All Digital Ultra-Fast Acquisition PLL

Frequency synthesizers find use in all high frequency applications such as communications, radar, digital communications, electronic imaging and spectroscopy [1]. There are three main techniques commonly used 1) PLL synthesizers based on a feedback mechanism to lock to a reference frequency. The popularity of the PLL is due to their extended tracking range and low cost. 2) Direct analog frequency synthesizers based on a main reference and group of frequencies derived by mixing, filtering, adding, subtracting or dividing as needed. The DA offers excellent spectral purity and fast switching speeds. 3) Direct digital synthesizers use digital techniques to create and modulate a signal that is eventually converted to analog form using a DAC. Applications in mm wave radio and Asynchronous Optical Sampling Spectroscopy would benefit from advances in frequency and resolution of a direct digital frequency synthesizer. Asynchronous Optical Sampling Spectroscopy [1] In conventional THz-TDR, mechanical delay limits the S/N ratio achieved in a given amount of time. ASOPS uses the time delay between 2 lasers, ramped from 0 to 1 f s at the rate given by  f R to eliminate the mechanical delay. Acquisition speed can be improved with high scan rates. Frequency resolution can be improved with better repetition rate stabilization. Time resolution is given by S=  f 1 f 2 , where = f 1 − f 2 and the signal appears expanded in the time axis by a factor f 2 . Thus stability and phase noise in  directly affects the accuracy of the time-base. Advantages of ASOPS 1) High acquisition speed (can capture dynamic events) speed improves as  increases 2) High spectral resolution resulting from capturing a temporal window the size of the THz pulse period, which would be the theoretical limit. resolution improves as f 2  increases Thus improvements in f 2  and = f 1 − f 2 that can be achieved using a DDFS in the laser control loop would greatly benefit ASOPS. A hybrid of a PLL and a DDS based frequency synthesizer could have added benefit. Figure 1 Basic direct digital frequency synthesis The basic DDFS was proposed by Tierney [2] in 1971 and the main elements are shown below. A summary of the advantages and disadvantages of the various approaches for direct digital frequency synthesis is well summarized in the Ref. [3]. For the phase-to-sinusoid amplitude converter (PSAC) an angular decomposition involves …