Digital filter

摘要

Methods and apparatus for variable-rate down-sampling filters for discrete-time sampled systems using a fixed sampling rate are disclosed. The Variable Rate Down-Sampling Filter allows a continuous range of sample rates to be derived from input samples at a fixed rate. The output rate does not have to be related to the fixed input rate in any integral or rational way, and in fact, the output rate can vary in time such as will occur when tracking a signal received from a station using a different timebase. A fixed sampling rate at the A/D Converter greatly simplifies the design of the analog front end. A single anti-aliasing filter can be designed and precisely matched to the fixed sampling rate. Used with a frequency-modulated numerically controlled oscillator (NCO), the Down-Sampling Filter keeps the entire frequency-synthesis and time-tracking loop in the digital domain. The need for an analog time-tracking loop to adjust the sampling instant at the A/D converter is eliminated. The need for an analog frequency synthesizer to generate a variable sampling rate is also eliminated. System design and performance analysis are simplified as well by eliminating the complication of hybrid analog-digital control loops. Details are disclosed. <IMAGE>