US 4,539,608 A · Low offset position demodular

Abstract

A low offset position demodulator that may be used in the positioning servo of a disk drive or similar device. A first stage of the demodulator circuit multiplies or switches a servo carrier signal, modulated with position information, with a synchronized gate signal. This first stage of the demodulator circuit includes a differential output and means for controlling the common mode operating point thereof with an external control voltage. A low pass filter, also having a differential output, is coupled to the differential output of the first stage of the demodulator. A differential-to-single-ended conversion stage is tied to the low pass filter output to provide a single position output signal. Also coupled to the low pass filter differential output is an averaging network that averages the signal appearing on one of the two differential signal lines with the signal appearing on the other differential signal line. The resulting averaged signal is compared to a zero voltage reference signal and the difference between these two signals is then used to derive the control voltage applied to the first stage of the modulator. Hence, the common mode operating point of the balanced demodulator is forced to assume and maintain a zero volt level.

Claims22

Dependent claims

A low offset position demodulator for providing, in response to a servo carrier signal, a position signal that indicates the radial position of a head assembly with respect to a given track boundary between adjacent tracks of a rotating disk, said tracks on said disk having information selectively encoded thereon that distinguishes at least every other track, the head assembly providing the servo carrier signal in response to reading the encoded information from at least one of said tracks, said position demodulator comprising:

means for generating a gate signal synchronized with selected transistions of said servo carrier signal;
means for multiplying said servo carrier signal by said gate signal, thereby generating a product signal;
means for determining whether said product signal represents an "On Track" condition, indicating a desired radial position of said head assembly with respect to a selected track boundary, or an "Off Track" condition, indicating a non-desired radial position of said head assembly with respect to said selected track boundary;
means for maintaining the offset of said product signal at a prescribed level;
and
conversion means for converting the findings of said determining means to said position signal, said position signal assuming a known value in response to an "On Track" condition.

3 more elements

An improved demodulator circuit that eliminates undesirable dc offset, said demodulator circuit comprising:

a balanced demodulator circuit including:
differential input signal lines to which a modulated carrier signal may be connected,
differential output signal lines from which a demodulated carrier signal may be recovered, at least one switching input signal line to which a switching gate signal for use in the demodulation process may be connected, and
a control line to which a control voltage may be connected in order to set the desired common mode quiesent operating point within said balanced demodulator circuit;
a low pass filter coupled to the differential output signal lines of said balanced demodulator circuit, said low pass filter also having differential output signal lines;
feedback means for averaging the signals appearing at the output signal lines of said low pass filter, comparing the resultant averaged signal to a reference signal, and coupling a signal representing the difference between said averaged signal and said reference signal to the control line of said balanced demodulator circuit;
and
a differential-to-single-ended conversion circuit coupled to the output signal lines of said low pass filter, said conversion circuit thereby providing a single output signal line upon which an output signal representative of the information contained in said modulated carrier signal may be recovered.

5 more elements

In a disk drive having a head that may be positioned radially with respect to a rotating disk under control of a positioning servo, a method for reducing the offset associated with a position demodulator used within said servo, said position demodulator being adapted to generate a position output signal that represents the relative location of said head with respect to servo tracks previously placed on said disk, said method comprising the steps of:

(a) modulating a carrier signal with position information that indicates whether an "On Track" or "Off Track" condition exists, an "On Track" condition meaning that the head is positioned as desired with respect to the servo tracks, and an "Off Track" condition meaning that the head is not positioned as desired with respect to said tracks;
and
(b) demodulating the carrier signal to recover the position information by
(1) generating a gate signal synchronized with selected transitions of said modulated carrier signal,
(2) multiplying the modulated carrier signal by the gate signal to form a product signal;
(3) analyzing the product signal to determine:
(a) whether an "On Track" or "Off Track" condition is indicated, and (b) whether undesirable dc offset is present therein,
(4) using automatic control system techniques to force and maintain any undesirable dc offset present within the product signal to zero, and
(5) converting the On Track/Off Track information contained within said product signal to said position output signal, said position ouptut signal having a zero level for an "On Track" condition, a positive level for an "Off Track" condition in one direction, and a negative level for an "Off Track" condition in the other direction.

6 more elements

Dependencies are inferred from the claim text.

Low offset position demodular

Life of this patent

Low offset position demodular

Life of this patent

Abstract

Claims22