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Fractional N Synthesizer


THE use of wireless products has been rapidly increasing the last few years, and there has been worldwide development of new systems to meet the needs of this growing market. As a result, new radio architectures and circuit techniques are being actively sought that achieve high levels of integration and low power operation while still meeting the stringent performance requirements of today’s radio systems. Our focus is on the transmitter portion of this effort, with the objective of achieving over 1-Mb/s data rate using frequency modulation. To achieve the goals of low power and high integration, it seems appropriate to develop a transmitter architecture that consists of the minimal topology that accomplishes the required functionality. All digital, narrowband radio transmitters that are spectrally efficient require two operations to be performed. The baseband modulation data must be filtered to limit the extent of its spectrum, and the resulting signal must direct modulation of VCO, and (c) indirect modulation of VCO. be translated to a desired RF band. This paper will focus on the issue of frequency translation, which can be accomplished in at least three different ways for frequency modulation. As illustrated in Fig. 1, the modulation signal can be (a) multiplied by a local oscillator (LO) frequency using a mixer, (b) fed into the input of a voltage controlled oscillator (VCO), or (c) fed into the input of a frequency synthesizer.

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