APPLICATION NOTE 3400

Abstract: The MAX1472, MAX1479, and MAX7044 300-450 MHz ASK Transmitter ICs are used in applications that require extremely small packages (Car Key Fobs, Tire Pressure Monitors). Very often, a small loop is the only antenna that will fit into one of these packages. Because these loops are very small compared to a wavelength at these frequencies, they are extremely high-Q and therefore they present a challenge to good impedance matching. This application note shows the typical impedance values for small loops and suggests matching networks for these impedances. It also shows the effectiveness of these networks in suppressing harmonics of the transmitted frequency. Most transmitter IC’s that serve these markets, such as the Maxim MAX7044, MAX1472, and MAX1479 are biased for maximum efficiency, rather than maximum linearity, which means that the harmonic content of the PA output may be very high. Additionally, regulatory agencies in all countries where these devices are used restrict spurious emissions. Hence, attenuating harmonic power from the power amplifier (PA) is very important.

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