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07-06 - Design of low Phase Noise Frequency S ynthesizer...

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Design of low Phase Noise Frequency Synthesizer for HF Search Receiver A. Himabindu Defence Electronics Research Laboratory, Hyderabad, India. [email protected] Abstract― Frequency synthesizers are electronics devices that produce frequencies coherent to a reference frequency. Such synthesizers are integral part of RF front end design for providing programmable local oscillator for the frequency translation process. This paper discusses the requirements, design features, a brief description and implementation aspects of the low phase noise frequency synthesizer (70- 100MHz) used in designing HF search receiver. Detail study of the conventional approach and modified approach are presented. The modified approach will give better phase noise performance. I.INTRODUCTION HF Search Receivers are widely used in Electronic warfare (EW) Systems for interception and Reconnaissance of hostile transmissions. The information they produce is used for the purpose of threat recognition and the tactical deployment of ECM equipment. Its purpose is to observe a given frequency band or bands for signal activity and acquisition. Capability of a search receiver is specified in terms of its frequency range of operation, scan rate, frequency resolution, sensitivity and dynamic range. Current generation state-of-the-art search receivers, employing Wideband Digital IF signal processing techniques are known as Digital Receivers . The Receiver comprises RF tuner, frequency Synthesizer & Controller and signal processor along with power supply units. The receiver shall be housed in a rack mountable enclosure with the Graphic User Interface running on a computer and connected via gigabit Ethernet link. The basic block diagram of is shown in Figure.1 Design of an EW receiver design is challenging when compared to a conventional receiver design. In a conventional receiver the signals of interest are known along with their properties where as in EW scenario, the signal characteristics like center frequency, bandwidth, type of modulation etc. are unknown. The receiver should have high sensitivity (ability to detect weak signal) of the order of -115dBm in 1.25KHz resolution bandwidth with 10dB Signal to Noise Ratio (SNR). To get high sensitivity double conversion super heterodyne architecture is used. The incoming HF Band signals (1-30MHz) are up- converted to 70MHz using low phase noise frequency synthesizer (70-100MHz) and subsequently down converted to 21.4MHz by mixing with a fixed frequency oscillator. During frequency translation process, the frequency synthesizer should not degrade the SNR (by add extra noise) to meet the sensitivity requirement, Phase noise is a measure of the purity of the frequency synthesizer. Phase noise is the ratio of output carrier power to the noise power in a 1-Hz bandwidth at a given frequency offset from the carrier. Expressed as log ratio, the units are dBc/Hz. Since phase noise directly affects the receiver sensitivity, to meet the sensitivity requirement, the synthesizer phase noise should be < -115 dBc/Hz @ 1 KHz offset. The search
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