A CIRCUIT FOR REMOVING UNWANTED TEMPORAL PORTIONS OF A VOLTAGE VARYING SIGNAL

Abstract: A circuit for removing unwanted temporal portions of a voltage varying signal on the occurrence of a reset signal and for differentiating a resultant corrected signal with respect to a reference voltage, has a switch (SW) responsive to the reset signal applied to a first input. The switch is responsive to the reset signal to divert a second input of the switch to a second output from a first output and thence to return the second input to the first output. An input capacitor (C1) receives the input signal. A first operational amplifier (U1-B) has positive and negative inputs and an output connected to the second input of switch (SW), the positive input being connected to receive a voltage representative of the current through the input capacitor (C1). A storage capacitor (C5) has one side connected to a reference voltage and has its other side in electrical connection to the negative input of the first operational amplifier (U1-B) and the first output of the switch (SW). An inverting amplifier (U2) is connected to receive the second output of the switch (SW). A second operational amplifier (U1-A) receives the corrected signal from the inverting amplifier (U2) and has a negative input which is connected through a first resistor (R3) and through the inverting amplifier (U2) to the second output of the switch (SW). The negative input of the second operational amplifier is also in electrical connection through the first resistor (R3) with the input capacitor (C1) and with the positive input of the first operational amplifier (U1-B). A positive input of the second operational amplifier is connected to a reference voltage, and provides a differentiated corrected signal at its output which is connected via a feedback resistor (R4) to the negative input.

Description: This invention relates to circuits for removing unwanted temporal portions of voltage varying signals and has particular application to deriving such signals for driving servo-mechanisms where the unprocessed signals contain undesirable portions. The present invention is particularly useful for signals in which a desired portion of a signal contains a leg of relatively mild slope and a rapid leg of relatively large opposite slope and has particular application to deriving useful velocity information from "tri-phase servo patterns". In the prior art, European Application EP-A-0 165 553 discloses a circuit for removing unwanted temporal portions of a voltage signal, without differentiating the signal. However, this circuit requires two switches in order to isolate the holding capacitor from a low impedance input and two different timing signals must be provided for the switches. Moreover, a high impedance device must be attached to the output of the circuit in order to prevent the holding capacitor from discharging. According to the present invention there is provided a circuit for removing unwanted temporal portions of a voltage varying signal on the occurrence of a reset signal and for differentiating a resultant corrected signal with the respect to a reference voltage, said circuit having switch means responsive to said reset signal applied to a first input; first operational amplifier means having positive and negative inputs and an output connected to a second input of said switch means; second operational amplifier means; storage capacitor means having one side connected to a reference voltage and having its other side in electrical connection to the negative input of the first operational amplifier means and a first output of the switch means, said circuit being characterised by input capacitor means for receiving said input signal, the positive input of the first operational amplifier means being connected to receive a voltage representative of the current through said input capacitor means; by said switch means, in operation, being responsive to said reset signal to direct a second input of the switch means to a second output from a first output and thence to return said second input to said first output; by inverting amplifier means connected to receive the second output of the switch means; and by the second operational amplifier means being arranged so as to receive said corrected signal from the inverting amplifier means and having a negative input which is connected through a first resistor and through the inverting amplifier means to said second output of the switch means, said negative input also being in electrical connection through said first resistor with said input capacitor means and with said positive input of said first operational amplifier means, a positive input connected to a reference voltage, and providing a differentiated corrected signal at its output which is connected via a feedback resistor to said negative input. Preferably the negative input of said second operational amplifier means is also connected to a reference voltage throught the first resistor and a filter capacitor which is connected between a second resistor and said first resistor, the negative input of said second operational amplifier means being in electrical connection through both said first and second resistors to said input capacitor means, to the output of said inverting amplifier means and to said positive input of said first operational amplifier means. In one embodiment the negative input of said second operational amplifier means is in electrical connection with its output via said feedback resistor and a compensating capacitor connected in parallel therewith. A third resistor may be connected between said second resistor and said input capacitor means, said input capacitor means being connected through said third resistor with said inverting amplifier means and with said positive input of the first operational amplifier means. Said reset signal may be generated from a tri-phase servo signal. Said reset signal preferably is generated from the same tri-phase servo signal which is used to generate said input signal. In one embodiment a storage resistor is connected in series between said storage capacitor means and said negative input of said first operational amplifier means. One side of the storage resistor may be connected to said first output of the switch means and said negative input of the first operational amplifier means and the other side is connected to said storage capacitor means. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a simplified diagram of one embodiment of a circuit according to the present invention for removing unwanted temporal portions of a voltage varying signal; Figure 1A is a detailed diagram of the circuit of Figure 1; Figure 2 is a graph with a set of voltage signals with respect to time to illustrate the operation of the circuit of Figure 1; Figure 3 is a diagram of a typical practical differentiator; Figure 4 is graph of a non-ideal input signal to the circuit of Figure 1; Figure 5 is a graph of a possible ideal output signal from the circuit of Figure 1; Figure 6 is a diagram of a circuit for deriving a reset signal from the tri-phase servo pattern for the circuit of Figure 1; Figure 7 is a graph of three signals, pairs of which are related to the circuit of Figure 6; Figure 8 is a graph of an output signal of the circuit of Figure 6; Figure 9 is a schematic diagram of a circuit for deriving a reset signal for the circuit of Figure 1; and Figure 10 is a timing diagram for use with Figure 9. Throughout the drawings like parts have been designated by the same reference numerals. Figure 1 is a simplified diagram of a circuit 10 according to the present invention for removing unwanted temporal portions of a voltage varying signal. The term "line" used herein is to denote electrical connection via wire or other means. An input signal is provided to the circuit 10 at a point S1 charging a capacitor C1 and thence through resistors R1,R2,R3 to a negative input of an operational amplifier U1-A. The line between the resistor R3 and the negative input to the operational amplifier U1-A is also linked to a resistor R4 and a capacitor C3 which are connected in parallel between a point 21 and a point 22, on a line 14. Thus the combination of the operational amplifier U1-A, the resistor R4 and the capacitor C3 constitutes a differentiator as shown in Figure 3. The line 14 is the output of the operational amplifier U1-A provided as output from the circuit 10 at a point S2. A positive input line 13 to the operational amplifier U1-A provides a reference voltage and is connected across a capacitor C2 to a point 12 on a signal input line 15. Between resistors R1,R2 on the signal input line, a point 16 connects a line 17 to the positive input of an operational amplifier U1-B. The line 17 is also connected to a line 18. The negative input of the operational amplifier U1-B is connected to ground (GND) through a resistor R5 and a capacitor C5 on a line 23. The output of the operational amplifier U1-B is selectively connected between: (a) a point on the line 23 between the negative input to the operational amplifier U1-B and the resistor R5, and (b) an approximate minus one or inverting amplifier U2. The selection is governed by a reset signal provided on a line 19 to a mechanical or electronic switch SW. On the occurrence of a reset signal on the line 19, the switch SW directs the output of the operational amplifier U1-B to the amplifier U2. After the reset signal indicates that the reset is no long occurring, the switch is set to provide the output of the operational amplifier U1-B to the line 23. The output of the amplifier U2 is provided on the line 18 to its connection to the line 17. The input signal charges the capacitor C1 and provides a voltage level V H at a point 16 which represents the charging current through the capacitor C1. Referring briefly to Figure 3, it will be noted that the resistors R2,R3 and the capacitor C2 are additions to this circuit which are not found in a typical practical differentiator of Figure 3. Note that the operational amplifier U1-A connected as shown will tend to stabilise itself at a point where the voltage on the negative input is brought up to a voltage level V REF on the positive input. Therefore, the output voltage of the operational amplifier U1-A will tend to be proportional to the voltage level V H . It should be recognised that because the changes in the voltage level V H may be very small (about 0.1 millivolt) any ambient current leakage will affect the circuit performance This fact must be considered in component choice. The voltage level V H on the line 17 is provided as a positive input to the operational amplifier U1-B. During normal operation of the circuit 10 when the wanted portion of the signal is provided as the input signal to the point S1 and no reset signal is present on the line 19, the output of the operational amplifier U1-B is provided to the line 23 as input to the negative input of the operational amplifier U1-B and through the resistor R5 to the capacitor C5 and to ground, or as in a preferred embodiment, may employ a 5 volt reference in place of ground. The resistor R5 can provide filtering to the voltage provided to the capacitor C5 if desired. On the occurrence of a reset signal, the switch SW transfers the output of the operational amplifier U1-B to the amplifier U2. Thus the voltage level on the capacitor C5 provides an error voltage to the amplifier U2 which provides a voltage level equal to the voltage level on the capacitor C5 on the line 18 to the line 17 and to the point 16, thus sustaining the voltage level to the negative input of the operational amplifier U1-A during the reset interval. A more detailed schematic diagram of the circuit of Figure 1 is provided in Figure 1A in which the resistance and capacitance values are provided and reference made to standard industrial use numbers for designating particular integrated circuit and other component parts. Referring now to Figure 1A the circuit is indicated by reference numeral 10A. Resistance, capacitance, and voltage values may be varied in accordance with known principles. Certain characteristics of some of the component parts shown, however, are important to prevent problems with ambient voltage affecting the performance of the differentiation characteristics of the operational amplifier U1-A. For instance, the amplifier U2 comprises a low leakage, high output impedance inverter amplifier when not operating so transistors Q3,Q4, for instance, must be low leakage transistors. The numbers 2N3906 and 2N3904 designates transistors of this type. Transistors Q1 and Q2 need not be low leakage transistors. Likewise, the operational amplifier U1-B, designated by the industry standard number LF412A (currently available through National Semiconductor Corporation) is a high speed operational amplifier with high input impedance (100 pico amp input), which will have no leakage current to affect the operation of the operational amplifier U1-A, which most likely will be on the same circuit board. Figure 4 illustrates a small segment of a possible non-ideal input signal 40. This signal will correspond to some portion of a signal WF₁ whose waveform is shown in Figure 2. Because this signal is originally generated by digital circuits, it takes on a stairstep form as shown. Due probably primarily to irregularities on the data disk itself, transients or spikes t₁,t₂,t₃ will often be found in the non-ideal input signal. These transients, as well as the stairstep configuration of the input signal 40, will be smoothed and filtered out by the circuit of Figure 1. The input signal 40 is indicated with reference to the voltage level V REF in Figure 4. It must be noted that the reference voltage levels need not be the same DC potential for any of the three points to which they are introduced into the circuit but the potential values must be constant. A reference voltage level of 5 volts for the positive input of the operational amplifier was found to be best in practice to achieve the greatest dynamic range for the power supplies used. Figure 5 describes by a curve 41 a possible ideal output signal. Segment 42 corresponds to a negative output signal portion indicated by signal WF₃ in Figure 2. There are five waveforms illustrated in Figure 2, namely: WF₁, an input signal with a negative going slope with a positive slope transition; WF₂, a reset signal consisting of a series of pulses indicating the time period for the slope transition; WF₃, a negative output signal provided at the point S2 for the input signal WF₁; WF₄, an input signal of positive slope and its corresponding output for a positive output WF₅. All these waveform signals occur over the same length of time and the voltage value is plotted against the voltage reference level V REF . Signals WF₁,WF₂,WF₃ may occur concurrently, and signals WF₄,WF₅,WF₂ may also occur concurrently, using a circuit according to the present invention. Referring now to Figure 6 there is shown a circuit diagram of a circuit 100 through which the reset signal may be derived from tri-phase signals. Three triangle wave signals A,B,C are provided on lines 31,32,33, respectively. The triangle waves are derived from tri-phase spaced signals located in a formatted data disk. The circuit 100 of Figure 6 provides an output signal from line 36 which forms the signal input at the point S1 of the circuit 10 of Figures 1 and 1A. A detailed representation of this output signal is found in Figure 8 and of the signals A,B,C is found at the top of Figure 7. Note that in Figure 7, three "bit" signals, 0, 1 and 2 (square wave signals or digital "pulses") are shown in their radial relation to the signals A,B,C which occur at the same time and radial position. These bit signals 0, 1 and 2 can be found on output lines B₀,B₁,B₂ of the circuit 100 respectively. The negative going slope of each one of these three signals produces a reset signal of a finite duration. This is preferably accomplished with a microprocessor decode circuit of the three bit signals or, if only generating reset signals during "head-in" (i.e. the head moving towards the centre of the disk) is acceptable, two EXCLUSIVE-OR gates such as gates 52,53 arranged and connected as shown can be used. A preferred decode circuit is explained with reference to Figure 9, which generates a reset signal during rapid transition periods in the servo position error signal, but other constructions to accomplish this are possible. The bit signals used to generate the reset signal are produced by the circuit 100 which acts in the following manner. The three phase signals A,B,C are compared by three comparators U8₁,U8₂,U8₃. Where the signal A on the line 31 is of higher voltage than the signal B on the line 32, the output of a non-inverting buffer circuit U2₁ is "high" and the output of an inverting circuit U1₁ is "low". Otherwise, the reverse is the case. The two other comparators U8₂,U8₃ and their associated inverting circuits U1₂,U1₃ and non-inverting buffer circuits U2₂,U2₃ function in the same way for their inputs. The signals A,B,C are input into the circuit 100 on lines 31, 32 and 33 respectively. In Figure 7 typical signals A, B and C are shown with respect to a reference voltage level V REF . The signals A,B,C are fed into the positive input of a respective one of the comparators U8₁,U8₂,U8₃. One of the other of the three signals A,B,C is input into the negative input of the comparators. "LM339" is the industry standard numerical designation for comparators used in a preferred embodiment and they are currently available from National Semiconductor Corporation. Each one of the signals A,B,C is also input to two of six analog switches U14 (1-6) to be passed through to inverting and non-inverting differential amplifier input lines 34,35, respectively, to create the servo position error signal SPE on line 36 as shown in Figures 7 and 8. The signals A, B and C input to the comparators U8₁,U8₂,U8₃ in turn drive the inverting circuits U1₁, U1₂,U1₃, and also drive the non-inverting buffer circuits U2₁,U2₂,U2₃. The output of these circuits provide the proper switching of the analog switches U14 (1-6) . Resistance/capacitor pairs 101,102,103 provide hysteresis to the respective comparators U8₁,U8₂,U8₃ to prevent multiple switching during noise transients which may appear in the signals A,B,C. A differential amplifier U19 produces a signal on the line 36 as shown in Figure 8. The alphanumeric indicators attached to the signal shown in Figure 8 (for "head-in" sequence) correspond to the decimal or SPE indicators shown in Figure 7. The bit signals of the comparators produced on the output lines B1,B2,B0, are shown in Figure 7 as 0, 1 and 2. Therefore, looking at the signals in a "head-in" direction in Figure 7, because the transition or unwanted signal portion occurs between areas designated 6 and 2, areas designated 3 and 1 and areas designated 5 and 4 (see Figure 5 and bottom of Figure 7) and because a downward going pulse occurs at each one of these area interfaces and because only one of the three bit signals is "high" at a downward going signal area, all that is necessary is to fire a reset signal at a downward going pulse. In a "head-out" direction each upward going pulse will signal that a reset signal is necessary; i.e., that the slope transition occurs at that interface. A microprocessor could receive these digital bit signals as input and based on a program generate a reset pulse at the proper upward or downward going transitions of each of the bit signals. It is simplest and most efficient to think of this in terms of the two EXCLUSIVE-OR gates 52,53 which will provide an output pulse of relatively short duration in conjunction with a digital one shot for each of the indicated transistors. The decode of the bit signals 0,1,2 is accomplished with the equivalent of three AND gates with an output each to one of three flip-flops whose outputs are OR'd together. The output of this OR would be the reset signal for both "head-in" and "head-out" orientations. In practice the reset signal generation is more complicated and is explained with reference to Figure 9, where a circuit 200 to accomplish this is shown. However, it should be noted that other decode schemes are possible and that reset signals may come from signals other than the tri-phase servo signals of this embodiment. In Figure 9, a clock input, which is a period length smaller than the desired duration of the reset signal, is provided to the circuit 200 at an input 201. The clock pulse should be a short portion of the clock period. The clock pulse (clock 1 in Figure 10) is provided as the clock input to a flip-flop F7. A second clock (clock 2 in Figure 10) provides a delayed clock to a NAND gate NA2, and its inverse is provided to a NOR gate N4. The Q output of the flip-flop F7 when "low" provides the reset signal used in the circuits 10 and 10A to control the switch SW. The circuit 200 receives the three bit signals from the circuit 100 of Figure 6 (shown in time relation to each other in Figure 7) as inputs. They are provided, inverted and non-inverted as input to NOR gates N1,N2,N3 respectively. The outputs of these NOR gates are input to respective flip-flops F4,F5,F6. The Q output of these flip-flops is input to NAND gate NA1 whose output is provided as an input to the NAND gate NA2. The output of the NAND gate NA1 clears the flip-flop F7. The NAND gate NA2 also receives the Q outputs of the flip-flops F7,F8 as inputs. The output of the NAND gate NA2 provides a clear signal to the flip-flops F4,F5,F6. The D inputs of the flip-flops F4,F5,F6,F8 are all tied to "high" signal or voltage level as indicated by arrows. It should be clear that the circuit 200 of Figure 9 and its associated timing diagram of Figure 10 is only one possibility for providing efficient decode for the reset signal.

Claims: A circuit for removing unwanted temporal portions of a voltage varying signal on the occurrence of a reset signal and for differentiating a resultant corrected signal with the respect to a reference voltage, said circuit having switch means (SW) responsive to said reset signal applied to a first input; first operational amplifier means (U1-B) having positive and negative inputs and an output connected to a second input of said switch means (SW); second operational amplifier means (U1-A); storage capacitor means (C5) having one side connected to a reference voltage and having its other side in electrical connection to the negative input of the first operational amplifier means (U1-B) and a first output of the switch means (SW), said circuit being characterised by input capacitor means (C1) for receiving said input signal, the positive input of the first operational amplifier means (U1-B) being connected to receive a voltage representative of the current through said input capacitor means (C1); by said switch means (SW), in operation, being responsive to said reset signal to direct a second input of the switch means (SW) to a second output from a first output and thence to return said second input to said first output; by inverting amplifier means (U2) connected to receive the second output of the switch means (SW); and by the second operational amplifier means (U1-A) being arranged so as to receive said corrected signal from the inverting amplifier means (U2) and having a negative input which is connected through a first resistor (R3) and through the inverting amplifier means (U2) to said second output of the switch means, said negative input also being in electrical connection through said first resistor (R3) with said input capacitor means (C1) and with said positive input of said first operational amplifier means (U1-B), a positive input connected to a reference voltage, and providing a differentiated corrected signal at its output which is connected via a feedback resistor (R4) to said negative input. A circuit as claimed in claim 1 characterised in that the negative input of said second operational amplifier means (U1-A) is also connected to a reference voltage through the first resistor (R3) and a filter capacitor (C2) which is connected between a second resistor (R2) and said first resistor, the negative input of said second operational amplifier means being in electrical connection through both said first and second resistors (R2,R3) to said input capacitor means (C1), to the output of said inverting amplifier means (U2) and to said positive input of said first operational amplifier means (U1-B). A circuit as claimed in claim 1 or 2 characterised in that the negative input of said second operational amplifier means (U1-A) is in electrical connection with its output via said feedback resistor (R4) and a compensating capacitor (C3) connected in parallel therewith. A circuit as claimed in claim 2 or 3 characterised in that a third resistor (R1) is connected between said second resistor (R2) and said input capacitor means (C1), said input capacitor means being connected through said third resistor (R1) with said inverting amplifier means (U2) and with said positive input of the first operational amplifier means (U1-B). A circuit as claimed in any preceding claim characterised in that said reset signal is generated from a tri-phase servo signal. A circuit as claimed in claim 5 characterised in that said reset signal is generated from the same tri-phase servo signal which is used to generate said input signal. A circuit as claimed in any preceding claim characterised in that a storage resistor (R5) is connected in series between said storage capacitor means (C5) and said negative input of said first operational amplifier means (U1-B). A circuit as claimed in claim 7 characterised in that one side of the storage resistor (R5) is connected to said first output of the switch means (SW) and said negative input of the first operational amplifier means (U1-B) and the other side is connected to said storage capacitor means (C5).

Inventors: JANZ, DONALD WALTER
Applicants: SEAGATE TECHNOLOGY INTERNATIONAL
Application year: 1987
Main classification: H03K4
IPC codes: H03K4, H03F1, H03K5, G11B21, G11C27