filtorX: computer-aided filter design and industry

Switched-capacitor transmit and receive filters for a CCITT compliant codec were designed at Mite1 Semiconductor employing hand-coded Pascal[ 12] programs and a variety of other tools. This approach required a significant investment in the designer’s time: to manually optimise biquadratic filter section performance; to write and debug the Pascal programs used and to generate simulation netlists. filtorX, written at the University of Toronto, is a user-programmable, computer-aided filter design tool intended for research and industrial design. It enables designers to perform diverse functions automatically, using one environment from which external simulation tools can be operated and is applied to the challenges of this project. 1 Problem Definition CCITT compliant transmit and receive filters for integration into a telecommunications PCM-codec chip were required. CMOS technology was to be used for voice-band frequencies, leading to switched-capacitor circuitry as the obvious choice. The transmit filter, shown in Fig. 1, required 1) low-frequency rejection to suppress line noise from AC power sources and 2) attenuation of signals above 3.4kHz as an anti-aliasing filter for the PCM encoder. The receive filter, an anti-imaging filter which follows the D/A converter, has the primary function of 3 0 0 50 6 0 200 3400 4600 frequency (Hz) Fig.1. The CCITT transmit specification. removing images above 3.4kHz and has no stringent requirements for low-frequency signals. This specification is plotted in Fig. 2. v-v0 50 60 200 3400 frequency (Hz) Fig.2. The CCITT receive specification. The receive low-pass filter was to be used to compensate for the passband distortion caused by sinc roll-off. The desired response for the receive filter would no longer be a flat passband, but would now become an inverse-sine compensated passband satisfying a period equal to the sampling rate of the system. The overall response would then meet the desired specifications. The specifications for both filters are challenging and pose a number of interesting difficulties for the designer. filtorX provides a versatile and systematic design solution.