Since the ADC is oversampled by 2X, only frequencies above 3,312 kHz will alias into the downstream band. In this case, the filter needs to deliver only about 12-dB rejection above 3,312 kHz. This is easily provided by a second-order Chebyshev filter. Meeting the design demands Today's system designers are mainly concerned with the performance, cost, and size of modems. By re-evaluating filtering techniques, designers can meet the same demands for the competitive ADSL market.
Integration is a technique that can be particularly beneficial to designers.
For lower cost and lower board area, these filters can be integrated with the analog chip. The main issue with integration is variation of the filter corner frequency because of processing variations. This can be handled by over-designing the filter to take care of the corner frequency variations or by having a calibration mechanism. About the Author Prakash Easwaran is a member of the technical staff for Texas Instruments' India operation. Prakash graduated from the Indian Institute of Technology, Madras in 1993.