Design And Analysis Of A Fractional Frequency Synthesizer Based On △-∑ Phase Discriminatior

The development of modern technology brings more and more difficult requirement for signal source. The output of signal source must have wide frequency range, high frequency resolution, low phase noise, high spurious restraining and can be controlled by program. Frequency synthesizer is the key technology to produce a great deal of frequency signal. It had been an important component of communication, radar, instruments, high-speed computer and navigation equipment.In a conventional PLL, the divide ratio N must be an integer and therefore the frequency resolution of the PLL is the reference frequency. Fine frequency resolution requires a small reference frequency and a correspondingly small loop bandwidth. Narrow loop bandwidth is undesirable because of long switching times, inadequate suppression of VCO phase noise, and susceptibility to noise.Fractional-N synthesis allows a PLL to achieve arbitrarily fine frequency resolution. Because the technique modulates the instantaneous divide ratio, fractional-N synthesis suffers from fractional spurs. The use of△-∑modulation results in a beneficial noise shaping for the phase noise introduced by fractional-N division. The technique has the potential to provide low phase noise, fast setting time, and reduced impact of spurious frequencies when compared with existing fractional-N PLL techniques. The methodology of the new synthesizer is similar to that of the 1 bit ADC technique for that the phase error detection is done by only determine whether the transition from DSFD (Delta-Sigma Frequency Discriminator) comes before or after the reference transition and the reference frequency must be much higher than the loop bandwidth. Besides, the sampling phase of the accumulator is actually the output phase of the synthesizer. This is a very useful feature for adaptive antenna array application. Using this feature it is possible to use several of these synthesizers to generate a group of high frequency signal of the same frequency but with digitally controllable phase shifts between each other. This signal group could be used to perform local oscillator phase shifting for each antenna cell, and greatly improve the beam forming architecture. As far as the author's knowledge, no other types of synthesizers have the similar functionality.This paper mainly focuses on researching the fractional frequency synthesizer which based on△-∑phase discriminator. Firstly, the characteristic and development of the frequency synthesizers and the basic theory of the traditional frequency synthesizers are introduced. And analyze the reason of bringing the frequency noise. Next, present the theory of the fractional frequency synthesizer which based on phase discriminator and educe the frame picture of it. At last contrast this new fractional frequency with the traditional one by simulating and realized it by FPGA, then conclude that the new one is more excellent than the old one. But for the speed of FPGA restriction, the result is not very satisfaction. We need to perfect the circuit realization in the future.