New mobile phones are being developed with the aim of integrating ever more frequency bands and operating modes while at the same time mini-mizing power consumption. This is the only way of ensuring that a mobile phone can meet all worldwide standards. In mid-2008, eleven frequency bands were in operation for W-CDMA systems, five of them also used for GSM (Table 1).

TABLE 1: FREQUENCY BANDS IN CURRENT USE FOR MOBILE PHONES

Status 08/2008

The frequency bands listed in the Table are used in the various regions of Europe, Asia and the USA. Their combination allows the use of a mobile phone in all these regions.

The combination of these bands and operating modes requires complex RF front ends, because each frequency band needs its own specific hardware. This means that the number of components as well as the space requirement on the circuit board increase, as does the power dissipation of the RF front end.

At the same time, mobile phones are being equipped with an increasing number of additional functions such as cameras, MP3 players, radios and TV tuners. As mobile phones are becoming ever smaller, the antennas incorporated in them must also be more compact.

Telephone users detune the antenna

Currently, planar antennas acting as a resonance circuit are largely used for this purpose. Their drawback is that their near field reacts with excessive sensitivity to external effects such as interactions with the telephone user. These change the antenna impedance considerably, with a correspondingly strong impact on the transmit and receive quality. Various telephone features such as flip or slider phones, movable keypads and displays further complicate the antenna’s performance because the varied common-ground loads also affect its impedance.

Figure 1 illustrates the response of the antenna’s input impedance to interactions with the user. The presence of the user’s hand in the antenna’s field of radiation reduces its resonant frequency, thus detuning the antenna.

	FIGURE 1: THE BODY EFFECT ON AN ANTENNA
Objects in the near field of the antenna lead to a reduction of the resonant frequency and thus detune the antenna.

State-of-the-art antennas are consequently developed with input impedances that - even under the worst conditions - do not exceed a voltage standing wave ratio (VSWR) of 3.5:1. This corresponds to a loss of about 1.6 dB or 30 percent of the reflected power at the antenna. If the larger number of duplexers and the corresponding switches are also considered, power is dissipated in the entire front end including the antenna, thus significantly shortening the standby life of the battery.

Fixed and tunable matching networks

The fixed matching networks previously used between the RF front end and antenna only permit precisely defined antenna impedances to be matched. They are also used to compensate small changes in antenna impedance.

In addition, fixed matching cannot compensate large impedance changes such as changes in the active resistance of the antenna impedance by a factor of four or a reactance increase from 3 to
50 Ω.

Tunable matching networks are a solution to this problem. They have the critical advantage of changeable impedance behavior. If, in addition, a feedback controller is implemented, the entire system can react adaptively to all impedance changes of the antenna. Such an adaptive tuning unit consists of four functional units (Fig. 2).

	FIGURE 2: FUNCTIONAL UNITS OF AN ADAPTIVE MATCHING NETWORK
Adaptive matching networks allow continuous correction of antenna impedance.

The function principle: a detector initially measures the transmitted RF signal. An algorithm uses the result to calculate – in real time – whether and which changes are needed in the adaptive matching circuit of the antenna, and passes this information on to a DC/DC controller. This driver sets the required voltages at the actuator (varactor) and forces the change in the impedance matching, which is carried out by varying the varactor capacitance. This process is repeated in steps until the desired impedance, for example 50 Ω, has been reached. All four functional units needed for this process can be integrated in an RF module.

The varactors currently in use are based on four distinct technologies: BST (barium strontium titanate), CMOS, semiconductor-based varactors and RF-MEMS.

Advantages of RF-MEMS technology

RF-MEMS offer many advantages over competing varactor technologies, especially linearity and power stability, but also a larger tuning range, giving this technology a very broad – indeed almost universal – range of applications.

EPCOS uses an electrostatically variable capacitive RF-MEMS switch (Fig. 3). Application of a DC voltage causes a movable plate – the top electrode – to switch between “open” and “closed” states. In the closed state, the top electrode comes into contact with a dielectric layer to produce a capacitance of a few picofarads in combination with a bottom electrode. In the open state, in contrast, the capacitance is very small, no more than a few femtofarads. The RF-MEMS switch thus toggles between a state of “high capacitance” and one of “low capacitance.” The relationship between these two states is known as the on/off ratio.

	FIGURE 3: OPERATING MODE OF AN RF-MEMS
The spacing between top and bottom electrodes may be variably set, thus changing the capacitance and with it the impedance.

A single capacitive RF-MEMS switch has a quality Q of up to 250 at 1 GHz (Fig. 4). This value clearly exceeds the results of other technologies by a factor of between 3 and 5.

	FIGURE 4: Q-CURVE OF AN RF-MEMS
With a value of up to 250, the quality of an RF-MEMS outperforms comparable technologies by a factor of 3 to 5.

To implement a switchable capacitance array, several switches are connected in parallel (see Fig. 5). As a rule, the switching process is binary coded. The use of five switches permits 32 capacitance values. Thanks to the large turn-on/turn-off ratio of an individual MEMS element, large tuning ratios can also be obtained.

	FIGURE 5: PRINCIPLE OF RF-MEMS ARRAYS
The required capacitances are set by binary-coded parallel circuits.

The overall tuning ratio is about 10:1. Such a high value cannot be achieved with BST- or semiconductor-based varactors (for instance with components having a hyper-abrupt doping profile).

First prototype successfully tested

A demonstrator was used to examine the functionality of the adaptive matching circuit for the antenna (Fig. 6). It consists of the functional units shown in Fig. 2 integrated in a single module.

	FIGURE 6: RF-MEMS DEMONSTRATOR
The complete adaptive matching network was implemented in a single module.

A simple series-connected LC matching network compensates the complex component of the variations in the antenna impedance. A binary weighted 5-bit RF-MEMS array is used for this purpose. A high-voltage driver generates the MEMS bias voltages, and the mismatch information is derived from the phase of the matched input impedance. A feedback loop brings the matched input impedance to the desired value. In the example shown, the control algorithm was implemented with hardware: the algorithm may be programmed in a microcontroller to increase flexibility.

Fig. 7 shows the measured changes of a planar-inverted F-antenna (PIFA) caused by the user.

	FIGURE 7: IMPEDANCE CHANGES OF THE ANTENNA
Various user interactions produce antenna impedance values that deviate from ideal values.

Selection of a suitable antenna allows only the complex (reactive) component of the input impedance to be changed – so that the real (resistive) component remains approximately constant. Interaction with the user makes the antenna characteristic more inductive, thus changing the resonant frequency. A series-connected capacitive RF-MEMS array with a tuning ratio of about 10:1 can compensate this strongly inductive response and thus correct the antenna impedance.

Fig. 8 shows the corrected antenna impedance (blue). Without adaptive antenna matching, the impedance would be strongly inductive (red). In this figure, the impedance of the unmatched antenna varies between 50 Ω and 50 * (1+j) Ω, which corresponds to a VSWR of 1:1 or 2.6:1 respectively. At this latter value, 20 percent of the power is already reflected and converted to heat. This power dissipation significantly shortens the service life of the battery.

	FIGURE 8: IMPEDANCE MATCHING WITH RF-MEMS
The use of tunable matching networks keeps the impedance of the antenna in the optimal range (blue line).

The use of an adaptive antenna-matching unit equalizes the VSWR to values of about 1.2:1, which corresponds to a reflected power of about one percent.

Table 2 summarizes the measured performance. The total power consumption of the adaptive tuner is currently about 4.4 mW, but can be reduced to values of less than 1 mW in the future.

TABLE 2: PERFORMANCE OF THE IMPLEMENTED RF-MEMS MODULE

The next step is to design a platform for future antenna-matching modules. Its first version will be controlled by a microcontroller, thus ensuring that the adaptive antenna-matching module operates autonomously in a standard mobile phone. The tuning network will be more complex in order to obtain a larger tuning area and allow operation of a greater number of different antenna types. The power dissipation and space requirement will also be minimized.

Authors:
Dr. Edgar Schmidhammer, Vice President R&D, SAW Mobile Communication
Maurice de Jongh, Senior Scientist, Product Development, SAW Mobile Communication