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Enabling next-generation mobile TV handsets

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

Keywords:mobile TV? RF? broadband?

By Tero Ranta and Rodd Novak
Peregrine Semiconductor

Fig. 1: Without the use of RF tunable technologies, mobile TV requires the use of an external whip antenna.

Designers faced with making ubiquitous mobile TV a reality have a fair number of challenges in front of them, including battery life, reception, range, screen size, and picture quality. The good news is that there have been several techniques and technologies introduced that are on track to address these challenges. However, a major remaining challenge is embedding the mobile TV antenna without compromising required RF performance.

Mobile TV antennas must support a wide range of voice and data frequencies in the same device, spanning 470MHz to 862MHz. The problem is that when embedded antennas are constrained to a small form factor, they cannot receive all of the required mobile TV bands at high efficiency without some sort of reliable tuning technology. In order to satisfy this specification, early versions of handsets equipped for mobile TV used an external whip antenna (Fig. 1). There are two major challenges involved with this approach. First, it is a potential point of failure because repeated handling and external forces can cause damage. Second, since mobile TV applications are new, many handset manufacturers and service providers have adopted a "push" marketing strategy: they include mobile TV functionality with the hope that consumers will eventually use it. Unfortunately, an external whip antenna is very low on the consumer list of "must-haves."

The market for mobile TV is promising, and analysts expect more than 50 million digital TV ready mobile handsets to ship next year . If handset manufacturers want to avoid the external whip antenna in their next-generation designs, then engineers need to find a better solutionand one that is ready now for high volume production. In order to embed the mobile TV antenna in the handset and meet the form factor restrictions, it will be necessary to use RF tunable technologies, but which technologies are robust enough and ready to meet the spec?

Design challenges
Global mobile TV standards, such as DVB-H and ISDB-T, require very broadband reception while delivering good coverage. This has to occur in a mobile handset, which is already a complex system with limited board space. These factors combine to make it very difficult to design a high-performance antenna for mobile TV.

Without the use of RF tunable technologies, mobile handset designers really have only two antenna choices: a passive internal antenna with poor performance, or an external whip antenna. For future designs, the market demands an embedded, internal antenna, so some type of antenna tuning is critical in order to ensure a good user experience. Mobile TV is experiencing good market penetration in Europe (DVB-H) and Japan (ISDB-T), and providers in these regions are particularly motivated to find a suitable antenna tuning technology.

Antenna tuning benefits
A mobile TV antenna's achievable bandwidth and input match is directly related to the physical size of the antenna and the mobile phone. In traditional internal mobile TV antennas, radiation efficiency suffers and voltage standing wave ratio (VSWR) gets very high (up to 6:1 across the 470MHz to 862MHz bandwidth). Therefore, the receiver loses 3dB of sensitivity simply due to antenna mismatch, which significantly reduces range and degrades reception quality of the mobile TV.

In contrast, a "tunable" internal antenna could cover a narrow section of the bandwidth, and this section could be "retuned" as the receiver channel changes. Using this type of RF tunable technology would deliver significantly better VSWR (ensuring that most of the signal power captured by the antenna makes it to the receiver). With a tunable architecture, the frequency of the antenna resonance could be moved up and down as needed in order to match the desired channel. Fig. 2 compares a traditional passive internal DVB-H antenna that covers the required bandwidth with very high VSWR (Fig. 2, left) with a narrowband tunable antenna that can be moved in frequency to cover the same bandwidth with significantly better VSWR (Fig. 2, right). Specifically, the VSWR of the antenna without tuning is 6:1. With tuning circuitry, matching is better than 2:1 across the entire band.

Fig. 2: The input impedance for an embedded mobile TV antenna with fixed matching (left) and with a tunable matching circuit (right).

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