The present invention relates to an analog filter for use in a signal readout system for a magnetic or magneto-optical disk, for example.
As magnetic/magneto-optical disk technologies have been remarkably developed in recent years, it has become increasingly necessary to further improve the signal processing technology applicable to reading signals therefrom.
Accordingly, an analog filter is designed such that its transfer function H(s) is given by the following Equation (1)
H(s)=(1−s2)/D(s)=(1+ω2)/D(jω) (1)
where s is a Laplace variable and D(s) is a function representing the denominator of the transfer function of the analog filter. In this case, the numerator of the transfer function H(s) has no imaginary part and therefore does not affect the phase characteristic of the analog filter. In addition, since the high-frequency gain is boosted by the term ω2, the gain-boosted characteristic such as that illustrated in
A filter with the gain-boosted characteristic such as that illustrated in
H1(s)=(gm1·gm2+sC2·gm1x)/(gm22+sC·gm3+s2C1C2) (2) (2)
Thus, the transfer function H(s) of the cascade of the two biquadratic filters shown in
H(s)={(gm1·gm2)2−s2}/(gm22+sC2·gm3+s2C1C2)2 (3)
In this manner, a transfer function having no imaginary part in its numerator and yet having the term ω2 can be obtained, thus easily realizing the gain-boosted characteristic.
A filter network implemented as a cascade of biauadratic filters, however, has its characteristic easily affected by the variation of its components.
As shown in
H(s)=Πk=1n1/(s−sk)
where sk is a vector representing the position of each pole on the Laplace plane. Thus, a frequency gain is an inverse of the product of the vector (s−sk). That is to say, the frequency characteristic of an analog filter is more likely to be affected by a relatively short vector (s−sk). In other words, the frequency characteristic of the filter is affected most by the position of a pole Sk that is closest to the imaginary axis. Also, the position of a pole displaces on the Laplace plane due to the characteristic variations of filter components.
In an analog filter network implemented as a cascade of biquadratic filters, a pair of poles is realized by each of these biquadratic filters. Thus, as shown in
An analog filter may also be implemented as a ladder filter. In a ladder filter, capacitors and inductors are connected together in a ladder shape and its input and output are terminated with resistors. In an LSI, an inductor is usually non-implementable, and therefore is replaced with an equivalent circuit including voltage-controlled current sources and capacitors, thereby constructing a ladder filter. In such a case, the ladder filter is implemented with plural biquadratic filters all coupled together.
Accordingly, in a ladder filter, the positions of all the poles are affected by the characteristic variations of its components. Thus, as shown in
However, the ladder filter is essentially a filter network of passive components. Thus, it has been widely believed that it is difficult to increase its gain to ½ or more or to realize the gain-boosted characteristic as illustrated in
It is therefore an object of the present invention to provide an analog filter exhibiting a gain-boosted characteristic, which is almost consistent even against the characteristic variations of its components.
Specifically, an inventive ladder filter includes multiple inductor sections, each being implemented by an equivalent circuit including voltage-controlled current sources and capacitors. A signal input to the ladder filter is provided to at least one of the voltage-controlled current sources by way of gain adjusting means. A gain obtained by the gain adjusting means is set to such a value as realizing a desired transfer function for the ladder filter.
In the inventive ladder filter, which exhibits a highly consistent filter characteristic even against the characteristic variations of its components, a signal input to the ladder filter is provided to at least one of the voltage-controlled current sources in the inductor sections by way of the gain adjusting means. As a result, even a transfer function, which has been hard to realize in a conventional ladder filter, is also realizable. For example, by setting the ratio of the gains obtained by the gain adjusting means to such a value that the transfer function of the ladder filter has a numerator consisting of only a term that is an even-numbered power of s, e.g., (1+s2), the ladder filter can vary only its gain characteristic while keeping its phase characteristic substantially constant.
In one embodiment of the present invention, a ratio of the gains obtained by the gain adjusting means is preferably set to such a value as making the ladder filter exhibit a desired gain-boosted characteristic independent of its phase characteristic.
In another embodiment, the inventive ladder filter may further include a first signal input terminal provided for a filtering process and a second signal input terminal provided separately from the first signal input terminal. The gain adjusting means preferably receives a signal that has been input to the second signal input terminal.
In this particular embodiment, the inventive ladder filter preferably further includes a variable-gain amplifier provided at a stage preceding the second signal input terminal.
In still another embodiment, a variable gain is preferably obtained by the gain adjusting means.
An inventive analog equalizer includes: a ladder filter including multiple inductor sections, each being implemented by an equivalent circuit including voltage-controlled current sources and capacitors; means for detecting an error between an output signal of the ladder filter and a reference signal; and means for changing a filter characteristic of the ladder filter by reference to the error that has been detected by the detecting means. A signal input to the ladder filter is provided to at least one of the voltage-controlled current sources by way of gain adjusting means, which obtains a variable gain. The changing means changes the gain, obtained by the gain adjusting means of the ladder filter, based on the error that has been detected by the detecting means.
An inventive signal readout system includes the analog equalizer of the present invention, reads out a signal from a recording medium such as a magnetic or magneto-optical disk and filters the signal using the analog equalizer.
a) illustrates the characteristic of a filter network implemented as a cascade of biquadratic filters; and
b) illustrates the characteristic of a ladder filter.
As shown in
Without the second signal input terminal IN2, the ladder filter 1 shown in
Thus, if the input signal Vin is provided to any of the voltage-controlled current sources 11a through 11g separately from the ordinarily input signal with its input gain appropriately controlled, then the transfer function of the ladder filter will have a freely modifiable numerator. As a result, a filter with any of various response characteristics will be obtainable in that case. That is to say, by taking advantage of a technical concept like this, a ladder filter according to this embodiment realizes a desired gain-boosted characteristic independent of its phase characteristic.
Unless the second signal input terminal IN2 is provided, the ladder filter 1 shown in
H(s)=0.5/Hr(s)=0.5/(1.000000000s7+5.233611506s6+19.69755040s5+45.91809198s4+76.50647398s3+84.06826807s2+57.09056406s+17.97359538) (4)
It should be noted that the cutoff frequency is normalized at 1 Hz for the sake of simplicity and
Supposing the gains obtained by the respective constant-ratio gain calculators 15a through 15c are denoted by gin, gm1 and gm2, the transfer function of the ladder filter 1 shown in
Hn(s)=1.219129594gm2s2+(1.142380774gm1+0.5335916099gm2)s+(0.5gm2+0.5gm1+0.5gin+0.5) (5)
If the 0th and 1st-order terms of the numerator Hn(s) are 0.5 and 0, respectively, then the gain-boosted characteristic is realized and the circuit shown in
1.142380774gm1+0.5335916099gm2=0
0.5gm2+0.5gm1+0.5gin=0 (6)
If these Equations are solved, then
gm2=−2.140927168gm1
gin=1.140927168gm1
∴Hn(s)=−2.61gm1s2+0.5 (7)
Thus, the ladder filter 1 shown in
H(s)=(−2.61gm1s2+0.5)/Hr(s) (8)
As can be seen from this transfer function H(s) equation, a desired gain-boosted characteristic is attainable by setting the gain gm1 obtained by the constant-ratio gain calculator 15b to an appropriate value and the boosted gain is changeable by adjusting the gain gm1.
By providing an additional input signal to a voltage-controlled current source separately from an ordinarily input signal and by appropriately controlling the gain ratio in this manner, this embodiment realizes a desired gain characteristic without disturbing its phase characteristic.
The ladder filter 1 shown in
In other words, to make only the gain-boosted characteristic controllable independently, (Vin+Vin·gin) should be input to the first voltage-controlled current source 11a. This is because if Vin·gin is input to the first voltage-controlled current source 11a, then it is impossible to control only the gain-boosted characteristic independently.
By adjusting the gain of the second variable-gain amplifier 21b, the intensity of the signal input to the second signal input terminal IN2 is controllable independent of the signal input to the first signal input terminal IN1. Accordingly, as can be seen from the transfer function H(s) equation (8), the boosted gain of the ladder filter 1 is easily changeable. That is to say, by dividing the variable-gain amplifier preceding the input stage into two, the boosted gain of the ladder filter is controllable independent of the gain control for an ordinarily input signal.
In addition, since the gains are controlled using the variable-gain amplifiers 21a and 21b, the boosted gain can be changed more smoothly compared to using a switch, for example.
The ladder filter 3 shown in
As described in the first embodiment, in a ladder filter with multiple inductors, each implemented as an equivalent circuit consisting of voltage-controlled current sources and capacitors, the input signal Vin is provided to any of the voltage-controlled current sources 11a through 11g separately from the ordinarily input signal. And its input gain is adjusted appropriately, thereby freely controlling the numerator of its transfer function. That is to say, a desired transfer function is realizable for the ladder filter 3 shown in
The ladder filter 3 shown in
The ladder filter 3 may have the numerator Hn(s) of its transfer function represented by a determinant shown in
A determinant shown in
Gm=(AT)−1×K
In the equalizer shown in
The respective elements of the matrix A can be obtained in the following manner. For example, the elements a66, a65, a64, a63, a62, a61 and a60 on the first row of the matrix A are coefficients for respective orders in the numerator of the transfer function when the input signal Vin is provided only to the seventh voltage-controlled current source 11g via the constant-ratio gain calculator 31g in the filter shown in
In the same way, the elements a55, a54, a53, a52, a51 and a50 on the second row of the matrix A are coefficients for respective orders in the numerator of the transfer function when the input signal Vin is input only to the sixth voltage-controlled current source 11f via the constant-ratio gain calculator 31f. Accordingly, in that case, the input signal Vin should not be input through the ordinary signal input terminal IN1, the gains of the first through fifth and seventh constant-ratio gain calculators 31a through 31e and 31g should be set to zero and only the gain of the sixth constant-ratio gain calculator 31f should be set to one. Then, the transfer function of the ladder filter 3 should be derived from the output signal Vout in that case. And the coefficients for respective orders in its numerator may be regarded as the elements a55, a54, a53, a52, a51 and a50 on the second row of the matrix A. The other matrix elements from the third row on can be obtained in a similar manner.
In contrast, according to this embodiment, an analog equalizer circuit is implementable just by providing another input terminal for the ladder filter separately from an ordinary signal input terminal and by inputting a signal through this additional input terminal to the respective voltage-controlled current sources by way of the gain converters. Thus, there is no need to add the differentiator or the like circuit hard to implement with analog components. Furthermore, according to this embodiment, an equalizer circuit can be constructed using the ladder filter with the consistent characteristic. As a result, the characteristic of the equalizer circuit can be further stabilized.
The analog equalizer according to this embodiment is easily applicable to a signal readout system for use in magnetic or magneto-optical recording by a partial response maximum likelihood (PRML) method, in which Viterbi decoding and partial response (PR) equivalent transformation are used in combination.
The PRML method is a promising signal reading method, because the SNR of the read signal can be improved compared to conventional magnetic recording techniques using a data slicer. Accordingly, a magnetic or magneto-optical disk signal readout system including the analog equalizer of the present invention can have its digital circuit section downsized compared to the conventional signal readout system including the digital equalizer. Thus, the present invention contributes to reduction in power dissipated and circuit size.
In the conventional signal readout system for use in PRML magnetic or magneto-optical recording, the equalizer often includes a digital filter. This is because no analog equalizers, qualified for the signal readout system, have been available so far. In contrast, the analog equalizer according to this embodiment uses the ladder filter with a low sensitivity as a basic circuit component and needs no differentiator that is hard to implement using analog components. Thus, compared to the conventional analog equalizer, the inventive analog equalizer attains much higher precision and requires much less area and power. Furthermore, an analog equalizer can reduce power dissipation more easily than a digital equalizer generally speaking.
Accordingly, a signal readout system including the analog equalizer of the present invention is much more advantageous in precision, area and power dissipation. It should be noted that the same effects are also attainable by applying the inventive analog equalizer to any system for reading out a signal from a recording medium other than a magnetic or magneto-optical disk.
As described above, the inventive ladder filter exhibits a filter characteristic highly consistent even against the characteristic variations of its components and provides its input signal to at least one of the voltage-controlled current sources for inductor sections via gain adjusting means. As a result, even a transfer function that has been hard to realize in the conventional filters is realizable. For example, by setting the ratio of the gains obtained by the gain adjusting means to such a value that the transfer function of the ladder filter has a numerator (1+s2), a desired gain-boosted characteristic is realizable independent of its phase characteristic.
In addition, by using such a filter, a downsized analog equalizer with stabilized characteristics can be obtained. Furthermore, a signal readout system including an analog equalizer like this is much more advantageous in precision, area and power dissipation.
Number | Date | Country | Kind |
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11-168872 | Jun 1999 | JP | national |
This application is a divisional of Application Ser. No. 09/594,153 filed Jun. 15, 2000 now abandoned.
Number | Name | Date | Kind |
---|---|---|---|
4381489 | Canning et al. | Apr 1983 | A |
4823092 | Pennock | Apr 1989 | A |
4839542 | Robinson | Jun 1989 | A |
5257286 | Ray | Oct 1993 | A |
5392003 | Nag et al. | Feb 1995 | A |
5596459 | Kovner et al. | Jan 1997 | A |
6144981 | Kovacs et al. | Nov 2000 | A |
6317016 | Kuo | Nov 2001 | B1 |
6369644 | Yoshizawa | Apr 2002 | B1 |
6838929 | Mitteregger | Jan 2005 | B2 |
7009446 | Yoshida et al. | Mar 2006 | B2 |
Number | Date | Country |
---|---|---|
50-43860 | Apr 1975 | JP |
6-61791 | Mar 1994 | JP |
6-78007 | Mar 1994 | JP |
6-164314 | Jun 1994 | JP |
8-274583 | Oct 1996 | JP |
Number | Date | Country | |
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20040070443 A1 | Apr 2004 | US |
Number | Date | Country | |
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Parent | 09594153 | Jun 2000 | US |
Child | 10680129 | US |