Apparatus for, and a method of, obtaining a measure of data detection error

Abstract: The width of the data window in a magnetic storage device is controllably varied in order to perform comparative data detection error rate measurements thereon is provided. More particularly, apparatus for obtaining a measure of data detection error in devices for detecting data signals, comprises: first means (9) for producing unambiguously defined timing signals in response to data signals; second means (11, 15) coupled to the first means for producing test signals including pulses having constant width in response to the timing and data signals; third means (15, 17), coupled to the first means, for producing control signals for controlling the width of a test window with respect to the timing signals; and fourth means (18), coupled to the second and third means, for producing a test window by selectively accepting or rejecting test signal pulses in response to the control signals. A continuously variable stressed data window can be generated symmetrically within, and in locked phase with, the full data window under the control of a single current digital-to-analog converter (DAC) in a voltage controlled oscillator (VCO).

Description: This invention relates to apparatus for, and a method of, obtaining a measure of data detection error in devices for detecting data signals, such as magnetic data recording and storage devices. In the data recovery process in a typical digital magnetic storage system, the interval during which a readback pulse representing a bit of data is detected is referred to as the "data window". In the ideal system, having a noiseless channel and no intersymbol interference, there is no "bitshift", i.e. all pulses occur in the centre of the data window. In real systems, however, particularly in high density storage devices, readback pulses representing data bits are shifted with respect to the centre of the data window as a result of the density of recorded data and the code patterns of the data in the data signal. See, for example, an article entitled, "Effect of Bitshift Distribution on Error Rate in Magnetic Recording", IEEE TRANSACTIONS ON MAGNETICS, Vol. MAG-15, No. 3, May 1979 by Eric R. Katz and Thomas G. Campbell. In addition to interaction-induced bitshift, readback pulses are also subject to additional noise-induced bitshift. If pulses are shifted enough to fall outside the data window, they are counted as errors. Such information may be used to predict the right error rate in lieu of a direct measure of such error rate which generally requires considerable time. A recording system may be evaluated directly, however, by measuring the error rate as a function of data window size. Historically, such a measurement has been tedious, time-consuming and costly, and the accuracy is typically unsatisfactory. US-A-4,394,695 describes two methods for deriving error rate. In the first method, raw data is applied to a window generator which generates a window of predetermined width at the data clocking rate. The raw data is also applied to a delay element which delays the data predetermined amounts with respect to the front edge of the data window. By selectively delaying the data signal, the data window is effectively narrowed. The narrowed window causes some data bits to fall outside the window and provide a measure of error rate. In the second method, data is applied directly to a plurality of window generators, each of which generates a window of different width, for making comparative measurements of error rate with respect to gradually decreasing window-widths. According to one aspect of the invention, we provide apparatus for obtaining a measure of data detection error in devices for detecting data signals, characterised by: timing means for producing unambiguously defined timing waveforms (Q, Z, N) phase locked to input data signals, the timing means (9) including a phase discriminator (11) and a voltage controlled oscillator (VCO) coupled in a phase locked loop, the timing waveforms (Q, Z, N) representing opposite and concurrent phases of the VCO output signals, the phase discriminator (11) being arranged to reshape the data signals so as to produce reshaped data signals (RD) including pulses of constant width; a blocked data window generator (17) incorporating a latch circuit, settable between a set and a reset condition, under the control of the timing waveforms (Q, Z, N), the latch circuit being operable to generate a blocked data window signal (W); and a stress data window gate (18) connected to receive the reshaped data signals (RD) and the blocked data window signal (W) and effective to selectively accept or block the reshaped data signals (RD) in response to the blocked data window signal (W); wherein the timing means (9) is adjustable to vary a DC voltage offset (M) between the timing waveforms (Q, Z, N) thereby to alter the timing of the latch circuit and so control the width of a test window applied by the stress window gate (188) to accept or block the reshaped data signals, the test window being symmetrically disposed within a data window pulse. According to another aspect of the invention, we provide a method of obtaining a measure of data detection error in devices for detecting data signals, the method including the steps of: employing a phase discriminator (11) and a voltage controlled oscillator (VCO) coupled in a phase locked loop to produce unambiguously defined timing waveforms (Q, Z, N) phase locked to input data signals, the timing waveforms (Q, Z, N) representing opposite and concurrent phases of the VCO output signals, employing the phase discriminator (11) to reshape the data signals so as to produce reshaped data signals (RD) including pulses of constant width; employing the timing waveforms (Q, Z, N) to control the setting and resetting of a latch circuit to generate a blocked data window signal (W); and applying the blocked data window signal to a stress data window gate (18) connected to receive the reshaped data signals (RD) to selectively accept or block the reshaped data signals (RD) in response to the blocked data window signal (W); and adjusting the DC voltage offset (M) between the timing waveforms (Q, Z, N) thereby to alter the timing of the latch circuit and so control the width of a test window applied by the stress window gate (188) to accept or block the reshaped data signals, the test window being symmetrically disposed within a data window pulse. within the full data window by controlling a single current. The stressed data window is generated symmetrically within, and in phase with, the full data window. A stressed data window is phase locked to a voltage controlled oscillator (VCO) and symmetrically disposed in time within the VCO clock window. Since the stressed data window tracks the VCO clock, the error rate measurement is far more accurate than that provided by prior art systems. The stressed data window is used by the VCO to selectively accept data bits which fall inside the stressed data window and reject all other data bits. The width of the stressed data window can be linearly changed by a current driven digital-to-analog converter (DAC). In this manner, a correlation between data detection error rate and data window width, and a data bit distribution within the data window, can be obtained. Thus, the stressed data window is useful for evaluation of such storage devices, manufacturing screen test of such magnetics and for field upgrade of such devices. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a system for generating a stressed data window, embodying the invention; Figure 2A is a schematic diagram of a VCO circuit modified for use in the system of Figure 1; Figure 2B is a schematic diagram of one part of the blocked data window generator circuit for use in the system of Figure 1; Figure 2C is a schematic diagram of another part of the blocked data window generator circuit for use in the system of Figure 1; Figure 2D is a schematic diagram of the stressed data window gate circuit for use in the system of Figure 1; Figure 3 is a timing diagram of the signal waveform produced at node N of the circuit of Figure 2; Figure 4 is a timing diagram of the signal waveforms produced at nodes Q and Z of the circuit of Figure 2; Figure 5 is a timing diagram of the signal waveforms of Figures 3 and 4, together with signal waveforms produced by the circuits of Figure 2B and Figure 2C; and Figure 6 is a timing diagram of the stressed data window and related waveforms generated by the system of Figure 1. Referring now to Fig. 1, a system embodying the invention, for generating a stressed data window comprises a phase-lock loop 9, a blocked data window generator 17 and a stress data window gate 18. Phase-lock loop 9 comprises phase discriminator 11, which provides phase difference between incoming data and the feedback clock produced by a voltage controlled oscillator (VCO) 15 and clock driver 16 and provides reshaped data pulses. Charge pump 12 charges or discharges filter 13 in response to signals from phase discriminator 11. Filter 13 provides required phase-lock loop stability. Buffer amplifier 14 provides low impedance driving capability for signals received from filter 13 to VCO 15 and clock driver 16. In particular, phase-lock loop 9 is similar to such loops described in "Analog Integrated Circuit Design" by Alan B. Grebene, Van Nostand Reinhold Company, 1972, pp. 298-326. Blocked data window generator 17 includes circuitry shown in Figs. 2B and 2C for generating the data blocking signals used to control stress data window gate 18 for deriving the stressed data window. The circuit for stress data window gate 18 shown in Fig. 2D produces logic control signals for generating data standardised to clock (DSTC) signals. Phase-lock loop (PLL) 9 receives input data bits to which it becomes locked. Signals produced by VCO 15 and clock driver 16 are thus unambiguously defined by such bits for as long as data is received. Blocked data window generator 17 provides control signals which are gated with the reshaped data signals from phase discriminator 11 to selectively accept data which falls within the stressed data window and to reject all which do not. Thus, the smaller the stressed data window, the higher the data detection error rate and vice versa. The reshaped data signals provided by phase discriminator 11 have constant pulse width and are derived as part of the phase discrimination process. VCO 15 comprises a conventional VCO circuit modified by the addition of a data window stressing circuit 22 as shown in Fig. 2A. The VCO 15 produces output signals in the same manner as such circuits described in "Analog Integrated Circuit Design" by Alan B. Grebene, Van Nostand Reinhold Company, 1972, pp. 313-315. In the modified circuit, transistors T1 and T3 form an emitter follower stage to transistor T6 such that the voltage at node N follows the voltage at node A. Similarly, transistors T2 and T4 form a corresponding emitter follower stage to transistor T7 such that the voltages at nodes Q and Z follow the voltage at node B. Resistor R1 is actively trimmed for symmetry of the stressed data window within the full data window which, in turn, is determined by the system clock frequency. Resistor R6 is passively trimmed to a nominal value. Thereafter R7 is actively trimmed for the desired VCO centre frequency when terminal 25 is grounded. During normal operation, V e , the feedback error voltage from buffer amplifier 14, is applied at terminal 25 to vary the VCO output signal frequency in accordance with the relation Δf ∝ V e . "Actively trimmed" refers to adjusting the value of the resistors while observing waveforms produced by an operating circuit. "Passively trimmed" resistors are adjusted to a fixed value while the circuit is inoperative. Adjustment of the current at the emitter of transistor T5 controls the width of the stressed data window with respect to full data window width. After R1 is appropriately trimmed, the voltage difference between the voltage at node N and the voltage at node Q (or at node Z) can be linearly varied by selectively controlling the current flowing in the emitter of transistor T5. The waveform of the signal at node N is shown in Fig. 3. Peak-to-peak voltage VT is the sum of voltages VD and VU. VD represents the capacitive regeneration at node B required for conventional operation of VCO 15 which the voltage node A tries to follow. Voltage VU represents capacitive boost developed at node A. Voltage level N₁, which is offset from the voltage level at node A by an amount approximately equal to -V BE -I s R₄, is the DC voltage of waveform N when the charging current for capacitor C is from node A to node B. VD is greater than VU owing to leakage through transistors T6 and T7, and ΔV is the difference between VD and VU, thus VD ≡ VU + ΔV, and VT = VD + VU. In Fig. 4, waveforms Q and Z represent the voltage waveforms at nodes Q and Z, respectively. The voltage levels Q1 and Z1, which are offsets from the DC voltage level at node B by amounts equal to -V BE -(I₁ + I₂) R₂ and -V BE -(I₁ + I₂) R₂ -I₁ R₃, respectively, are the DC voltages of their respective waveforms when the charging current to capacitor C is from node B to node A. Fig. 5 shows the timing relationship of the signals of Figs. 3 and 4 during operation of the circuits of Figs. 2A, 2B and 2C to produce the stressed data window. Waveforms WS and WR of Fig. 5 are produced by the circuit of Fig. 2B in response to waveforms N, Q and Z. Inputs T₁, T₃ and , T₁ T₃ are the differential signals that determine when the system is operating in stressed data window mode. When the system is operating in stressed data window mode, signal T1 (T3) is "high" and T1 (T3) "low". Signal WS is "high" and WS "low" only when node N is at a higher voltage than node Q. Similarly, signal WR is "high" and WR "low" only when node Z is at a higher voltage than node N. Waveform W of Fig. 5 is produced by the circuit of Fig. 2C in response to waveforms WS, WS , WR and WR . The circuit is a latch circuit. The "set" condition is w(set) = WS · T₃, and "reset" condition is w(reset) = WR · T₃ + T₃ . The trickle bias suppresses the effects of transient signals on operation of the circuit. Referring again to Fig. 5, the width of the waveform W, which is directly related to the stressed data window, is less than the width of a full data window or one cycle of system clock signal CL. The width of the stress data window is in fact equal to the width of W plus (PW-WT)+WT. Where PW is the reshaped data pulse width and WT, the minimum overlap of RD and W that results in producint of the corresponding GD and hence DSTC. Overlap of RD and W less that WT is ignored. The width of waveform W can be continuously varied by changing the offset, ΔM, between waveform N on the one hand and waveforms Q and Z on the other. Offset ΔX, the difference in DC voltage level between waveforms Q and Z is constant, being fixed by the value of resistors R1 and R3 through which current I₁ flows. As stated earlier, the pulse width of waveform W is varied by changing the offset ΔM. Offset ΔM is changed by varying the DC level at nodes Q and Z with respect to the DC level at node N. The relative DC voltage level at nodes Q and Z is set by adjusting the value of R1. Adjusting ΔM provides continuous control of the width of the stressed data window. Referring again to Fig. 2, adjustment of ΔM is provided by stress window control signal 25 at the emitter of transistor T5. Control signal 25 is derived from a conventional current digital-to-analog converter (DAC), such as an HP model 3497A Data Acquisition Control Unit, manufactured by Hewlett-Packard Co., Palo Alto, California. Referring now to Fig. 6, to detect data within the stressed data, reshaped input data is combined with the blocked data window signal (i.e. waveform W) to selectively accept or block a data bit. If the data bit is accepted, control signals to produce corresponding Data-standardised-to-clock (DSTC) pulses will result. Conversely, if a data bit is blocked, no corresponding DSTC signal results. The blocking effect requires constant pulse width of input data pulses. As mentioned elsewhere in this document, such constant pulse width is produced by phase discriminator 11 as part of its normal operation. DSTC pulses are generated in response to GD signals produced by stressed data window gate 18. Generation of DSTC pulses forms no part of the present invention. A circuit for stressed data window gate 18 is shown in Fig. 2D for producing logic control signals in response to reshaped data pulse RD and waveform W. In operation, when the reshaped data, RD, is "high" and overlaps the blocked data window signal W by at least WT (defined later), the output GD will go "high" and RD is accepted. If the overlap between the RD and W signals is less than WT, then GD will remain "low" and RD is blocked. If width PW of data pulse A is greater than WT and overlaps waveform W at the time data pulse A arrives, then the leading edge of data pulse A will define the leading edge of the stressed data window. Similarly, the leading edge of data pulse B defines the trailing edge of the stressed data window if PW is greater than WT and overlaps waveform W as shown. The pulse widths of data pulses A and B are equal for symmetry. Since the system would interpret a wider pulse width of data pulse A, such as PW′, as falling in two consecutive windows, the following relation must be imposed ΔWB min + 2WT ≧ PW > WT, where PW is constant reshaped pulse width of data pulses, WT is the minimum overlap of RD and W, and ΔWB min is the minimum data blocking interval. Referring again to Figs. 2 and 5, since the charge and discharge currents in VCO 15 are constant and of equal magnitude, the voltage and time of waveforms Q, Z and N are linearly related. For symmetry, the time difference between the leading edge of the clock window boundary and the leading edge of data pulse A, namely, ΔA, equals the time difference between the trailing edge of the clock window boundary and the leading edge of data pulse B, namely, ΔB. Data pulses A and B are reshaped data pulses RD at two different instants of time which overlap waveform W by WT just enough to trigger the stress data window logic. Thus, for ΔA ≡ ΔB, K(VU+ΔM)+D-(PW-WT)=K(VD+ΔM)-ΔX-D+WT Then, ΔX=KΔU+PW+2WT-2D=constant > 0, where D is the delay of latches comprising the circuit of Fig. 2C in which equal delays have been assumed for "set" and "reset" transitions, and k is a constant representing the slope of the charging/discharging rate of capacitor C. Since ΔV equals VD - VU, ΔX must equal a constant greater than zero to maintain symmetry. The stress data window may be trimmed to be symmetrical with respect to the full data window by adjusting the value of R1 to control current I₁. Stress data window symmetry will remain independent of current I₂. With continuing reference to Figs. 2 and 5, ΔM is given by ΔM = -(I₁+I₂)R₂-(-I₃ R₄)=I₃R₄-I₁R₂-I₂R₂ The percentage reduction of the full data window achieved for the stressed data window may be computed as follows: The percentage data window reduction is therefore linearly proportional to the control current I₂. Symmetry is achieved by actively trimming resistor R1 of Fig. 2. Referring now to Fig. 5, this trimming will alter the DC offset Δx, between waveforms Q and Z. For a constant current level of I₂, it is precisely the ability to alter the value of Δx that provides a degree of freedom by external control for symmetry. The stressed data window is formed within the full data window by symmetrically blocking RD at either end thereof. RD is blocked when the ΔWB min portion of W overlaps RD and is combined by stress data window gate 18. Unblocked data, GD, therefore, represents the stressed data falling within the stressed data window. Such stressed data, in turn, provides the basis for correlating data detection error rate with data window width and data bit distribution within the data window.

Claims: Apparatus for obtaining a measure of data detection error in devices for detecting data signals, characterised by: timing means for producing unambiguously defined timing waveforms (Q, Z, N) phase locked to input data signals, the timing means (9) including a phase discriminator (11) and a voltage controlled oscillator (VCO) coupled in a phase locked loop, the timing waveforms (Q, Z, N) representing opposite and concurrent phases of the VCO output signals, the phase discriminator (11) being arranged to reshape the data signals so as to produce reshaped data signals (RD) including pulses of constant width; a blocked data window generator (17) incorporating a latch circuit, settable between a set and a reset condition, under the control of the timing waveforms (Q, Z, N), the latch circuit being operable to generate a blocked data window signal (W); and a stress data window gate (18) connected to receive the reshaped data signals (RD) and the blocked data window signal (W) and effective to selectively accept or block the reshaped data signals (RD) in response to the blocked data window signal (W); wherein the timing means (9) is adjustable to vary a DC voltage offset (M) between the timing waveforms (Q, Z, N) thereby to alter the timing of the latch circuit and so control the width of a test window applied by the stress window gate (188) to accept or block the reshaped data signals, the test window being symmetrically disposed within a data window pulse. Apparatus as claimed in claim 1, wherein the number of accepted test signal pulses is proportional to the data detection error as a function of the width of the test window. A method of obtaining a measure of data detection error in devices for detecting data signals, the method including the steps of: employing a phase discriminator (11) and a voltage controlled oscillator (VCO) coupled in a phase locked loop to produce unambiguously defined timing waveforms (Q, Z, N) phase locked to input data signals, the timing waveforms (Q, Z, N) representing opposite and concurrent phases of the VCO output signals, employing the phase discriminator (11) to reshape the data signals so as to produce reshaped data signals (RD) including pulses of constant width; employing the timing waveforms (Q, Z, N) to control the setting and resetting of a latch circuit to generate a blocked data window signal (W); and applying the blocked data window signal to a stress data window gate (18) connected to receive the reshaped data signals (RD) to selectively accept or block the reshaped data signals (RD) in response to the blocked data window signal (W); and adjusting the DC voltage offset (M) between the timing waveforms (Q, Z, N) thereby to alter the timing of the latch circuit and so control the width of a test window applied by the stress window gate (188) to accept or block the reshaped data signals, the test window being symmetrically disposed within a data window pulse.

Inventors: CHAN PATRICK MENG-HO, CHEN SHIN CHUNG, CHAN, PATRICK MENG-HO, CHEN, SHIN CHUNG
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: G11B20
IPC codes: G11B20