Mobile 5G device antenna tuning revealed

火辣西米秀

Mobile 5G device antenna tuning revealed [Copy link]

Designing multiple mobile antennas? Facing efficiency degradation? Here’s a guide to developing products with superior antenna performance using COFF capacitor devices to improve system efficiency and range.

5G technology is driving a significant increase in the number of new frequency bands that mobile antennas must support. Due to the complexity of new phone designs, phone designers are using more and more aperture tuners on a single antenna. Adding aperture tuners helps optimize overall antenna performance across frequency bands, but sometimes at the expense of antenna efficiency. If antenna efficiency and antenna performance across frequency bands are not balanced, overall device performance and range suffer.

Every antenna has an inherent resonant frequency at which maximum antenna efficiency is achieved. Aperture tuning is achieved by placing a shunt capacitor (to lower the resonant frequency) or a shunt inductor (to raise the resonant frequency) across the antenna. Using multiple capacitors and inductors, the antenna can be tuned to multiple frequencies using antenna tuner switches, as shown in the figure below.

RON and COFF Interpretation

Aperture tuning mainly uses tuner switches and tunable capacitors. The main quality factors of these switches are on-state resistance (RON) and off-state capacitance (COFF), as shown in the figure below. For tunable capacitors, it is important to have a wide range of tuning capacitance and a good Q factor (quality factor). RON and COFF can significantly affect antenna efficiency. RON has a greater impact at low voltages; COFF has a greater impact at high voltages; and a switch layout strategy with low RON or low COFF can optimize tuning for different frequencies. [attach
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In the off state, the COFF of the aperture tuner affects the capacitive loading on the antenna, thereby lowering the resonant frequency. The higher the COFF of the tuner, the greater the deviation of this frequency from the natural resonant frequency of the antenna.

Figure 1 below shows the impact of the COFF of a single-pole double-throw (SPDT) switch on the simulated efficiency of an inverted-F antenna (IFA). The baseline measurements shown were taken without the SPDT placed in the tuning position. After adding the SPDT, the COFF was set to 100 fF and 200 fF at each port.


Figure 1.

COFF Antenna Tuning

When switching from the baseline antenna to the low COFF switch, a frequency shift of 40 MHz and a 0.3 dB drop in peak efficiency were observed. When switching from the low 100fF COFF to the high 200fF COFF switch, an additional 40 MHz shift occurred and the peak efficiency dropped by 0.85 dB. The final frequency shift compared to the baseline was 80 MHz and the total efficiency dropped by 1.15 dB.

Figure 2.

To counteract the frequency shift caused by the tuner COFF, one port of the SPDT can be used to switch in an inductor to re-tune the antenna to its natural resonant frequency. In this case, the antenna is tuned twice: once by the tuner’s COFF and once by the inductor, which recalibrates the antenna to its original resonant state. However, this approach comes at the expense of efficiency and bandwidth, as shown in Figure 3, which shows the efficiency of the antenna when retuned to its original resonant frequency of 890 MHz.

Figure 3.

While the COFF of the SPDT can be considered, the high COFF switch results in a 0.47 dB drop in peak efficiency from the baseline measurement. Aperture tuning is necessary to facilitate antenna band swapping in mobile phones.

Our mobile devices are clearly becoming more and more complex. This places some challenges on the antenna and its capabilities to meet all the needs of the user. Unfortunately, not just any antenna tuning component can meet the needs of these complex device systems. As discussed in this blog post, using a high COFF tuner that does not meet the requirements can result in significant frequency shifts, which detune the antenna and reduce the overall antenna efficiency. Therefore, when balancing antenna efficiency and band requirements, a low COFF switch is the best choice.