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Enabling next-generation mobile TV handsets

Posted: 18 Dec 2008 ?? ?Print Version ?Bookmark and Share

Keywords:mobile TV? RF? broadband?

Power handling requirements
VSWR is not the only performance parameter of concern for a mobile TV antenna. In fact, when a mobile TV antenna and a GSM cellular antenna are embedded in the same phone, the mobile TV tuning element must handle high levels of coupled GSM power. Isolation between the GSM and the DVB-H antennas inside a mobile phone can be as low as 7dB, and GSM antennas transmit at power levels up to +33dBm. So, when the GSM transmitter is operating at full power, the tuning element could experience coupled power levels up to +26dBm. If the tuning circuitry cannot withstand these high power levels, then the coupled power could modulate the tuning, changing the center frequency of the mobile TV antenna or generating high levels of harmonics.

It might seem like a logical conclusion to switch off the GSM transmitter when the mobile TV receiver is being used, or engineer a time-scheduling method to avoid simultaneous operation. These are not good solutions for a number of reasons. First, the GSM handset transmits periodic location updates to its base stations, and this would disrupt mobile TV reception. Second, in some countries, the handset sends billing information over the cellular line when the mobile TV service is in use. Finally, the user might simply receive a phone call while watching TV. Alternatively, the handset designer could use a tuning method that allows simultaneous operation of GSM and mobile TV, but the tuning method must withstand challenging power handling requirements up to +26dBm or higher.

Antenna tuning technologies
Any tuning circuit connected to the antenna must be extremely linear in order to avoid harmonics or inter-modulation distortion. Other important specifications include: a tuning ratio of at least 3:1, power consumption of less than 1 mA, small size, low insertion loss, and high quality factor (Q). In addition, it must be rugged and reliable.

In addition, since mobile TV is a receive-only system, an "open-loop" antenna tuning method must be used. In this technique, the center frequency of the antenna is tuned based on a look-up table. The antenna center frequency is then moved in order to cover specified frequencies between 470MHz and 862MHz. To be successful, the tuning technology must quickly retune the antenna whenever the receive channel is changed. Since the UHF band for DVB-H is divided into 48 channels 8MHz apart, 16 or 32 tuning states will be necessary to provide ample fine resolution and high-quality reception for any channel in the mobile TV band.

Table 1: Requirements for tunable elements used in mobile TV antenna tuning

Over the years, designers have considered several RF tunable technologies and methods, including varactor diodes, switch-based tuning and FET switched capacitor banks. Now, there is a new solution: DuNE digital tunable capacitors (DTCs) from Peregrine Semiconductor.

Some of the earliest mobile TV designs proposed a varactor diode approach, which offers the benefits of being straightforward and cost effective. Varactor diodes are analog components, so they would require a dedicated analog tuning voltage output for the mobile-TV chipset. Unfortunately, varactor diodes do not typically meet the +26dBm power handling requirement to support simultaneous cellular and mobile TV operation.

In response, some designers have proposed a switch-based tuning approach, which uses a GSM SP4T switch with a tuning circuit on each of the four legs of the switch. While this addresses the power handling issue, it only provides very coarse tuning (nowhere near the 16 to 32 recommended tuning states), so, while it is a clever method, it will likely not be sufficient to guarantee good performance for all mobile TV channels.

As a next step, designers considered FET switched capacitor banks that were based on bulk CMOS technology. Early experiments confirmed that this method showed promise as the technology to satisfy this challenging list of requirements for mobile TV antenna tuning . However, despite the fact that they have a high quality factor, their linearity is typically on the order of IIP3=+25dBm, which suggests less than +15dBm of power handling, meaning that bulk CMOS FETs fell short of the requirement for simultaneous cellular and mobile TV operation.

There was yet another solution using FETs, however. Peregrine's UltraCMOS FETs have the fundamental advantage of stacking due to the fully insulating sapphire substrate. So, it was possible to use multiple low-voltage FETs in series to handle the high-power signals encountered in simultaneous GSM and mobile TV operation.

A new approach
DuNE technology is a patent-pending design methodology that leverages the UltraCMOS process and Peregrine's HaRP design innovations. The first devices using this methodology are tunable capacitor circuits referred to as digitally tunable capacitors (DTCs). Due to the fundamental advantages of UltraCMOS, DuNE technology can be scaled to handle any power level without degrading Q or tuning ratio. As a result, it can be scaled to handle power levels anywhere from +20dBm up to +40dBm or higher. For mobile TV applications, DuNE DTCs have been designed for +28dBm nominal power handling and can deliver IIP3>+62dBm (Fig. 3).

Fig. 3: Measured IIP3 and power handling as a function of state for a DuNE DTC for mobile TV.

(For devices optimized for GSM/W-CDMA applications, power handling is better than +38dBm). Peregrine's DTCs for mobile TV are housed in a 2mm x 2mm package with a built-in 3-wire serial interface (Fig. 4).

Fig. 4: Peregrine's DuNE DTC for mobile TV applications is housed in a 2mm x 2mm 8L DFN package (right).

DuNE mobile-TV DTCs were designed for 5 bits of resolution or 32 tuning states, delivering the level of fine resolution required to tune an antenna across the entire mobile TV band. Fig. 5 shows a 1.36 to 6.3pF capacitance range, which results in a 4.6:1 tuning ratio, for a 5bit mobile-TV DuNE DTC.

Fig. 5: Measured capacitance as a function of state for a DuNE DTC mobile-TV antenna tuning circuit.

Mobile TV antenna tuning circuits need to demonstrate low insertion loss in order to maximize total antenna efficiency. As a result, the DuNE mobile TV DTC has been designed for Q range of 40-70 at 470 MHz to 862MHz (Fig. 6). And, the current consumption for this device is 11?A at +2.75V.

Fig. 6: Measured quality factor as a function of state for a DuNE DTC for mobile TV.

Preparing for high-volume mobile TV
Addressing all of the required performance points for embedding a tunable mobile TV antenna, DuNE DTCs for DVB-H and ISDB-T applications are sampling already to alpha customers, with volume production planned for Q3 09. These self-contained UltraCMOS RFICs communicate directly with the mobile TV receiver chipset, integrating a digital communication interface and other required functionality on the die, which simplifies implementation as well as minimizes connections between the two devices.

Table 2: Key specifications for the Mobile TV DuNE DTC device

Because DuNE DTCs are based on UltraCMOS technology, a proven manufacturing process that is the foundation for millions of RFICs shipping per week to the handset industry, mobile handset designers now have a way to design in compact, tunable embedded antennas for mobile TV with confidence.

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