An electrically programmable transversal filter

A real time electrically programmable analog transversal filter is presented, based on a novel functional multiplying structure utilized in the standard silicon gate MOS process. The analog multiplication of signals is achieved by digitally selecting the application of a controlled analog signal voltage on the semiconductor inversion layer side of an MOS capacitor. The digitized tap data controls the selection of seven binary area-ratioed MOS capacitors per tap position, for seven-bit tap accuracy. The multiplying capacitances are connected in common across the chip to achieve summation without the necessity for amplifiers. The filter has 16 analog taps and is modularized, i.e., chips may be cascaded to implement larger filters. The experiments presented will show a 96 tap cascaded filter with excellent performance. Discussion of possible applications will be presented, including interaction with a microprocessor for adaptive filtering.