Optimizing Smartphone Signals with RF Switches

Smartphones Represent the forefront of RF personal communications and one of the most challenging RF product designs. These third generation (3G) cellular multimode, multiband devices may operate in three or four frequency bands when based on EDGE/GSM (2.5G) standards, and up to three or four frequency bands when based on 3G WCDMA/HSPA standards. As smartphone versions evolve and WCDMA and 3GPP Long Term Evolution (LTE) standards are adopted, the number of operating bands may increase to 10 or 11. One of the key components to support so many frequency bands and operating modes in a small device is the radio frequency (RF) switch, which is used in many ways throughout the front-end circuitry of a smartphone.

To ensure wireless communication at high data rates, whether through cellular or mobile connectivity networks and external devices, a typical smartphone (Figure 1) integrates many antenna components and multiple wireless data streams. Amplifying, filtering, and switching the required RF signals requires the use of a variety of RF components ( the RF switch section is highlighted in blue in the figure).

Modern smartphones are complex devices that combine computing and communications technologies in a handheld package. These devices must provide a variety of consumer and business-related functions through an intuitive operator interface. The close proximity of electromagnetic (EM) field-generating computer circuits and wireless circuits that are bound to receive other EM signals is just one of the design challenges facing smart handheld devices. The complexity of smartphones also extends to the RF analog/mixed-signal area of ​​these multiple RF transmitters.

Figure 1: This block diagram shows a typical smartphone architecture that includes embedded Bluetooth, GPS, and FM radio capabilities in addition to multi-band cellular.

Looking at the wireless modules inside these new mobile phones reveals the inherent complexity of multiple radio signals operating and coexisting in a small physical volume. This complex signal routing and RF switching requirements pose a great challenge to smartphone design. The RF switch configurations in smartphones vary widely, from relatively simple single-pole double-throw (SPDT) configurations to more complex single-pole ten-throw (SP10T) configurations, sometimes with even higher numbers of throws. The driving force behind the development of all these switches is the richness of radio communication technology and the multiple frequencies and antennas in smartphones.

A variety of RF switching components are used in cellular communication systems (Figure 2). These switching components include the main antenna transmit/receive (T/R) switch, which is used to use the main device antenna for cellular transmit and receive functions in a time division duplex (TDD) transceiver, such as the transceiver used to provide full-band GSM service. In addition, the band mode switch is mainly used to direct wideband CDMA signals from the output of the power amplifier to the frequency selective band duplexer. Diversity switches are used to connect a second or diversity cellular antenna. Diversity antennas are very common in data card applications and are becoming more popular in smartphone applications, especially for data-centric systems such as LTE.

Figure 2: Cellular handsets require a large number of RF switches for antenna switching, frequency band switching, and diversity switching.

The main function of the main antenna switch is to connect the over-the-air RF radio communication signals to the main transmit and receive functional blocks in the communication modem. There are many different implementations of this antenna switch, ranging from single-pole seven-throw (SP7T) to single-pole ten-throw (SP10T), depending on the number of RF bands supported by the smartphone.

The primary antenna switch is tasked with maintaining signal linearity and providing isolation between the transmit and receive chains while minimizing insertion loss . The transition from previously voice-only communications to wireless data communications has led to the development of higher-order modulation schemes such as Orthogonal Frequency Division Multiplexing (OFDM) and the closely related Orthogonal Frequency Division Multiple Access (OFDMA). These complex modulation schemes produce waveforms with a wide range of amplitude variations, resulting in signals with high peak-to-average ratios (PAPRs), which in turn place greater dynamic range requirements on the components that process them, and require RF switches with outstanding linearity to minimize distortion in the RF signal path .

In order to maintain good linearity at higher and higher signal power levels, most antenna switches use increasingly complex design topologies to meet these more stringent linearity requirements. For example, a charge pump is often integrated into the antenna switch module to boost the battery voltage to control the field effect transistor ( FET ) that makes up the switch. This approach is usually a reasonable compromise because it reduces the need for larger FETs, improves switch insertion loss and isolation performance, and enhances the robustness of the switch compression point. The switch compression point is defined as the output power point where the gain is compressed by 0.1dB or 1dB (P0.1dB or P1dB).

Depending on the system partitioning approach, the switch may include GSM transmit harmonic frequency filtering. This configuration is often referred to as an antenna switch module (ASM). Some advanced switches may also include surface acoustic wave (SAW) filters , making for a complete switch/receive filter in a micromodule package. Whether it is an ASM or a switch without GSM filters, or a complete transmit/receive switch and SAW module, the main channel switch always includes the main switch and control logic functions in a compact module.

High-throw-count switches for smartphone applications are typically manufactured using a GaAs pseudomorphic high electron mobility transistor ( pHEMT ) process, although these products may also be developed using standard silicon CMOS and silicon-on-insulator (SOI) technologies. Each approach has advantages and disadvantages, and the choice is usually determined by the end user's requirements.

Skyworks Solutions has recently developed several high-throw-count switches, including a single-pole, ten-throw switch for multi-band operation. For example, the single-pole, ten-throw (SP10T) switch model SKY13362-389-LF supports up to five 3G cellular communication bands. This switch is controlled by four CMOS/TTL-compatible control voltages, integrates a CMOS decoder and GSM transmit filter, operates from a single +2.5V to +3.0VDC power supply, and operates from 0.4GHz to 2.2GHz. The typical insertion loss from the antenna port to the transceiver port is 0.5dB, the insertion loss from the antenna port to either GSM transmit port is 1.1dB, and the insertion loss from the antenna to the receive port is also 1.1dB. Isolation At least 17dB, up to 32dB (depending on which two ports are measured). The switch features a 5μs switching speed and an input third-order intercept point of +61dBm or better when tested in UMTS mode. The switch is available in a 3.0x3.8mm QFN package.

The front-end module model SKY14152 launched by Skyworks Solutions includes an integrated insulator silicon switch and controller, a GSM transmit filter and a single-band differential SAW receive filter. The switch is a single-pole eight-throw (SP8T) component with an operating frequency range from 0.4GHz to 2.17GHz. The transmit and receive ports of the switch can withstand at least +27dBm of power, and the typical insertion loss is below 0.7dB. The isolation between any two ports is at least 30dB. The SKY14152 module is optimized for data card applications and is supplied in a 20-pin, 3.2x3.2mm multi-chip module (MCM) package.

In some handheld applications, it is useful to allow direct connection to the phone battery (with the correct supply voltage range). Future designs may also move from a GPIO-based control interface to an industry-standard programmable interface to accommodate high-throw-count antenna switch module applications.

As 3G multimode and multiband architectures become more prevalent, the need for band/mode switches is increasing. Highly linear band and/or mode switches can switch wideband code division multiple access (WCDMA) signals from the power amplifier (PA) output to the appropriate frequency duplexer before the signal is transmitted through the antenna switch module. The requirements for band or mode switches depend on the selected PA architecture. In single-mode architectures, since the GSM transmit path and the linear wideband CDMA path are separate, the band switch is only required to pass the WCDMA signal from the PA output to the duplexer. In multimode, multiband signal path architectures, since GSM and WCDMA signals are transmitted through the same PA, the mode switch must be able to handle the higher power saturated GSM modulated signal. Both PA architectures generally have a high-band and a low-band amplifier chain. In single-mode architectures, the switch is only used to select one of multiple wideband CDMA duplexers.

Depending on the number of wideband CDMA bands that need to be supported, the band or mode switch is usually a single-pole double-throw (SPDT) or single-pole three-throw (SP3T) configuration. However, in designs where space is at a premium, this function can be combined into a double-pole four-throw (DP4T) switch or a double-pole five-throw (DP5T) switch, which can handle up to five bands. The requirements for these switches depend on the RF signal levels that the switch must handle. The goal is to provide sufficient linearity without adding switch-induced signal distortion to the system.

Band switches can be used for both pre-amplifiers and post-amplifiers as bands are added or merged in multi-band platforms. The choice between pHEMT and SOI is usually determined by the actual control voltage. pHEMT technology is a good choice when the control voltage is 2.8V or above. SOI technology is more suitable when the RF subsystem uses lower voltage logic signals (such as 1.8V). Unlike high-throw-count switches, low-throw-count GaAs devices are usually not used with charge pump circuits due to size constraints.

Skyworks Solutions offers band/mode switch products including both SOI and GaAs pHEMT devices for low logic levels (1.8V control logic). SOI switch devices include SPDT models SKY13330-397LF, SP3T models SKY13345-368LF, and double-pole, four-throw (DP4T) models SKY14155-368LF. GaAs pHEMT integrated circuits (ICs) include SPDT switches models SKY13351-378LF, SKY13320-374LF, and SKY13321-360LF, SP3T switches models SKY13309-370LF and SKY13317-373LF, and SP4T switches model SKY14151-350LF.

In emerging wireless handset and data card applications, a second or diversity receive antenna and associated receiver signal chain are often used. Diversity receive techniques are used to increase data rates, especially where data throughput is most critical. Due to cost and power considerations, practical diversity techniques are only applied to the receive side of the end device and do not include the transmit signal path. Depending on the number of wideband CDMA bands required to be supported, an appropriate switch is required at the diversity antenna. In most cases, an SP3T, SP4T, or single-pole five-throw (SP5T) switch is required. These RF switches have lower power handling requirements than the switches used for the main path antenna because they do not need to pass very high power transmitter signals.

Skyworks has designed several products for diversity applications. The SKY13322-375LF SP4T switch, SKY14153-368LF SP4T switch, SKY13345-368LF SP4T switch and SKY14151-350LF SP4T switch are some of the most popular devices suitable for diversity applications. Future products will also include the SP5T switch SKY13358-388LF. For example, the GaAs FET SP4T switch model SKY13322-375LF can operate in the 0.1GHz to 6.0GHz frequency range, with a typical insertion loss of 0.45dB at 1GHz and a typical isolation of 28dB at 1GHz. The switch has a 1dB compression point (P1dB) of +30dBm and is packaged in a 10-pin micro lead frame dual package (MLPD) with a size of 2x3mm. The diversity switch model SKY14151-350LF SP4T can operate in the frequency range of 0.1GHz to 2.5GHz, with an isolation of 29dB at 1GHz and a typical insertion loss of 0.4dB at 1GHz. This switch also integrates a decoder and uses only two TTL-compatible DC control lines to control the four switch ports. The switch can handle 900MHz RF power up to +34.5dBm and 1.8GHz RF power up to +31.5dBm. This is a symmetrical switch optimized for GSM/WCDMA/EDGE, which can operate on a single power supply of +2.5 to +3.0VDC and is supplied in a 3x3mm QFN package.

In addition, the 4G standard provides for a second antenna assembly for multiple-input multiple-output ( MIMO ) operation. In simpler implementations, MIMO can improve the overall received signal strength by selecting the stronger received signal or combining the signals received by two independent antenna ends, thereby facilitating increased data rates. In more complex implementations, MIMO can be used to simultaneously transmit two (or more) independent data streams between the base station and the mobile phone. The introduction of LTE, the fourth generation cellular standard, will drive more diversity path configurations as smartphones expand the data throughput capabilities of portable devices.

In mobile connectivity, switches are critical for signal routing and connecting different wireless signals to the antenna. The Bluetooth radio, Wi-Fi connectivity, and mobile TV reception in smartphones all require additional switching functionality. The attach rate (the rate at which more features are added to the phone) for mobile connectivity in smartphone-class devices has increased significantly in recent years. The attach rates for Bluetooth and FM radio are greater than 50%, and the attach rates for Global Positioning System (GPS) and Wi-Fi are approaching 20%. The introduction of chipsets that combine multiple common mobile connectivity technologies (Bluetooth, Wi-Fi, and FM radio) in a single chip has further driven the popularity and penetration of these additional features.

Mobile wireless connectivity options are generally implemented in two ways: designed using discrete components and chipsets, or provided by third-party module manufacturers. Regardless of the implementation details, there are multiple applications for RF switches on the mobile connectivity side of smartphones.

Figure 3: This diagram shows some mobile connectivity applications for smartphones.

In embedded WiFi applications, the frequency band coverage of the handset manufacturer will determine the RF switch requirements. If limited to 2.4GHz operation, a switch device of SPDT or SP3T is generally required, depending on whether Bluetooth signals are present. The switch interface depends to some extent on the chipset manufacturer of the WiFi solution. Chipsets that provide a regulated 2.8V output to the switch are generally suitable for pHEMT devices. An increasing number of new WiFi chipsets are optimized for portable applications that use low-level logic voltages in the 1.8V range. In cases where only 1.8V is used, an insulator silicon switch may be a better choice to avoid the performance degradation that may occur when pHEMT devices operate at voltages below 2.7V.

When choosing GaAs pHEMT devices, Skyworks has a rich product portfolio suitable for this type of application. Recommended options include SKY13323-378LF SPDT and SKY13351-378LF SDPT (suitable for 5.8GHz band) switches, SKY13317-373LF SP3T switches (suitable for 5.8GHz band). If lower operating voltage is required, the insulator silicon devices SKY13330-397LF and SKY13345-368LF can be used.

The SKY13323-378LF SPDT switch is a pHEMT GaAs FET device that operates from 0.3GHz to 3.0GHz with a typical insertion loss of 0.35dB and a typical isolation of 27dB at 3GHz. The device is available in a 6-pin, 1x1mm MLPD package. The SKY13351-378LF SPDT switch operates from 2GHz to 6GHz with an insertion loss of 0.35dB and an isolation of 24dB at 2.45GHz. The device is also available in a 1x1mm MLPD package. The device achieves a rise/fall time of 40ns and an input third-order intercept point (IP3) of +32dBm at 2.45GHz. The SKY13317-373LF SP3T switch can be used to control signals from 0.1GHz to 6.0GHz, with an insertion loss of 0.5dB at 2.5GHz, 0.9dB at 6GHz, and 25dB isolation at 6GHz. The switch is available in a 1.5x1.5mm MLP package with a 1dB compression point level of +27dBm, making it ideal for high linearity applications.

With the launch of WiMAX networks, more devices can work on WiMAX and cellular networks. WiMAX devices usually send higher output power than corresponding WiFi network devices, so higher linearity switches are required to avoid signal distortion. Skyworks devices suitable for WiMAX include SKY13370-374LF, SKY13299-321LF and SKY13348-374LF.

Improvements in battery technology, display screens, and processing power/speeds have made it possible for consumers to enjoy a richer experience on today’s smartphone mobile platforms. New applications are emerging that can take advantage of the new generation of devices, with streaming video and video highlights becoming prominent features of these applications. A major challenge facing broadcast TV reception in handheld devices is the physical size of the antenna operating at very low frequencies. Mobile TV and FM radio chipsets typically cover the frequency range from tens of MHz to 800MHz. Given the traditional relationship between the physical size of the antenna and the wavelength of the received signal, mobile phones with FM radio or TV reception capabilities need to rely on some type of RF tuning capabilities to receive these relatively low frequency signals using compact embedded antennas. By switching in impedance adjustment and impedance matching circuits and components, RF switches can be used to adjust the electrical length of embedded antennas so that mobile phones using these embedded antennas can meet consumer expectations for good performance at a reasonable device size.

FM radio is one of the most popular wireless connectivity technologies and has evolved from receiving only to including FM transmit capability. The explosive growth of in-car and home MP3/FM radio pass-through devices has put more demands on FM transmit capability. Typically, SPDT switches are used to select between FM receive and FM transmit. The SKY13322-375LF (SP4T) and SKY13309-370LF (SP3T) switches are designed to meet the requirements of mobile TV applications. The SKY13323-378LF is ideal for both FM transmit and receive applications.

Modern smartphones are complex technological innovations that effectively integrate communications and computing functions, simplifying many different applications for consumers and commercial enterprises alike. To provide consumers with error-free, high data rate communications at all times, modern mobile phones need to use multiple frequency bands and configurations, which places higher demands on high linearity and low signal distortion. Complex architectures require routing of RF signals, which is a key driver for the increasing popularity of MPMT RF switches.