Method and apparatus for the coherent demodulation of an amplified modulated carrier using a dual track and hold amplifier and the application of said method and apparatus to measurement of magnetic media performance

A test apparatus for demodulating a read signal generated by a magnetic head that senses a magnetic field of a magnetic disk, wherein the read signal has a plurality of peaks which have a varying amplitude, comprising:

• track and hold means for providing a continuous DC output signal that tracks the varying amplitude of the peaks of the read signal, wherein said track and hold means include a first track and hold circuit which provides an output signal that tracks the amplitude of the read signal when in a tracking state, and provides a constant first output signal approximately equal to the amplitude of the read signal when said first track and hold circuit, said second track and hold circuit provides an output signal that tracks the amplitude of said constant first output signal when in a tracking state, and switches to a hold state to provide said continuous DC output signal approximately equal to said constant first output signal, said second track and hold circuit switches between said tracking and hold states at a phase that is different from when said first track and hold circuit switches between said tracking and hold states and, a control circuit that switches the states of said first and second track and hold circuits;
• a comparator which compares said continuous DC output signal with a threshold signal to determine whether said continuous DC output signal exceeds said threshold signal.

A method for demodulating a read signal generated by a magnetic head that senses a magnetic field of a magnetic disk, wherein the read signal has a varying amplitude, comprising the steps of:

• a) tracking the amplitude of the read signal to a peak amplitude with a first track and hold circuit;
• b) providing a first constant output signal that corresponds to the peak amplitude of the read signal;
• c) tracking said first constant output signal with a second track and hold circuit;
• d) tracking the amplitude of the read signal with said first track and hold circuit while providing a second constant output signal with said second track and hold circuit that corresponds to said first constant output signal;
• and,
• e) repeating steps (a)-(d).

An apparatus for demodulating an amplitude modulated signal that has a varying amplitude, comprising:

• a first track and hold circuit which provides an output signal that tracks the amplitude of the amplitude modulated signal when in a tracking state, and provides a constant first output signal approximately equal to the amplitude of the amplitude modulated signal when said first track and hold circuit switches to a hold state;
• a second track and hold circuit that is in series with said first track and hold circuit, said second track and hold circuit provides an output signal that tracks the amplitude of said constant first output signal when in a tracking state, and switches to a hold state to provide a second output signal approximately equal to said constant first output signal, said second track and hold circuit switches between said tracking and hold states at a phase that is different from when said first track and hold circuit switches between said tracking and hold states;
• and,
• a control circuit that switches the states of said first and second track and hold circuits.

A test apparatus for demodulating an amplitude modulated signal that has a varying amplitude, comprising:

• first track and hold means for providing an output signal that tracks the amplitude of the amplitude modulated signal when in a tracking state, and switches to a hold state to provide a constant first output signal approximately equal to the amplitude of the amplitude modulated signal when said first track and hold means switches to said hold state;
• second track and hold means for providing an output signal that tracks the amplitude of said constant first output signal when in a tracking state, and switching to a hold state to provide a second output signal approximately equal to said constant first output signal, said second track and hold means switches between said tracking and hold states at a phase that is different from when said first track and hold means switches between said tracking and hold states;
• and,
• control means for switching the states of said tracking and second track and hold means.

A method for demodulating an amplitude modulated signal that has a varying amplitude, comprising the steps of:

• a) tracking the amplitude of the amplitude modulated signal to a peak amplitude with a first track and hold circuit;
• b) providing a first constant output signal that corresponds to the peak amplitude of the amplitude modulated signal;
• c) tracking said first constant output signal with a second track and hold circuit;
• d) tracking the amplitude of the amplitude modulated signal with said first track and hold circuit while providing a second constant output signal with said second track and hold circuit that corresponds to said first constant output signal;
• and,
• e) repeating steps (a)-(d).