Today's mobile phones need to not only support cellular frequencies, but also non-cellular features such as those used for mobile TV, Bluetooth, WLAN, and location-based services. As the real estate available for the antenna shrinks with each handset generation, antennas are wrapped around camera and keyboard circuitry and re-pathed, which causes them to lose efficiency. Some of this lost performance can be recovered with antenna tuning, which is when the system uses dynamic impedance tuning techniques to optimize the antenna performance for both the frequency of operation and environmental conditions. However, the challenge is that any successful tuning scheme needs to be low-loss, highly linear, able to handle very high RF signal levels, and consume low power.

Antenna Tuning Architectures
An open-loop antenna tuning system is often used when a passive antenna can no longer meet performance requirements, such as when bandwidth requirements increase, handset designs becomes more complex, and there is less space for antennas. In an open-loop system, the tunable element fine-tunes the performance of the antenna at set frequency bands and modes of operation (Figure 1), taking into account static information such as transmit/receive frequencies, modulation schemes, or use case. Open-loop systems do not measure the operation of the antenna in real time, however, so they cannot take into account environmental conditions.

Environmental conditions are very important in a mobile device, and they change regularly as users walk, drive, or move their fingers. To address these conditions, antenna designers can use adaptive closed-loop antenna tuning; in this case, a mismatch sensor provides consistent feedback by tracking the antenna's operation.

The mismatch sensor compares the VSWR (the amplitude of the power that is reflected back to the antenna) to the transmit power, and it makes an adjustment to the impedance tuning circuitry. The tuning algorithm forces the tunable elements to constantly track and adjust to the optimal setting (Figure 1) in all use cases.

Tuning Challenges
Theory is helpful, but the biggest roadblock to implementing adaptive antenna tuning in cellular handsets has been the absence of a high-performance, electronically-tunable reactive component that is low loss and has an adequately wide tuning ratio. In terms of "high performance," the most challenging component requirements are power handling and linearity. For example, GSM antennas typically must handle transmit power up to +33dBm, but under mismatch conditions, the tunable component actually needs to handle RF signal levels measuring a surprising 30Vpk or +40dBm.

The quest for the best tuning material has resulted in a fair amount of research over the years. For instance, some researchers have worked with micro-electromechanical systems (MEMS) and ferroelectric materials technologies (such as barium strontium titanate; BST) in order to implement tunable antennas and filters. These techniques show promise, but they still face significant technological and manufacturing hurdles. In order to adequately address the need for antenna tuning, designers need a technique that already supports high-volume manufacturing, and, preferably, uses proven technology.

Antenna Complexities
Antennas are complex devices, and antennas embedded in mobile handsets are no exception. Since the RF transceiver of the mobile handset is designed for 50Ω impedance, the handset antenna would, ideally, also demonstrate 50Ω impedance across the entire frequency band. In reality, this rarely occurs because the laws of electromagnetics dictate that small handsets have an inherently narrow antenna bandwidth, poor matching, and low radiation efficiency.

Antennas, then, are usually designed at non-50Ω impedance across the whole band, with a typical VSWR between 2:1 or 3:1 for multi-band antennas. The antenna's impedance is also affected by other factors, such as how the phone is being held (the so-called "head and hand effect"). The subscriber's body also causes absorption of the power, further limiting the antenna's radiation efficiency. Generally speaking, handset antennas operate at VSWR of better than 3:1, but if, for instance, the subscriber rests a finger on top of the antenna radiator, this mismatch could increase up to a 9:1. Considering that all devices in the signal chain were designed to operate at VSWR of 1:1, this could be problematic. Figure 2 traces the impact of the "hand effect," or the detuning of the antenna when a hand is near the antenna radiator. This effect changes the resonant frequency of the antenna, causing it to be badly mismatched at its intended operating frequency.

When an antenna port is in mismatch, the RF performance degrades quickly. Specifically, if the antenna is in a state of VSWR=3:1 (a common design target for multi-band antennas), approximately 1.25dB of power is immediately lost due to reflection. If the VSWR reaches 5:1, then the mismatch loss is 2.55dB. Such a mismatch also causes the power amplifier output power to drop, which further reduces radiated power. A mobile handset's narrowband duplex or receive filters can also experience ripples on the passband when they are not terminated to their characteristic impedance, which could cause up to 2dB of loss in addition to the mismatch loss. In Figure 3, the green line shows how a typical WCDMA duplexer transmit filter performs at 50Ω impedance. The red line is the specification, and the blue lines show the filter response when the antenna has VSWR of 5:1 across all phase angles; note that insertion loss reaches 5dB at its worst.

Head and hand effects, mismatch loss in the antenna, ripples in the RF filter passband, and reduction in the PA output power all combine to severely challenge the amount of power that can be radiated out of the handset's antenna. The result of this detuning is a decrease in battery life, degradation of the link budget (range) and call quality, and an increase in the number of dropped calls. To address this concern, many service providers have developed "Total Radiated Power (TRP)" and "Total Isotropic Sensitivity (TIS)" specifications. To meet these specifications, it is necessary to test the handset by simulating actual use cases (with head and hand) rather than simply performing a conducted measurement in 50Ω environment or testing the phone in free space.

Adaptive antenna tuning promises to be a good method for satisfying these new TRP and TIS specifications because an antenna tuner can force the antenna to appear 50Ω despite environmental effects, allowing the remainder of the system to operate under optimal conditions. Even though an antenna tuner causes some additional insertion loss, adaptive antenna tuning will significantly improve the overall insertion loss from the tuner input to antenna input as compared to an uncorrected situation (Figure 4), which will lead to performance improvements.

For antennas in multi-band transmit/receive systems, then, what is needed is an adaptive antenna tuning circuit that guarantees performance all the way to the band edges, actively tracks detuning, and retunes the antenna quickly. This tuning circuit must be extremely linear to avoid generating harmonics or inter-modulation distortion. It should be small, rugged, and have a tuning ratio of at least 3:1, and the whole circuit should consume less than 1 mA. In order to effectively improve performance, it must have low insertion loss, so the quality factor (Q) should be a minimum of 50.

New Digitally Tunable Capacitors
Designers at Peregrine Semiconductor (San Diego, CA) have developed DuNE technology, a patent-pending design methodology that leverages the company's proven UltraCMOS™ process and HaRP™ design innovations [1]. DuNE digitally tunable capacitors (DTCs) were designed to satisfy the needs of antenna tuning requirements and feature high-Q capacitors, a serial interface, low bias voltage, and high linearity. Available with flip-chip packaging, the DuNE DTC for GSM/WCDMA handsets measures 1.36x0.81mm (Figure 5)

Measured Performance
A major advantage of UltraCMOS field effect transistors (FETs) is that, unlike bulk CMOS and SOI technologies, they can be stacked to handle high RF power levels, due to a fully-insulating sapphire substrate. This allows power handling to be scaled to more than +40dBm to handle high RF power levels without degrading the Q or tuning ratio.

DuNE DTCs have the potential to be developed for numerous applications and operating conditions. Currently, they can be designed with capacitance values from 0.5pF to 10pF, with tuning ratios ranging from 3:1 to 6:1 with 5 bits or 32 states of resolution (Figure 6). In these DTCs, Q values can be set from 40 to 80 at 1 to 2GHz (Figure 7). In addition to >+38dBm of power handling in 50Ω (Figure 8) and switching speed of better than 5s, these new DTCs demonstrate power consumption of about 100μA (which is orders of magnitude lower than some alternative tuning technologies).

Based on proven building blocks and process technologies that are already shipping in the millions of units per week to the handset industry, DuNE technology is poised to dramatically change the options available to handset designers. All parameters of the DTC (capacitance values, tuning ratio, quality factor, and power handling) can be changed by circuit design instead of materials engineering, making it very fast to generate new application-specific designs. DuNE DTCs for cellular as well as mobile TV applications are already sampling, with volume production planned for 2009-2010. This latest technology innovation has led to the monolithic integration of the complete adaptive antenna tuner system, which promises to dramatically improve the performance of antennas in new handset designs.

Reference
[1] "UltraCMOS Process Technology" http://www.psemi.com/content/foundry/foundry_process_tech.html and "HaRP Technology Innovations" http://www.psemi.com/content/foundry/foundry_harp.html

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