A new on-line multiply-add M A ) module is designed for implementing Seconb-Order Direct-Form IIR filters. The M A module is implemented an a 0.7-micron HCMOS gate array technology. A n array of M A modules is developed t o perform the secondorder recursive computations at 128Msamples/second with two levels of scattered lookahead. A new conventional (bit-parallel) design using similar lookahead is implemented in the same technology. The 104Msam-ple/second conventional design implements a carryfree recursive loop and uses radix-4 recoding to reduce the number of partial products. The on-line array has a sampling rate higher than the conventional implementation for 1% or more bits.
Abstract. Designs using conventional and on-line arithmetic for performing the recursive computations of SecondOrder Direct-Form IIR filters are implemented in a 0.7/z HCMOS gate array technology. The new conventional (bit-parallel) design using two levels of scattered lookahead achieves a rate of 104 Msamples/second. The design eliminates carry propagate addition from the recursive loop and uses radix-4 receding to reduce the number of partial products. The on-line design is an improved radix-4 multiply-add (MA) module 9 Two arrays based on the on-line MA module are developed to achieve rates of 128 Msamples/second with two levels of scattered lookahead. Performance and cost comparisons of the designs provide insights into the most appropriate designs to be used for a given word length.
A dynamic scaling (DS) method is proposed as a cost-effective means of eliminating overflow and limit cycle oscillations in fixed-point direct-form recursive filters. It is implemented by adding a DS unit to a fixed-point on-line module without modifying latter. On-line modules consume inputs and produce outputs digit serially, most significant digit first. The DS method introduces a shared exponent into the fixed-point computation at reasonable cost. Implementation in a 1.5 gate array shows that the DS method is twice as cost effective as the previously known precision extension method. The need for scaling between filter sections is also eliminated.Index Terms-On-line arithmetic, oscillations, recursive digital filters.
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