A method of eliminating oscillations in high-speed recursive digital filters

We propose a dynamic scaling (DS) method as a cost-effective means of eliminating overflow and limit cycle oscillations in fixed-point direct-form recursive filters. The method is implemented by adding a DS unit to a fixed-point on-line module. On-line modules consume inputs and produce outputs digit-serially, most significant digit first. Except for precision adjustment, easily achieved by adding bit-slices, the on-line module is unmodified. The DS method introduces a shared exponent into the fixed-point computation, providing floating-point features to the internal computation at reasonable cost. The on-line nature of the operands is exploited by the DS method which normalizes and de-normalizes operands by deleting or adding digit delays in the operand path. Implementation in a gate array shows that the DS method is twice as cost effective as the precision extension method. The need for scaling between filter sections is also eliminated.