Mixed-Signal Tone Detection for Modem Wakeup in ADSL

Tone detection is an integral part of the wakeup sequence of a DSL modem. As the modem sits in low power state waiting to be awakened by the central office, a low power detection mechanism is needed for this operation. In this paper, a low power mixed-signal non-linear tone detector is introduced. The detector has to distinguish multiple tones received at specific frequencies and be able to operate over a large dynamic range of 67 dB corresponding to loop lengths from Oft to 18 kft. The detection can be performed via hard-limiting the input signal, then cross correlating the signal with three different tones. To average out the inter-modulation components, two levels of integration/detection are used and system programmability is utilized to allow accurate detection depending on the loop length.