Method and apparatus for the coherent demodulation of an amplified

Abstract

An apparatus and method for demodulating an amplitude modulated read signal generated by a magnetic head of a hard disk drive. The apparatus includes a pair of serially connected track and hold circuits that receive the read signal. The track or hold state of each circuit is controlled by a phase lock loop circuit which also receives the read signal. The first track and hold circuit initially tracks the read signal to a peak value. The circuit then switches to a hold state and provides a constant output signal that corresponds to the peak voltage of the read signal. While the first track and hold circuit is in the hold state, the second track and hold circuit is in a tracking state. The second track and hold circuit tracks the output of the first circuit. Because the output of the first circuit is a constant value, the second circuit also provides a constant output signal in the tracking state. The first circuit is switched back to the tracking state to again track the amplitude of the read signal, and the second circuit is switched to the hold state. The process is repeated to create a 'stepped' demodulated signal. The output signals are compared to a threshold value to detect bit errors in the signal. The constant output signals provide additional energy to the detection circuit to minimize the number of false error signals from the comparator circuit.