Tuning components enable better wireless performance
Specifically, as more bands are added to the network to support the increasing demand for data capacity, the components in the wireless system must support wider bandwidths with lower loss. As a result, the latest mobile terminals require more filtering and RF front-end components that offer better linearity. By deploying mobile devices with tunable components, operators can improve the link between the radio and the base station, get a better data connection, reduce the number of required base stations, and more effectively manage a network.
Over the past several years, numerous technological approaches have shown promise to satisfy the need to tune everything from mobile handset antennas, to filters and frequency synthesizers. In today’s wireless market, designers can immediately reduce circuit complexity by using tunable matching networks to extend the frequency range and performance of a single antenna, or by implementing tunable low-pass, band-pass, and notch filters with selectable rejection ratios. A viable tuning technology can dramatically improve wireless radio performance.
In addition to improving radio performance, tuning circuitry can reduce radio complexity. This is particularly important for military radios, software-defined radios, and UHV/VHF radios. Modern radios require multiple RF filters to support various bands. These applications can benefit from tunable low-pass and band-pass filters, tunable notch filters, and production centering. In the past, tunable filters have been implemented using a switchable filter bank to select the desired frequency band. Today, a large, switchable filter bank can be replaced by a single tunable DTC filter, reducing radio complexity and cost.
Success in the high-volume cost-sensitive wireless market requires any selected technology to be low-cost, repeatable, reliable, and low power. One such approach has been taken by Peregrine Semiconductor. By applying proven, patented UltraCMOS® process and HaRP™ switch technologies, engineers at Peregrine developed DuNE™ tuning technology, a new design methodology used to develop Digitally Tunable Capacitors (DTCs). These DTCs are monolithic solid-state, digitally controlled variable capacitors targeted for the 100 MHz to 3 GHz range.
Tuning advantages
While wireless devices have long benefitted from tuning, the addition of multiple radios (Bluetooth, RFID, Wi-Fi, and cellular) and the increasing number of frequency bands now make tuning a necessity. Multiple modes and bands require an antenna with broadband coverage. This is nearly impossible to achieve in a small form factor because handset antennas must trade off RF performance for form factor (the smaller the antenna, the lower the performance). Poor handset antenna performance leads to reduced network coverage area, low data rates, dropped calls, and poor battery life. Unfortunately, small antennas often have high mismatch loss on band edges, which reduces the power delivered to the antenna and the radiated power. This, in turn, reduces RF performance.
The ways in which users hold a mobile handset can also degrade antenna performance. This causes increased filter losses, reduced power amplifier output, and increased current consumption. Adding tuning circuitry, such as a tunable matching network, improves radio performance, which translates to “more bars” on the mobile handset. It also simplifies radio complexity, reducing the number of required components and improving cost and footprint.
For example, the antenna in Figure 1 loses 84% of power delivered to it simply due to mismatch at 700 MHz. If this cellular device is required to radiate at a certain power level, it must, therefore, boost its output power to compensate for the energy lost due to mismatch. This reduces transmission efficiency and battery life. Fortunately, this problem can be addressed using tuning technologies, providing a mechanism where energy lost due to mismatch can be reduced.
After adding a tunable matching network, the power delivered to the antenna is increased for a fixed output power for the cellular device, demonstrating an improvement of 0.5-5 dB (see Figure 1). This translates to better radio efficiency and longer battery life. Further, in comparing a device with a tunable matching network to a network without tunable matching, at 700 MHz the device with the matching network can achieve nearly 3x the data rate at the same distance from a base station.
Evolution of tuning
Historically, tunable RF components have been large, mechanical, and costly. The earliest variable capacitors were realized by sizeable mechanical capacitors that were rotated by a motor (or manually). Then, trimmer capacitors were used to fine-tune circuit performance in a production environment. Today’s growing need for tuning technologies did not take the industry by surprise, so several alternatives have been in development to satisfy anticipated needs. Currently, the tuning technologies with the most promise include MEMS switched capacitor banks, BST ferroelectric capacitors, and DTCs based on FET switches.
Selecting a tunable technology
Since tuning may be new to many designers, it is important to understand the figures of merit for a successful implementation in a wireless application. For example, applications such as mobile handsets and military radios require a tunable capacitor with a performance level that is challenging to achieve:
- High power handling: +33 to +42 dBm;
- High linearity: harmonics less than -36 dBm, IMD3 less than -105 dBm;
- Low-loss: Q = 30-100;
- Capacitance: 0.5-20 pF,
- Tuning ratio 3:1-10:1;
- Low total power consumption <1 mA;
- Fast switching speed: 10 μs;
- Reliable, rugged, suitable for high volume production
Specifically, for antenna applications, the tunable component must provide a wide impedance tuning ratio and a high quality factor, while handling RF power levels up to 2 W and meeting the stringent harmonics and IMD distortion requirements for 3G/4G operation [1]. Given the maximum GSM transmit power of +33 dBm and the voltage multiplication that can occur in matching networks under mismatch conditions, the tunable component must also linearly tolerate RF voltages of up to 30 Vpk, corresponding to over +39 dBm in 50 Ω.
To support cellular GSM RX/TX tuning, the tunable component must be reliable over 1012 switch cycles and have a <10 μs switching speed. The cellular handset application environment also requires small size, low cost, high reliability, and high yield for economical high volume mass production. Until the development of DuNE DTCs, the lack of electrically tunable reactive components meeting these very tough specifications prevented the adoption of tuning matching networks in mobile wireless devices.
Integration is another important consideration in the selection of tunable component technology. In addition to economies of scale, the silicon-on-sapphire process used in DuNE DTCs enables the monolithic integration of multiple tunable components on one die together with analog circuitry and a shared digital control interface. The die can then be flip-chip mounted on module laminate. This is a significant improvement over GaAs processes that require a separate CMOS control chip and wire-bond assembly. A module containing the single die implementation of multiple DTCs with a serial interface, coupled with SMD inductors, enables a low cost implementation with a reduced module footprint and assembly cost (see Figure 2).
DuNE™ DTCs
The DTC shown in Figure 2 is based on a solid-state, monolithically integrated CMOS switched capacitor bank, where each switch is realized as a stack of field effect transistors (FETs) instead of a single FET. This stacking approach enables high RF power handling and high linearity, and it uses the same RF switch technology with an insulating sapphire substrate that is in high volume production in 2G/3G/4G handsets.
The DuNE DTC is programmed through a 2-or 3-wire serial interface, designed using existing UltraCMOS switch FETs combined with MIM capacitors. The DTC can be used either in a series or shunt configuration, enabling the use of this component in many different tunable circuits.
Tunable matching networks
Figure 3 shows a schematic for a single-path, tunable matching network using DuNE DTCs. This particular design targets multi-band LTE/WCDMA/GSM applications on UMTS-FDD Bands I, II, III, IV, V, VIII and XII. It is implemented with three tunable components and biasing and control circuitry integrated onto a single die. This design controls the DTCs through a serial interface. By employing a reconfigurable coupled resonator topology widely used in band-pass filters and impedance-matching networks, this tunable matching network provides wide impedance coverage in the operating bands of 698 – 960 MHz and 1710 – 2170 MHz. Each DTC in this design is implemented using 4-bit resolution as a tradeoff of resolution versus number of individual tuning states available for the network (4096 for 3 x 4 bit DTCs). This device measures approximately 3 mm2, so it is suitable for integration into a 3.5 x 3.5 mm module that includes passive components, such as the high-Q wire-wound inductors.
To evaluate the performance of the tunable matching network in the system, a metric called the power delivered improvement (PDI) has been proposed [2]. PDI is a measure of the performance improvement (in dB) that the cellular device will achieve with a tunable matching network for a given antenna VSWR and phase angle. Figure 4 shows the PDI for various load VSWR conditions and frequencies. When the antenna impedance is at high VSWR (12:1), using the UltraCMOS tuning circuitry improves power delivered to the load by 4 dB or more. The break-even point in power delivery occurs when the load VSWR is approximately 2:1, below which the tuner dissipative losses are higher than mismatch loss without tuner.
Click image to enlarge.
Antenna performance can be significantly improved with the addition of a tunable matching network. This will increase the power delivered to and radiated out of the antenna. Initially, we can expect to see tunable matching networks as well as low-pass and band-pass filters, tunable notch filters, and production centering for wireless devices. In addition, the reduced radio complexity through tuning will benefit military radios, software defined radios, and UHV/VHF radios.
References
[1] T. Ranta, J. Ella and H. Pohjonen, “Antenna Switch Linearity Requirements for GSM/ WCDMA Mobile Phone Front-ends”, 8th European Conference on Wireless Technology Proceedings, Paris, France, Oct. 2005, pp. 23–26.
[2] R. Whatley, T. Ranta, and D. Kelly. “CMOS Based Tunable Matching Networks for Cellular Handset Applications,” IMS2011 Proceedings.
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