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Harnessing the Benefits of Adaptive RF Tuning
A high-level open-loop RF tuning solution called the Radio Antenna Frequency Tuner (RAFT) can be used to optimize antenna impedance.By Dr. Paul McIntosh, Paratek, Inc.
While today’s portable wireless devices embody dizzying arrays of features and capabilities, they still are beholden to the basics of RF transmission theory. The
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Figure 1. The basic difference between an open-loop and a closed-loop solution |
paradigm is essentially the same as sticking a set of rabbit ears atop a 50” plasma HD television, because the antenna ends up being the limiting element. We all inherently recognize that wireless handhelds are able to maximize their potential only when a very good impedance match exists at the antenna. We also recognize that this is very tough to accomplish when mobile devices are used and carried in an almost limitless number of ways. The comfortable 50W target is an awfully tough one to maintain when environmental changes and variable use-cases come into play. The handset that performs flawlessly in the laboratory can often become a poor performer in actual use.
What To Do?Fortunately, there has been a considerable amount of attention paid to the whole concept of ‘RF tunability.’ The wireless industry supply chain — including carriers, handset makers, and device makers — recognizes that achieving workable, cost-effective RF tunability will be a major differentiator. Having the ability to bring RF tunability to the handset is a remarkable differentiator. First, carriers can lessen their infrastructure build-out by offering self-tuning handsets. Better-performing handsets mean fewer tower build-outs and lower capital investment for carriers. Second, handset makers can enjoy higher factory yields because the RF tuning function can correct for parameters such as Total Radiated Power (TRP) that cause a phone to fail. The iconic ‘slim and thin’ handsets that consumers favor most will not come at the expense of performance, since small-footprint antennas can instantly become better-performing ones. Third, consumers can enjoy an overall better user experience thanks to RF-tunable handsets. Battery life increases, talk time increases, and link-margin increases. Far fewer dropped and missed calls lead to happier customers who stick with their carriers.?
With so many obvious advantages all across the wireless industry, RF tunability is not a new technical challenge. In fact, several different approaches have been tried, each with its own advantages. Some approaches are mechanical in nature and others are electronic, but the solutions that rise to the top are those that are commercially viable (ready now), cost-effective, and meet the criteria seen as most meaningful to the wireless industry. Size, economy, and performance are crucial factors that RF tuning solutions seek to optimize, as are the RF loss and reliability.?Foundational Material Gains Traction in WirelessA purely electronic tuning solution using a semiconductor-like manufacturing process is gaining traction in the wireless industry. The foundational material for the production of these highly miniaturized RF-tunable circuits is a proprietary-doped combination of Barium, Strontium, and Titanium Oxide (BST) called ParaScan™. The ParaScan material outperforms pure BST by exhibiting a breakdown voltage that is 1.7 times higher. This leads to better device reliability and low-loss characteristics, which leads to Q values at or above 100 in many phone applications. Finally, the leakage current of ParaScan devices is reduced by an order of magnitude over pure BST, leading directly to higher reliability and lower power consumption.
The devices formed from this thin-film material are highly miniaturized passive tunable ICs (PTICs), which are purely electrical in nature and accomplish their RF tuning function without mechanical components. They exhibit tuning ratios of 3:1 and operate over a voltage range from 2 to 20 V. PTICs are produced using a semiconductor-like process, which makes them economical. But because they are not semiconductor diodes, they don’t exhibit any unwanted forward conduction characteristics under high RF swing. PTICs have the ability to ‘hot switch,’ which means that they can change their capacitance values while loaded with an RF signal. Also, since the devices are easy to bias and work electrically rather than mechanically, there are no latch-up or stiction issues. PTICs are typically around 0.5 mm x 0.5 mm in size while Micro-Electro-Mechanical Systems (MEMS) devices, by comparison, can reach 2 mm x 2 mm in size.?Electrical vs. Mechanical RF Tuning SolutionThe debate between electrical and mechanical RF tuning solutions continues, because there are clear merits to both approaches. However, there is no debate that RF-tunable handsets can revolutionize the wireless industry. One of the most vexing problems facing the entire wireless industry is how to make handsets perform optimally regardless of operating conditions or use-cases. For example, placing a finger over the antenna can cause an impedance mismatch. Placing the phone in a pants pocket, a purse, or even against the head can detune the handset as well. Symptoms of this condition include dropped or missed calls, reduced battery life, and shorter talk time. A poorly-performing power amplifier, reduced Total Radiated Power (TRP), and reduced Total Isotropic Sensitivity (TIS) are other undesired effects of an impedance mismatch.
Because PTICs have the ability to change capacitance, they become a linchpin for producing the best possible impedance match at the antenna. A fully-integrated RF tuning solution can be created by combining the PTICs with Paratek’s High Voltage Application Specific Integrated Circuit (HVASIC). The HVASIC provides the step-up voltages from the lower system voltages and provides a controlled output voltage that drives the PTICs. It also provides an SPI bus for communicating with the phone. PTICs can be used in an antenna tuner in either a closed-loop or an open-loop design (see Figure 1.)RAFT Open-Loop Antenna Tuning SolutionA closed-loop solution comprises an RF tuner section along with the HVASIC and the sense and control circuitry that provides the feedback mechanism necessary for adaptive RF tuning. An Adaptive Impedance Matching Module (AIMM) is one example of a closed-loop, fully integrated product. An open-loop solution would comprise the RF tuner section and the HVASIC, but omit the sense and control circuitry. An example of this type of open-loop antenna-tuning solution is Paratek’s Radio Antenna Frequency Tuner (RAFT).
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Lacking the sense and control circuitry, RAFT is a less complex solution and typically quicker to deploy. Using different tuning states, which are sets of voltages on the PTICs, it optimizes the impedance match based on instructions received from the handset.
The RAFT circuit can have as many different tuning states as the application calls for, which gives designers a wide degree of flexibility. For instance, transmit and/or receive frequency bands can be split into sub-bands, and optimal tuning states can be established for each sub-band. Also, different use cases can be addressed by monitoring inputs that indicate in which physical configuration the handset is currently being used. For example, a flip phone has a detector that monitors its ‘closed’ or ‘open’ position. This input could be used to adjust the tuner to a different tuning state. Similarly, the presence of audio in the earpiece speaker could be used to determine if the handset is positioned next to the users’ head. Slider phones have similar detectors indicating the physical configuration of the handset which would allow the tuning network to adjust accordingly.
Any or all of these different tuning states can be defined and utilized by the handset designer in order to meet specific performance objectives. For a particular handset having quad-band GSM and UMTS WCDMA capabilities and only one physical configuration (candy bar shape factor), it may be desirable to define 18 different tuning states as shown in Table 1. In order to utilize these 18 tuning states, the handset software would need to identify which frequency band the radio was currently operating in (GSM, UMTS, or WCDMA), and which physical configuration it was in.
The physical configuration can be determined by the presence of earpiece audio. If audio is present, the phone is assumed to be held by the head. If audio is not present, it is assumed to be in free space. Frequency band information is available from the handset’s baseband.
In order to use the 18 states optimally, the handset baseband would write an SPI command to the high voltage ASIC before every transmission or reception. Specifically, in a GSM/GPRS or EDGE system, the appropriate message would be sent to the ASIC prior to every transmit burst and prior to every receive time slot. For WCDMA in the UMTS band, for example, the appropriate SPI command would be written to the ASIC prior to any voice or data call initiation as well as prior to paging channel monitoring. These commands would send the appropriate DAC settings needed to set the passive tunable ICs to the correct DC voltages and thus accomplish the chosen tuning state for that particular band and use case. These correct voltages would be determined during the design process for the particular handset. Correspondingly, optimal tuning states would be defined by the specific DC voltages applied to the tunable ICs.
Additionally, designers can choose to further segment transmit or receive bands into additional tuner states if desired. Here, the handset baseband controller takes into account the specific channel the radio is operating on and sets the tuner to the appropriate sub-band of channels (defined by the handset designer).
Paul McIntosh is the director of engineering at Paratek Microwave Inc., Nashua, NH. E-mail Paul at pmcintosh@paratek.com.
Paratek Microwave Inc.
http://www.paratek.com
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2012
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