In order to ensure that the aliased noise is below the noise floor of the ADSL signal, the HomePNA signal has to be attenuated by more than 72 dB before sampling. Since the gain of this signal is increased before reaching the anti-aliasing filter, the noise specification of this filter is relaxed. This filter must also be calibrated to account for variations in the corner frequency due to processing variations. One way to reduce the order of the anti-aliasing filter is to maintain a higher oversampling ratio. In this case, digital filters can do most of the filtering.
Digital filters are easier to implement, consume lower power, and can be made into a linear phase.
Without accounting for the HomePNA signals, the energy above 2,208 kHz comes primarily from the side lobes of the downstream signal itself. 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.