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High Performance Cosine Filter With Digital K Selection

IP.com Disclosure Number: IPCOM000100699D
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 4 page(s) / 127K

Publishing Venue

IBM

Related People

Jaquette, G: AUTHOR

Abstract

A circuit which provides a wide bandwidth cosine filter which can have its boost (or K) adjusted via a digital input is described. This filter block can be used to provide high frequency linear phase boost as part of an equalizer. The ability to change the high frequency boost without changing the phase response of the filter is particularly useful in disk file equalizers. Such equalizers are adjusted to accommodate changes in the channel transfer function such as can occur when reading data recorded at different lineal densities.

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High Performance Cosine Filter With Digital K Selection

       A circuit which provides a wide bandwidth cosine filter
which can have its boost (or K) adjusted via a digital input is
described.  This filter block can be used to provide high frequency
linear phase boost as part of an equalizer.  The ability to change
the high frequency boost without changing the phase response of the
filter is particularly useful in disk file equalizers.  Such
equalizers are adjusted to accommodate changes in the channel
transfer function such as can occur when reading data recorded at
different lineal densities.

      Cosine filters, such as the one shown in Fig. 1, are commonly
used in disk files.  The characteristics of the filter shown give a
voltage transfer function of:
Av(w) = G(1-K * COS(wT))                 (1)
where:
K = R2/(R1+R2)  and  G = -RC(R1+R2)/(R1 * R2)       (2)

      One technique that can be used to switch the effective K of the
filter is to use a FET analog switch to put a resistor in parallel
with either R1 or R2.  This technique brings with it the parasitic
capacitance of the switch which degrades the performance of the
filter.

      Another technique which can be used is to essentially have two
cosine filters in parallel which use the same delay line (see Fig.
2).  By setting the resistors in the two filters differently and
multiplexing the output to select one of the two filters in response
to a digital input, K switching is achieved.  The current steering
selection stage provides cascode isolation at the collector of
transistor in the differential amplifier, improving the bandwidth of
the filter.  This circuit provides independent gain and K settings of
the two filters.  In addition to selecting one of the two filters,
another K setting is available by selecting both filters
simultaneously (V1 and V2 greater than VR).  This gives:
K = R2 * R4 * (R1 + R3)/((R1+R2)*R3*R4+R1*R2*(R3+R4))   (3)
G = -RC*((R1+R2)/(R1*R2) + (R3+R4)/(R3*R4))

      A third technique which uses fewer components than the parallel
implementation is shown in Fig. 3.  The characteristics of the filter
depend on the state of the differential voltage, Vs, which selects
the state of the current steering:
For Vs>0.2V,  K = R2/(R1+R2)  and   G = -(RA+RB) (R1+R2)/(R1xR2)  (4)
For VS<-.2V,  K = ((RBxR2)-(RAxR1))/(RAxR1)+RBx(R1+R2))  and (5)
G = -((RAxR1)+RBx(R1+R2))/(R1xR2)

      Note that the K and G are inte...