Analysis and implementation of low-cost FPGA-based digital pulse-width modulators

Abstract

This paper describes the architecture and operating principles of two digital pulse-width modulator (DPWM) implementations for low-cost field-programmable gate arrays (FPGAs). Both architectures are based on a countercomparator block to process the most significant bits (MSB) portion of the reference input, enriched with additional elements to enhance duty-cycle resolution according to the less significant bits (LSB). The first architecture described has already been reported, it uses the on-chip PLL blocks to generate fixed delays and a selector to choose the one corresponding with the desired duty-cycle. Post-fitting adjustment of PLL delays are required to compensate delay differences between the diverse signal paths across the selector. In the second architecture described, a serializer-deserializer (SERDES) module is used to serialize a thermometer-coded representation of the LSB portion of the input. This serialization technique is commonly used for data transmission on high-speed serial I/O data transmission standards like LVDS and is extensively supported by FPGA providers. Experimental results are presented for both architectures synthesized on standard low-cost FPGA chips, showing very good linearity and resolutions up to 1ns. The first architecture provides a moderately better resolution. The second architecture, on the other hand, is a much more robust solution as it requires no post-fitting delay adjustments.