CA1153465A - Constant bandwidth automatic gain control - Google Patents
Constant bandwidth automatic gain controlInfo
- Publication number
- CA1153465A CA1153465A CA000377177A CA377177A CA1153465A CA 1153465 A CA1153465 A CA 1153465A CA 000377177 A CA000377177 A CA 000377177A CA 377177 A CA377177 A CA 377177A CA 1153465 A CA1153465 A CA 1153465A
- Authority
- CA
- Canada
- Prior art keywords
- servo
- signals
- gain
- loop
- gain control
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/58—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B5/596—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on disks
- G11B5/59605—Circuits
- G11B5/59622—Gain control; Filters
Abstract
Abstract of the Invention An improved automatic gain control circuit suitable for use in the servo loop controlling the position of magnetic read/write heads with respect to magnetic disk data storage media is disclosed. The AGC circuit features a fast loop and a slow loop each of which loops contain gain varying elements, which in the preferred embodiment comprise field effect transis-tors. By using a reference voltage to control the operating point of the variable gain element in the fast loop, the effects of mechanical resonances on the servo data can be substantially eliminated while the overall AGC system response exhibits a substantially constant bandwidth.
Description
STC-121 ~534~5 - CONSTANT BANDWIDTH AUTOMATIC GAIN CONTROL
Field of the Invention This invention relates to a method of automatic gain control. In particular, this invention is described with re-ference to an automatic gain control used in a servo system controlling the position of the read/write head of a rotatable magnetic disk storage system.
Background of the Invention In the prior art, automatic gain cont~ol has been used in the head positioning servo loop of a magnetic disk storage system. The servo system uses prerecorded head posi-tion information on one surface of a stack of rotating magnetic platters to generate a regular signal for control. The infor-mation is read from a servo head flying above the servo surface.
As the servo head is mechanically ganged to data heads flying over the other surfaces of the stack of rotating magnetic platters via a carriage mechanism, the servo position informa-tion indicates the position of the data heads. The head posi-tion information must be amplified, filtered and decoded (or peak detected) to provide a position error signal used to drive a linear motor attached to the carriage mechanism such that the data heads are correctly positioned over their respec-tive data cylinders. ~An automatic gain control is used in the head positioning servo decoding system to remove signal ampli-tude variations due to media variations, head variations, amplification variations and flying height variations which otherwise cause inaccurate head positioning.
The bandwidt~ or response time of the automatic gain control is the ability to follow a time varying input signal~
''~
One such time varying signal is that of vertical mode instabil-ity or mechanical resonance of the servo head itself. As the servo head moves up and down with respect to the disX, the out-put signal from the head changes in amplitude because the output amplitude of the servo head is a function of its flying height.
Thus, if the automatic gain control had a bandwidth or response time in excess of the frequency of the vertical mode instabil-ity, the amplitude variations wouid eEfectively be removed.
` However, this solution would also prevent ~he system from operating on the servo data.
Ob`ects of the Invention It is accordingly an object of the invention to pro-vide a constant bandwidth automatic gain control ~ircuit for a magnetic disk storage system.
I 15 A further object o the invention is to provide an automatic gain control circuit which is not affected by varia-tions in its input amplitude due to mechanical resonances in the system which it is used to control but which maintains sufficient bandwidth to provide useful servo control.
A further object of the invention is to provide a constant band~idth automatic gain control which is substantial-ly unafected by nonlinearities in the components used to make up the circuit.
Still a further object of the invention is to provide constant bandwidth automatic gain control which is substantial-ly insensitive to its incoming signal level and to nonlinear-ity in its variable gain element but which can be implemented with relative ease and at little expense.
Summary of the Invention The above listed objects of the invention and needs of the art are satisfied by the present invention which provides a constant bandwidth automatic gain cont.rol which has a response time that is insensitive to the incoming signal level and to nonlinearities in the response of the variable gain elements used to provide variable gain to the circuit. This is accom-plished by providing irst and second servo loops, one fast and one slow, each comprising a variable gain element and appropriate filtering so that substantially low frequency variations in the signal are corrected for in accardance with the object of accurate control of a magnetic head but high frequency instabilities in the signal due to servo loops are separately controlled in a loop having a much faster response time. In this way, the mechanical resonances are precludea from affecting the accuracy of the servo posi~ioning o the head.
Brief Description o the Drawings The invention will be better understood if reference is made to the accompanying drawings, in which:
FIG. 1 represents a block diagram of a typical prior art automatic gain control circuit;
FIG. 2 represents a diagram useful in analyzing the bandwidth of the prior art automatic gain control;
FIG. 3 shows a graphical representation of system reponse;
FIG. 4 shows a block diagram of the automatic gain control according to the invention comparable to FIG. l;
FIG. 5 shows a diagram comparable to FIG. 2, useful STC-121 ~53~
in analyzing the bandwidth of the automatic gain control accord-ing to the invention.
Description of the Preferred Embodiments Reference will now be made to FIG. 1, which as men-tioned abov~ is a diagram of a prior art servo system~
Servo information is ta~en ~rom lower most disk 100 of a disk pack indicated generally at 102 by a read head 104 which is ganged to a plurality of data read heads 106 by means of a carriage 108. The data is written and read in any conven-tional manner; this does not relate to the present invention.
The servo data is passed to a servo preamp 110 which amplifies it; it is then passed to a variable gain element 112. The replacement of the variable gain element 112, which amplifies the signal from the servo preamp to an ~ssentially constant level to enable accurate servoing of the movement of the carriage 108 with respect to the disks 102 is the subiect of this application.
The signal from the variable gain element 112 is then passed to a low pass filter 114 and a buffer amplifier 116 and then to a pair of peak detectors 118, 120 which are gated to be out of phase so that their combined output, when applied ~o a summing ampliier 122, provides an average of the peak height o alternate pulses which are supplied from the amplifier 116.
The output of the summing amplifier I12 is thus an average of the height of the amplified and filtered servo data. This is then compared in a comparator 124 with a reference voltage supplied according to the teachings of the prior art. The out-put ~rom this element is then integrated in an integrator stage 126 which generates the control voltage signal used to operate STC-121 1153~65 the variable gain element 112. In this ~ay a-servo loop is provided or variable gain of the servo signal so that the amplitude of the servo signal is maintained at a substantially constant level. The gain-controlled servo signal is according-ly also passed from the peak detectors 118 and 120 to a aif~er-ence amplifier 126 which measures the difference between even and odd servo pulses. This amounts, of course, to a signal indicative of whether the servo head velocity is increasing or slowing down. The output of the difference ampli~ier 126, therefore, comprises a position error signal which can be passed to a compensator 128 and thence to a power ampliier 130 which in turn provides the power to the linear motor 132 which moves the carriage 108 and the heads, including both read/write heads 106 and the servo data head 104 back and forth with respect to the disk pack 102.
It will be appreciated tha,t the system described above provides automatic gain control to the servo signals of a servo system used to control the position of read/write heads with respect to the disk pack. The bandwidth of this system is its ability to ollow a time varying input signal. It will be appre~iated that the signal varies both as the speed of the carriage varies with respact to the disk pack and also if the head 104 from which the servo signals are derived moves with respect to the disk pack 102. The heads which are used in magnetic disk storage drives are extremely delicate assemblies which fly only a few millionths of an inch above the surface o the data storage disk and, accordingly, are somewhat suscept-ible to high frequency mechanical resonances. If these reson-ances are of a frequency outside the bandwidth o the AGC loop, servo head positioning is inaccurate.
As discussed above, it is important, therefore, that the automatic gain control circuit not bP affectea by these reson-ances.
The bandwidth of the prior art automatic gain control can be analyzed through the use of FIG. 2. Since the corner frequency of the low pass filter is high in comparison ~o the bandwidth of the automatic control loop, it can be ignorea and the loop gain of the system can be simplified to that of a first order system. The loop gain of the system is then simply the product of multiplication of the individual gain of each of the elements of the loop or: ~
Equation 1:
Gain = Vin a KLPFK~AKpdKi ~1) S
15 where:
Vin = output of the servo preamp 110;
~gain of variable gain element 112;
Vout ~LPF = gain of filter 114;
20KBA = gain of bu~fer amp 116;
Kpd = gain of peak detectors 118, 120; and Ki ~ gain of integrator 126.
S a j ~
` It will be appreciated that the response time of the loop.is related to the bandwidth of the AGC loop. By perturb-ing the AGC loop with a step-like change in the Vin term, the loop performance can be gauged by observing the output response time. A typical prior art respo,nse i9 shown in FIG.
3, in which FIG. 3(a) is the stepwise input V(in) and FIG.
3(b) is the system's response V(out). The time constant T is STC-121 11534~5 equal to the reciprocal of the closed loop bandwidth for the first order system described by Equation 1, i.e., is also proportional to ~ . The output signal response is within 37~ of its steady state value after one time constant as illus-trated in FIG. 3.
The loop gain, the automatic gain control bandwidth, is proportional to the "Vin~ " product. Due to media and head variations, a 7-fold variation in the signal amplitude o~ Vin is not uncommon. The variation in ~ is also large, due to the large variation in operating points in variable gain con-trol voltage, and the non-linearity of the usual gain control element. One solution to limit the resultant variation in loop gain would be to set the worst case minimum bandwidth of the automatic gain control higher than the vertical mode instability mechanical resonance frequency of the servo hea~.
This would effectively remove the amplitude v`ariation due ~o the resonance effect, but then the maximum worst case bandwidth of the automatic gain control would be in violation of the sampling rate criteria of the encoded servo position informa-~0 tion on the disc. Therefore, it would clearly be desirable to devise an AGC circuit in which the loop gain function did not depend on Vin or on a. Such a circuit would provide a con-stant bandwidth automatic gain control loop.
The circuit-of the invention, which is a co~stant bandwidth automatic gain control circuit, is shown in FIG. 4~
Gain control is accomplished by use of two variable gain ele-ments to control two AGC loops, a "fast" loop and a "slow" loop.
The signal from servo head 1 is first preamplified in preamp 2, and then passed to a first variable gain element 20. This is used in an attenuator configuration consisting o~ two attenua-STC-121 llS3465 tor resistors 3 and a P-type j~nction field effect transistor (J-FET) 4 used as a variable resistance device. The other variable gain element 21, comprises an N-type J~FET 8, used as the emitter degeneration resistance of a differential amplifier S encompassed within a wide bandwidth or "video" amplifier 5.
The constant bandwidth automacic gain control according to the invention can be implemented with other forms of varia~le gain elements such as transconductance-amplifiers, multipliers or any other square-law device, and is not limited to field effect transistors. The variable gain element 20 used in the attenua-tor configuration is set to a fixed DC attenuation by a,refer-ence voltage Vref supplied at 15 when an AC signal i5 passed to the voltage controlling port ~the gate of J-FET 4) throu~h,the action of a high pass filter 14. Thus, for steady-state input signals from preamp 2, the gain control element 20 in the attenuator configuration provides a constant attenuation for low-frequency signal variation.
Under the circumstance o a signal of steady-state amplitude received from preamp 2, the second gain control element, J-FET 8, used between two emitter legs of a differen-tial amplifier contained in the video amplifier 5 is controlled via the slow loop to adjust the gain of the video amplifier until the sum of peaX detectors 9 (which are as in the prior art embodiment of FIG. 1) equals a predeter~ined reerence voltage supplied to a summing network 11~ Thus, no addition-al change of the output of inteqrator 12 takes place. m e response time of the automatic gain control or slow loop con-sisting o video amplifier 5, low pass fi,lter 6, (for noise filtration) buffer amplifier 7, peaX detectors 9, summing amplifier 10, summing network 11, integrator 12, lag-lead com-r STC--121 ~Ll.534~;5 pensator 13 (i.e. a compensator havlng a gain varying with in-put signal frequency) and N-FET 8, is not constant, but can be set such that the maximum worst case banawidth is much lower than the fast loop bandwidth. T~e linear motor is controlled S as in the circuit of FIG. 1, by the signals present at the out-put of the peak detectors 9.
The fast loop automatic gain control circuit uses most of the slow loop, and consists of the attenuator action of attenuator resistors 3, field effect transistor 4, fixed average gain video amp 5, low pass ilter 6, buffer amplifier 7, peak detectors 9, summing amplifier 10, summing network 11, integra-tor 12 and high pass filter 14.
A block diagram of the fast loop for small signai perturbation of Vin is illustrated in FIG. 5; from the block diagram in FIG. 5, the loop gain expression is calculated ~y multiplying all blocXs around the loop.
KIKHKvKLpFKBKpDa~ 2) Loop Gain =
S Tp t- 1 where S is j~ and Tp is the corner frequency of the high pass ilter 14.
For a constant aVin product the loop gain cross-over requency is fixed which would result in the desirea constant closed loop bandwid~h and a constant step response for a small change in Vin amplitude.
For small signal perturbation, the average value at Vin is defined by Equation 3.
_ g _ r 3~
VREF
VIN = (3) KA}cvKLpFKBKpD
where KA is the gain oE variable gain element 20.
Since all terms in Equation 3 are constants, Vin can be treated as a constant. -The term ~ is calculated by partial differentiation of attenuator 4 output voltage with respect to high pass filter 14 output voltage. Since the reference voltage (VREF 15) is fixed, ~ is substantially constant, and is independent of Vi~.
The ~ Vin product is there~ore constant for small signal pertur-bation of Vin; this results in the desired constant bandwidth AGC system.
Since ~ for the system is the reciprocal of closed loop bandwidth, ~quation 4 defines ~ .
. ~
KA Tp ~ 4 ) =
K l KH (XVREF
Equation 4 is obtained by substituting E~uation`3 into Equation 2 and then taking the reciprocal o~ the gain crossover frequency in radians/sec.
The placement of the variable gain control elements is not limited to the preferred embodiment. The fast loop `variable gain element could be interchanged with the slow loop ~5 variable gain element and the overall system for small per-turbations o~ Vin would exhibit constant bandwidth.
It will be appreciated that there has been described an automatic gain control circuit which Eeatures fast and ~low loops each controlling a variable gain element by the appropri-r ~3~
ate use of low and high pass filters. One loop (the slow loop) responds to substantially low frequency (i.e. below 1 kHz) varia-tions in the input signal, while the other, comparatively fast loop responds to high frequency (1-20 kHz) per-turbations in the signal. In this way the efect of these perturbations on the control signal used to servo the motion of ~he read head with respect to magnetic disks can be eliminated and improved accuracy in the control of these positions can be achieved~
` In the preferred embodiment a video amplifier is used to respond to the slowly varying dc component of the input sig-nal. This is compared with a reference voltage which is used in turn to generate a servo signal used to drive the linear motor which moves the carriage carrying the heaas back and'forth with respect to the disk. At the same time, fast oscillations in the system caused by mechanical resonances and the like are controlled by means of a high pass filter to control the con-ductance of a J-FET which serves to attenuate the incoming sig-nal to seduce the effects of these perturbations on the servo loop. In this way, increased stability and accuracy of opera-tion of the servo loop and hence of the magnetic disk storage apparatus within which it is used are improved. In the partic-ularly pre~erred embodiment which is shown with respect to FIGS.
4 and 5, the nonlinear response characteristics of the J-FET 4 used to attenuate the-input signal is controlled by biasing the J-FET 4 by use of the reference voltage Vref supplied at 15. In this way the nonlinearity of this circuit element is de-emphasize and removed from the gain equation. Similarly, the slow loop is used for correction of large low-frequency variations in Vin, again so as to ensure stability to the circuit and to allow for constant closed loop bandwidth, which in turn permits constant ~ 11 --stepwise response for changes in the amplitude of the incoming signal Vin Therefore, it will be understood by those skilled in the art that t'nere are other modifications and ernbodiments of the invention which nevertheless fall within its spirit and scope which is as defined by the following claims.
.
Field of the Invention This invention relates to a method of automatic gain control. In particular, this invention is described with re-ference to an automatic gain control used in a servo system controlling the position of the read/write head of a rotatable magnetic disk storage system.
Background of the Invention In the prior art, automatic gain cont~ol has been used in the head positioning servo loop of a magnetic disk storage system. The servo system uses prerecorded head posi-tion information on one surface of a stack of rotating magnetic platters to generate a regular signal for control. The infor-mation is read from a servo head flying above the servo surface.
As the servo head is mechanically ganged to data heads flying over the other surfaces of the stack of rotating magnetic platters via a carriage mechanism, the servo position informa-tion indicates the position of the data heads. The head posi-tion information must be amplified, filtered and decoded (or peak detected) to provide a position error signal used to drive a linear motor attached to the carriage mechanism such that the data heads are correctly positioned over their respec-tive data cylinders. ~An automatic gain control is used in the head positioning servo decoding system to remove signal ampli-tude variations due to media variations, head variations, amplification variations and flying height variations which otherwise cause inaccurate head positioning.
The bandwidt~ or response time of the automatic gain control is the ability to follow a time varying input signal~
''~
One such time varying signal is that of vertical mode instabil-ity or mechanical resonance of the servo head itself. As the servo head moves up and down with respect to the disX, the out-put signal from the head changes in amplitude because the output amplitude of the servo head is a function of its flying height.
Thus, if the automatic gain control had a bandwidth or response time in excess of the frequency of the vertical mode instabil-ity, the amplitude variations wouid eEfectively be removed.
` However, this solution would also prevent ~he system from operating on the servo data.
Ob`ects of the Invention It is accordingly an object of the invention to pro-vide a constant bandwidth automatic gain control ~ircuit for a magnetic disk storage system.
I 15 A further object o the invention is to provide an automatic gain control circuit which is not affected by varia-tions in its input amplitude due to mechanical resonances in the system which it is used to control but which maintains sufficient bandwidth to provide useful servo control.
A further object of the invention is to provide a constant band~idth automatic gain control which is substantial-ly unafected by nonlinearities in the components used to make up the circuit.
Still a further object of the invention is to provide constant bandwidth automatic gain control which is substantial-ly insensitive to its incoming signal level and to nonlinear-ity in its variable gain element but which can be implemented with relative ease and at little expense.
Summary of the Invention The above listed objects of the invention and needs of the art are satisfied by the present invention which provides a constant bandwidth automatic gain cont.rol which has a response time that is insensitive to the incoming signal level and to nonlinearities in the response of the variable gain elements used to provide variable gain to the circuit. This is accom-plished by providing irst and second servo loops, one fast and one slow, each comprising a variable gain element and appropriate filtering so that substantially low frequency variations in the signal are corrected for in accardance with the object of accurate control of a magnetic head but high frequency instabilities in the signal due to servo loops are separately controlled in a loop having a much faster response time. In this way, the mechanical resonances are precludea from affecting the accuracy of the servo posi~ioning o the head.
Brief Description o the Drawings The invention will be better understood if reference is made to the accompanying drawings, in which:
FIG. 1 represents a block diagram of a typical prior art automatic gain control circuit;
FIG. 2 represents a diagram useful in analyzing the bandwidth of the prior art automatic gain control;
FIG. 3 shows a graphical representation of system reponse;
FIG. 4 shows a block diagram of the automatic gain control according to the invention comparable to FIG. l;
FIG. 5 shows a diagram comparable to FIG. 2, useful STC-121 ~53~
in analyzing the bandwidth of the automatic gain control accord-ing to the invention.
Description of the Preferred Embodiments Reference will now be made to FIG. 1, which as men-tioned abov~ is a diagram of a prior art servo system~
Servo information is ta~en ~rom lower most disk 100 of a disk pack indicated generally at 102 by a read head 104 which is ganged to a plurality of data read heads 106 by means of a carriage 108. The data is written and read in any conven-tional manner; this does not relate to the present invention.
The servo data is passed to a servo preamp 110 which amplifies it; it is then passed to a variable gain element 112. The replacement of the variable gain element 112, which amplifies the signal from the servo preamp to an ~ssentially constant level to enable accurate servoing of the movement of the carriage 108 with respect to the disks 102 is the subiect of this application.
The signal from the variable gain element 112 is then passed to a low pass filter 114 and a buffer amplifier 116 and then to a pair of peak detectors 118, 120 which are gated to be out of phase so that their combined output, when applied ~o a summing ampliier 122, provides an average of the peak height o alternate pulses which are supplied from the amplifier 116.
The output of the summing amplifier I12 is thus an average of the height of the amplified and filtered servo data. This is then compared in a comparator 124 with a reference voltage supplied according to the teachings of the prior art. The out-put ~rom this element is then integrated in an integrator stage 126 which generates the control voltage signal used to operate STC-121 1153~65 the variable gain element 112. In this ~ay a-servo loop is provided or variable gain of the servo signal so that the amplitude of the servo signal is maintained at a substantially constant level. The gain-controlled servo signal is according-ly also passed from the peak detectors 118 and 120 to a aif~er-ence amplifier 126 which measures the difference between even and odd servo pulses. This amounts, of course, to a signal indicative of whether the servo head velocity is increasing or slowing down. The output of the difference ampli~ier 126, therefore, comprises a position error signal which can be passed to a compensator 128 and thence to a power ampliier 130 which in turn provides the power to the linear motor 132 which moves the carriage 108 and the heads, including both read/write heads 106 and the servo data head 104 back and forth with respect to the disk pack 102.
It will be appreciated tha,t the system described above provides automatic gain control to the servo signals of a servo system used to control the position of read/write heads with respect to the disk pack. The bandwidth of this system is its ability to ollow a time varying input signal. It will be appre~iated that the signal varies both as the speed of the carriage varies with respact to the disk pack and also if the head 104 from which the servo signals are derived moves with respect to the disk pack 102. The heads which are used in magnetic disk storage drives are extremely delicate assemblies which fly only a few millionths of an inch above the surface o the data storage disk and, accordingly, are somewhat suscept-ible to high frequency mechanical resonances. If these reson-ances are of a frequency outside the bandwidth o the AGC loop, servo head positioning is inaccurate.
As discussed above, it is important, therefore, that the automatic gain control circuit not bP affectea by these reson-ances.
The bandwidth of the prior art automatic gain control can be analyzed through the use of FIG. 2. Since the corner frequency of the low pass filter is high in comparison ~o the bandwidth of the automatic control loop, it can be ignorea and the loop gain of the system can be simplified to that of a first order system. The loop gain of the system is then simply the product of multiplication of the individual gain of each of the elements of the loop or: ~
Equation 1:
Gain = Vin a KLPFK~AKpdKi ~1) S
15 where:
Vin = output of the servo preamp 110;
~gain of variable gain element 112;
Vout ~LPF = gain of filter 114;
20KBA = gain of bu~fer amp 116;
Kpd = gain of peak detectors 118, 120; and Ki ~ gain of integrator 126.
S a j ~
` It will be appreciated that the response time of the loop.is related to the bandwidth of the AGC loop. By perturb-ing the AGC loop with a step-like change in the Vin term, the loop performance can be gauged by observing the output response time. A typical prior art respo,nse i9 shown in FIG.
3, in which FIG. 3(a) is the stepwise input V(in) and FIG.
3(b) is the system's response V(out). The time constant T is STC-121 11534~5 equal to the reciprocal of the closed loop bandwidth for the first order system described by Equation 1, i.e., is also proportional to ~ . The output signal response is within 37~ of its steady state value after one time constant as illus-trated in FIG. 3.
The loop gain, the automatic gain control bandwidth, is proportional to the "Vin~ " product. Due to media and head variations, a 7-fold variation in the signal amplitude o~ Vin is not uncommon. The variation in ~ is also large, due to the large variation in operating points in variable gain con-trol voltage, and the non-linearity of the usual gain control element. One solution to limit the resultant variation in loop gain would be to set the worst case minimum bandwidth of the automatic gain control higher than the vertical mode instability mechanical resonance frequency of the servo hea~.
This would effectively remove the amplitude v`ariation due ~o the resonance effect, but then the maximum worst case bandwidth of the automatic gain control would be in violation of the sampling rate criteria of the encoded servo position informa-~0 tion on the disc. Therefore, it would clearly be desirable to devise an AGC circuit in which the loop gain function did not depend on Vin or on a. Such a circuit would provide a con-stant bandwidth automatic gain control loop.
The circuit-of the invention, which is a co~stant bandwidth automatic gain control circuit, is shown in FIG. 4~
Gain control is accomplished by use of two variable gain ele-ments to control two AGC loops, a "fast" loop and a "slow" loop.
The signal from servo head 1 is first preamplified in preamp 2, and then passed to a first variable gain element 20. This is used in an attenuator configuration consisting o~ two attenua-STC-121 llS3465 tor resistors 3 and a P-type j~nction field effect transistor (J-FET) 4 used as a variable resistance device. The other variable gain element 21, comprises an N-type J~FET 8, used as the emitter degeneration resistance of a differential amplifier S encompassed within a wide bandwidth or "video" amplifier 5.
The constant bandwidth automacic gain control according to the invention can be implemented with other forms of varia~le gain elements such as transconductance-amplifiers, multipliers or any other square-law device, and is not limited to field effect transistors. The variable gain element 20 used in the attenua-tor configuration is set to a fixed DC attenuation by a,refer-ence voltage Vref supplied at 15 when an AC signal i5 passed to the voltage controlling port ~the gate of J-FET 4) throu~h,the action of a high pass filter 14. Thus, for steady-state input signals from preamp 2, the gain control element 20 in the attenuator configuration provides a constant attenuation for low-frequency signal variation.
Under the circumstance o a signal of steady-state amplitude received from preamp 2, the second gain control element, J-FET 8, used between two emitter legs of a differen-tial amplifier contained in the video amplifier 5 is controlled via the slow loop to adjust the gain of the video amplifier until the sum of peaX detectors 9 (which are as in the prior art embodiment of FIG. 1) equals a predeter~ined reerence voltage supplied to a summing network 11~ Thus, no addition-al change of the output of inteqrator 12 takes place. m e response time of the automatic gain control or slow loop con-sisting o video amplifier 5, low pass fi,lter 6, (for noise filtration) buffer amplifier 7, peaX detectors 9, summing amplifier 10, summing network 11, integrator 12, lag-lead com-r STC--121 ~Ll.534~;5 pensator 13 (i.e. a compensator havlng a gain varying with in-put signal frequency) and N-FET 8, is not constant, but can be set such that the maximum worst case banawidth is much lower than the fast loop bandwidth. T~e linear motor is controlled S as in the circuit of FIG. 1, by the signals present at the out-put of the peak detectors 9.
The fast loop automatic gain control circuit uses most of the slow loop, and consists of the attenuator action of attenuator resistors 3, field effect transistor 4, fixed average gain video amp 5, low pass ilter 6, buffer amplifier 7, peak detectors 9, summing amplifier 10, summing network 11, integra-tor 12 and high pass filter 14.
A block diagram of the fast loop for small signai perturbation of Vin is illustrated in FIG. 5; from the block diagram in FIG. 5, the loop gain expression is calculated ~y multiplying all blocXs around the loop.
KIKHKvKLpFKBKpDa~ 2) Loop Gain =
S Tp t- 1 where S is j~ and Tp is the corner frequency of the high pass ilter 14.
For a constant aVin product the loop gain cross-over requency is fixed which would result in the desirea constant closed loop bandwid~h and a constant step response for a small change in Vin amplitude.
For small signal perturbation, the average value at Vin is defined by Equation 3.
_ g _ r 3~
VREF
VIN = (3) KA}cvKLpFKBKpD
where KA is the gain oE variable gain element 20.
Since all terms in Equation 3 are constants, Vin can be treated as a constant. -The term ~ is calculated by partial differentiation of attenuator 4 output voltage with respect to high pass filter 14 output voltage. Since the reference voltage (VREF 15) is fixed, ~ is substantially constant, and is independent of Vi~.
The ~ Vin product is there~ore constant for small signal pertur-bation of Vin; this results in the desired constant bandwidth AGC system.
Since ~ for the system is the reciprocal of closed loop bandwidth, ~quation 4 defines ~ .
. ~
KA Tp ~ 4 ) =
K l KH (XVREF
Equation 4 is obtained by substituting E~uation`3 into Equation 2 and then taking the reciprocal o~ the gain crossover frequency in radians/sec.
The placement of the variable gain control elements is not limited to the preferred embodiment. The fast loop `variable gain element could be interchanged with the slow loop ~5 variable gain element and the overall system for small per-turbations o~ Vin would exhibit constant bandwidth.
It will be appreciated that there has been described an automatic gain control circuit which Eeatures fast and ~low loops each controlling a variable gain element by the appropri-r ~3~
ate use of low and high pass filters. One loop (the slow loop) responds to substantially low frequency (i.e. below 1 kHz) varia-tions in the input signal, while the other, comparatively fast loop responds to high frequency (1-20 kHz) per-turbations in the signal. In this way the efect of these perturbations on the control signal used to servo the motion of ~he read head with respect to magnetic disks can be eliminated and improved accuracy in the control of these positions can be achieved~
` In the preferred embodiment a video amplifier is used to respond to the slowly varying dc component of the input sig-nal. This is compared with a reference voltage which is used in turn to generate a servo signal used to drive the linear motor which moves the carriage carrying the heaas back and'forth with respect to the disk. At the same time, fast oscillations in the system caused by mechanical resonances and the like are controlled by means of a high pass filter to control the con-ductance of a J-FET which serves to attenuate the incoming sig-nal to seduce the effects of these perturbations on the servo loop. In this way, increased stability and accuracy of opera-tion of the servo loop and hence of the magnetic disk storage apparatus within which it is used are improved. In the partic-ularly pre~erred embodiment which is shown with respect to FIGS.
4 and 5, the nonlinear response characteristics of the J-FET 4 used to attenuate the-input signal is controlled by biasing the J-FET 4 by use of the reference voltage Vref supplied at 15. In this way the nonlinearity of this circuit element is de-emphasize and removed from the gain equation. Similarly, the slow loop is used for correction of large low-frequency variations in Vin, again so as to ensure stability to the circuit and to allow for constant closed loop bandwidth, which in turn permits constant ~ 11 --stepwise response for changes in the amplitude of the incoming signal Vin Therefore, it will be understood by those skilled in the art that t'nere are other modifications and ernbodiments of the invention which nevertheless fall within its spirit and scope which is as defined by the following claims.
.
Claims (10)
1. In apparatus for the control of the position of a recording head with respect to a magnetic data storage media, said position being controlled by drive means, said drive means being controlled by a servo system, said servo system receiving signals from one of said magnetic disks and comprising an auto-matic gain circuit for control of amlification applied to said servo data, the improvement which comprises said automatic gain control circuit comprising first and second gain control loops, each of said loops comprising a variable gain element, one of said variable gain elements responding to substantially slower variations in the amplitude of said servo signals and one of said loops responding to substantially faster variations in the amplitude of said input signals whereby the automatic gain con-trol circuit of said servo loop is of substantially constant bandwidth.
2. The apparatus of claim 1 wherein said variable gain elements comprise field effect transistors.
3. The apparatus of claim 2 wherein the field effect transistor in said fast loop is supplied with said servo signals after said signals have been filtered by a high pass filter.
4. The apparatus of claim 2 wherein the field effect transistor in said slow loop is connected between emitter legs of a differential amplifier comprised in a video amplifier used to amplify said servo signals.
5. The apparatus of claim 4 in which said field effect transistor is controlled by means of a lag lead compen-sator.
6. A method of applying automatic gain control to servo signals used to control the motion of magnetic read and write heads with respect to magnetic data storage media com-prising the steps of amplifying said servo signals read from said magnetic disk media by a servo read head, said amplifica-tion being varied by automatic gain control means, said varia-tion being performed by a plurality of gain control elements, one of said gain control elements being adapted to control said gain in response to relatively lower frequency variations in the amplitude of said servo signals and another of said gain control elements being controlled to vary the gain applied to said servo signals in response to substantially higher frequency variations in the amplitude of said servo signals.
7. The method of claim 6 wherein said variable gain control elements comprise field effect transistors.
8. The apparatus of claim 7 in which said high frequency signals are used to control the first one of said field effect transistors by application of a high pass filter to said signals.
9. The apparatus of claim 7 wherein said low fre-quency signals are used to control one of said field effect transistors by application of a lag lead compensator to said signals.
10. The method of claim 6 in which the one of said variable gain elements responsive to said low frequency signals is biased by a substantially constant reference voltage so that its response is substantially linear.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/147,724 US4326226A (en) | 1980-05-08 | 1980-05-08 | Constant bandwidth automatic gain control |
| US147,724 | 1980-05-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1153465A true CA1153465A (en) | 1983-09-06 |
Family
ID=22522662
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000377177A Expired CA1153465A (en) | 1980-05-08 | 1981-05-08 | Constant bandwidth automatic gain control |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US4326226A (en) |
| JP (1) | JPS576901A (en) |
| CA (1) | CA1153465A (en) |
Families Citing this family (21)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2472304A1 (en) * | 1979-12-19 | 1981-06-26 | Cii Honeywell Bull | METHOD FOR CONTROLLING THE GAIN OF CIRCUITS AMPLIFYING SIGNALS DELIVERED BY A READING HEAD ASSOCIATED WITH AN INFORMATION CARRIER AND DEVICE FOR IMPLEMENTING THE SAME |
| US5153786A (en) * | 1982-05-10 | 1992-10-06 | Digital Equipment Corporation | Extended range servo system for positioning a disk drive head over a selected track |
| ATE39588T1 (en) * | 1982-05-10 | 1989-01-15 | Digital Equipment Corp | POSITIONING CONTROL SYSTEM WITH BOTH CONTINUOUS AND INSERTED SERVO INFORMATION FOR A MAGNETIC DISK MEMORY. |
| US5136440A (en) * | 1982-05-10 | 1992-08-04 | Digital Equipment Corporation | Track identification and counting in a disk drive positioning system |
| US5187619A (en) * | 1982-05-10 | 1993-02-16 | Digital Equipment Corporation | High speed switched automatic gain control |
| US5153787A (en) * | 1982-05-10 | 1992-10-06 | Digital Equipment Corporation | Combination embedded and dedicated servo system including embedded servo waiting |
| US5202802A (en) * | 1982-05-10 | 1993-04-13 | Digital Equipment Corporation | Methods of writing and detecting dibit servo encoding |
| US5099367A (en) * | 1982-05-10 | 1992-03-24 | Digital Equipment Corporation | Method of automatic gain control basis selection and method of half-track servoing |
| US5095471A (en) * | 1982-05-10 | 1992-03-10 | Digital Equipment Corporation | Velocity estimator in a disk drive positioning system |
| US5220468A (en) * | 1982-05-10 | 1993-06-15 | Digital Equipment Corporation | Disk drive with constant bandwidth automatic gain control |
| US5109307A (en) * | 1982-05-10 | 1992-04-28 | Digital Equipment Corporation | Continuous-plus-embedded servo data position control system for magnetic disk device |
| US5115359A (en) * | 1982-05-10 | 1992-05-19 | Digital Equipment Corporation | Fault tolerant frame, guardband and index detection methods |
| US5115360A (en) * | 1982-05-10 | 1992-05-19 | Digital Equipment Corporation | Embedded burst demodulation and tracking error generation |
| EP0097208B1 (en) * | 1982-06-18 | 1987-03-11 | International Business Machines Corporation | Head positioning system with automatic gain control |
| GB2231976A (en) * | 1989-04-24 | 1990-11-28 | Mitsubishi Electric Corp | Position control |
| US5050016A (en) * | 1989-10-12 | 1991-09-17 | Conner Peripherals, Inc. | Disk drive servo system using gain limited high-frequency track-following compensator |
| JP2672699B2 (en) * | 1990-08-28 | 1997-11-05 | 株式会社東芝 | AGC control device in magnetic disk device |
| US5533031A (en) * | 1994-06-24 | 1996-07-02 | International Business Machines Corporation | Adjustable read channel equalization for error processing in a recordable media drive |
| US6710959B1 (en) * | 2000-02-25 | 2004-03-23 | Texas Instruments Incorporated | Input pole compensation for a disk drive read head |
| US6583671B2 (en) * | 2000-12-01 | 2003-06-24 | Sony Corporation | Stable AGC transimpedance amplifier with expanded dynamic range |
| US7596193B2 (en) | 2005-11-09 | 2009-09-29 | Via Telecom, Inc. | Variable bandwidth automatic gain control |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5858690B2 (en) * | 1977-12-28 | 1983-12-27 | 株式会社東芝 | Speed control method |
| JPS54126515A (en) * | 1978-03-24 | 1979-10-01 | Nippon Shiyuuhenki Kk | Magnetic disc apparatus |
| US4172267A (en) * | 1978-04-03 | 1979-10-23 | Digital Equipment Corporation | Dynamic filter for a moving head disk storage system |
-
1980
- 1980-05-08 US US06/147,724 patent/US4326226A/en not_active Expired - Lifetime
-
1981
- 1981-05-07 JP JP6886481A patent/JPS576901A/en active Granted
- 1981-05-08 CA CA000377177A patent/CA1153465A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| JPS576901A (en) | 1982-01-13 |
| JPH027081B2 (en) | 1990-02-15 |
| US4326226A (en) | 1982-04-20 |
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