LNA WITH BYPASS MODE LNA With a Bypass Mode Improves Overload Resistance for Mobile TV

Television (TV) stations broadcast in the very high frequency (VHF; 30-300 MHz) and ultra high frequency (UHF; 300-900 MHz) bands. Opposite to the small cell philosophy popularized by modern wireless telephone service, free-to-air TV broadcast is historically driven by the need to extend the area of coverage, as doing so will garner a larger audience. So, commercial TV transmissions utilize tall aerial towers and power level in the 10-50 kW range in order to maximize their radio horizon. In comparison, the transmit power of a cellular repeater is many magnitude orders lower at ~50 W. Mobile TV allows the commuting public to watch TV programs on hand-held or portable devices [1]. The mobile TV functionality can be incorporated into notebook personal computers, Personal Digital Assistant (PDA), InCar-Entertainment (ICE) system [2], and cellular handsets, etc. Technically, the mobile TV poses a unique problem in that the distance between the transmitter and receiver can change considerably as the user travels to different locations. Given that only miniature aerials (i.e., low gain aerials) can be embedded in these portable products, the TV receiver must understandably be designed with very high sensitivity to permit clear reception at the coverage fringes. Unfortunately, the large RF gain will result in overloading of the mixer and IF stages when the user moves into the proximity of high power TV transmitters. Once the signal waveform is distorted by an over-driven amplifier or a mixer, it cannot be corrected by simple filtering or signal processing. Although it is technically possible to engineer higher overload tolerance in the mixer and IF stages, the required hardware will be too prohibitively expensive for this price-driven product category. Practical implementations of the mobile TV receiver almost universally rely on reducing the overall gain in the presence of strong signals as this is a far more cost effective solution to the overloading problem—hence the need for gain-controlled RF and intermediate frequency (IF) amplifier stages. The ability to vary RF gain greatly relaxes the linearity requirement in the mixer stage [3] and therefore allows low cost radio This article is a case history describing the design choices and tradeoff considerations required to meet a particular set of customer specifications Figure 1 · Block diagram of receiver frontend with a variable-gain LNA stage. Mixer performance is reflective of consumergrade TV receiver RFICs.