طراحي و پياده سازي يك مدولاتور دلتا- سيگما قابل استفاده در سنتز كننده هاي فركانس كسري

چكيده انگليسي :

Design and Implementation of a delta sigma modulator applicable in Fractional N Frequency Synthesizers Khalil Farrokhiyan Farrokhiyan@ec iut ac ir Date of Submission April 8 2009 Department of Electrical and Computer engineering Isfahan University of Technology Isfahan 84156 83111 IRAN Degree M Sc Language Farsi Supervisor Rasoul Dehghani Dehghani@cc iut ac irAbstractFrequency synthesizers are one of the main parts of transceivers The quality of the transmitted or receivedsignal directly depends on the quality of the frequency synthesizer output spectrum used as the transceiverlocal oscillator Among the three frequency synthesizing methods direct analog indirect digital using PLLsand direct digital the indirect digital frequency synthesizing using PLLs is widely used in low cost portablemultiband transceivers The integer N type of PLL based frequency synthesizers was firstly used in wirelesstelecommunication Their structure is fairly simple but they can synthesize just integer multiples of thereference frequency which is equal to the channel spacing In the designing of integer N frequencysynthesizers there is a severe trade off among frequency resolution spectral purity and PLL dynamicbehavior Fractional N frequency synthesizers which can synthesize fractional multiples of referencefrequency are a solution for the mentioned trade off They eliminate the need for the reference frequency tobe equal to channel spacing Fractional N synthesizers use dual or multi modulus prescaler to generatefractional multiple of the reference frequency Main problem of the fractional N synthesizers is spurioustones caused by repeated operation of prescaler By randomizing the input control of the prescaler in such away that its division factor is equal to the digital control word the systematic fractional sidebands could beconverted to random white noise High pass noise shaping also can be used to improve signal to noise ratio Delta sigma modulators which have been using in data converters are known as a tool which can do therandomization and noise shaping together The delta sigma modulators in fractional N frequencysynthesizers act like digital to analog data converters without the decimation filter The delta sigmamodulators for this purpose should have the following characteristics low complexity to reduce powerconsumption long repetition duration and high dynamic range Delta sigma structures for frequencysynthesizers are just reported as a part of a whole synthesizer and there are few works concentrated on thissubject distinctively Two mostly used structures of delta sigma modulators are MASH and single loop thatthe second one introduces better noise characteristics This work introduces a new delta sigma structureconsisted of two stages based on previous works The first stage is a second order error feedback modulatorwhich decreases the effective number of input control word bits and the second stage is a Ritchie based thirdorder single loop modulator that acts as a concentrator to provide a single bit output stream Noise shaping inthis modulator is performed by the concentrator Decreasing the effective number of bits causes complexityreduction in the concentrator structure which corresponds directly to its input bit number Also thequantization noise of the first stage acts like dither for the concentrator To remove the first stage limit cycles the LSB of the input word is set to one The simulation and FPGA implementation results show higher SNRwith lower OSR and some more hardware resources in comparison with similar works Key WordsRF Transceiver Local Oscillator Frequency Synthesizers PLL Fractional N SpuriousTones Phase Noise Delta Sigma Modulation

فرخيان، خليل

طراحي و پياده سازي يك مدولاتور دلتا- سيگما قابل استفاده در سنتز كننده هاي فركانس كسري

ادوات مخابراتي , فركانس جعلي , نويز فاز , PLL