Adaptive Tuning of Cellular Phone Power Amplifier Antenna Interface

Current wireless product design principles involve designing the power amplifier (PA) and low noise amplifier (LNA) to have a 50Ω impedance independent of the antenna, which is also designed to have a 50Ω impedance as much as possible over the required frequency band. The concentration of multiple wireless applications in a single device requires the antenna to operate over more frequency bands, which affects performance. Wireless data applications introduce additional conductors and dielectrics into the antenna environment, such as wooden and metal desktops when the phone is used as a modem. These antenna environmental factors affect its performance through two ways: absorption and reflection.

Two common methods to solve the reflection effect are to use a load-insensitive PA or a continuously tunable antenna. However, the disadvantages are: although the load-insensitive PA can improve the performance to 4:1 voltage standing wave ratio (VSWR) mismatch, the noise source insensitive LNA used in the receiver part of the transceiver is not a standard supply product; the continuously tunable antenna provides ideal tuning over the channel bandwidth in a laboratory environment, but this is at the expense of increased material cost and larger circuit board area.

A compromise solution

This article proposes a compromise solution that requires less circuitry and less board area while achieving the best benefit of a complete solution, i.e. matching almost all impedances. Cellular radio antenna modules have been developed that incorporate the PA and front-end module RF electronics inside the antenna. The best match between the antenna and the RF circuitry is achieved for the antenna loading conditions. This is a low-cost approach to tunable matching circuits that handle the different loading conditions experienced by the antenna. This approach can achieve a good return loss of 20dB for these impedance matches, but not the ideal 50Ω impedance. At the same time, this approach cannot achieve a good return loss for all possible impedance matches, but can match the impedances experienced by the antenna under the above-mentioned harsh environmental conditions. This will achieve most of the performance improvements of the ideal 50Ω impedance tunable circuit while significantly reducing the cost, i.e. board area, software development cost, and BOM.

The developed tunable matching circuit can achieve good return loss for the impedance matching of the antenna under the most severe application conditions. As shown in Figure 1, simulations were performed on the circuit to observe the different impedance matching levels that can be achieved, namely 20dB and ideal 50Ω impedance range.

Figure 1. Range of a simple single-tuning element matching circuit.

Figure 2 shows the antenna impedance under six application conditions in this range.

Figure 2. Tuning circuit range with added antenna impedance

All six conditions were well handled by the tunable matching circuit. However, unlike using a more complex and expensive tunable matching circuit, the 20 dB matching criterion was abandoned in the other two conditions. More than 20 different conditions were tested in both the high and low frequency bands.

Bandwidth Capacity of Tunable Matching Circuits

The first metric of interest is the bandwidth capability of the tunable matching circuit. This metric is important for two reasons. The first reason is in cellular receivers. Although adaptive tuning is intended for the transmitter of a mobile device, it must not degrade the performance of the receiver. The receiver operates in different frequency bands in Global System for Mobile Communications (GSM) applications. The second consideration is wideband modulation applications such as W-CDMA, WiMAX, or WLAN.

As shown in Figure 3, a simple low-cost single-tuned element matching circuit has a large bandwidth capability, which can theoretically cover both the DCS 1800 and PCS 1900 bands, as shown in the upper left curve of Figure 3. However, in practice, as shown in the lower two curves of Figure 3, the return loss over the complete DCS or PCS band can be improved by 10dB using the single-tuned element matching circuit.

Figure 3 Improvement of return loss performance of tuned circuit in GSM high frequency band

This has two main consequences. The first has been mentioned above. The circuit exhibits the ability to handle wide bandwidth signals, i.e. 100MHz signals at 1.75GHz or 1.88GHz. The second consequence concerns implementation. The tuning required across the spectrum is not very different. This means that the handset is not required to make large adjustments to the tuning voltage during handoff or other frequency changes, so the dynamic response of the control loop is significantly reduced. There is no need to worry about the control loop deviating to a poor tuning voltage during part of the burst when making adjustments.

Figure 4 shows the case for the GSM low frequency band, although it is not necessary, but in theory it is possible to cover the full 800MHz and 900MHz bands at the same time.

Figure 4 Improvement of return loss performance of tuned circuit in GSM low frequency band

Although the performance improvement in the low frequency band is less than that in the high frequency band, it is still favorable. However, the return loss performance of the full 800MHz GSM or 900MHz GSM band can be improved by more than 5dB using a simple single tuned element matching circuit. Moreover, the metal element shown in Figure 4 is not an antenna. In this case the antenna is inside the phone/plastic case. The metal element is a balanced-unbalanced (BALUN) device. BALUN can eliminate the wire effect or the current radiated from the wire.

With the antenna placed on top of the phone facing away from the head and the hand resting on top of the phone, all possible impedances of the PA are presented. These impedances are all possible impedances generated by the tuning voltages applied to the tuning elements during the tuning process. When the loop detects a good matching condition, the loop stops working, so the loop is not tuned to every tuning voltage mentioned above. In addition, the loop will not over-regulate the voltage under the same conditions during frequency hopping, which may occur when switching. However, the worst case presented to the PA under this condition is a 3dB return loss. Since the matching circuit will not exceed this value at any tuning voltage, the control loop will no longer present this worst impedance. This does not affect the ability of commercial power amplifiers to withstand 10:1 VSWR at all phase angles. In addition, the control loop can be designed to have two loop dynamic responses, and the fast loop can be used first to find a good return loss, and then the loop can switch to a slower response.

Although the return loss improvement is achieved, the designer is most concerned about the overall RF performance. The tuning circuit must improve the overall efficiency of the antenna and power amplifier system while maintaining or improving the linearity performance. The antenna impedance after the tuning circuit was measured by setting the same voltage on the tuning element and connecting the power amplifier and the tunable matching circuit with wires. The test results show that at all power levels, the return loss and overall efficiency have been significantly improved with the same or better linearity. The same technology can be used for other linear modulation technologies such as 3G data cellular applications. The idea of ​​this article is to simplify and miniaturize the tuning circuit. This also has the same effect on 3G applications and can be extended to include harmonic tuning for linearity control, making these applications more efficient.

Finally, the performance was checked in an antenna measurement room with and without tuning using a simulated head and hand. This measurement room requires a continuous signal, and the power amplifier cannot withstand continuous operation at high power levels. This requires reducing the radiated power level, and under this condition, the tuning performance improvement is not obvious. However, an overall efficiency improvement of 1dB is still obtained in the three test cases, as shown in Table 1.

Table 1 Comparison of results with and without tuning

Tuning Component Characteristics

The final consideration is the tuning element used in the matching circuit. If the requirements for this element are too stringent to be achieved, the solution will be too costly to be considered. Table 2 shows the performance of the tuning element.

Table 2 Tuning element characteristics

The tuning circuit topology requires a tuning range slightly less than 10:1, which limits the supplier range, but there are suppliers that can meet the range requirements shown in the table. All other parameters meet or exceed the requirements of the tuned circuit components and cellular phone voice applications and 3G data applications. In addition, the frequency range and power handling performance of the tuning components are better than the range shown in the table. The limitations in the table are caused by the limitations of the test equipment.

Conclusion

The simplified tunable matching circuit presented in this article can improve the return loss presented by the antenna to the PA under various harsh environmental conditions in cellular applications. Compared with more expensive and larger tunable systems that can match almost any impedance to the ideal 50Ω impedance, this simplified tunable circuit can achieve most of the performance of the tunable system with only one tunable element while reducing cost and size. In addition, the circuit has sufficient bandwidth capacity for GSM as well as wideband modulation systems. RF performance testing shows that the circuit can significantly improve efficiency under various harsh test conditions while maintaining good linearity performance. When tested under head and hand blocking conditions, this tuning technique can also be used to meet the broadband industry requirements for multiple wireless applications connected to a single antenna. The ability to match the characteristics of multiple frequency bands can reduce the return loss of the antenna even in free space conditions. Finally, designing the antenna and power amplifier together still helps control the interface impedance even without tuning. This is particularly important for harmonics.