the center frequency can be tuned by the slot width Ws2, slot length Ls2, and figure number N2
with little effect on the impedance matching.