Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
1999-09-30
2001-02-27
Ton, My-Trang Nu (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S558000
Reexamination Certificate
active
06194958
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to filters and more particularly to methods and apparatus for tuning the bandwidth, or cut-off frequency, of such filters. Still more particularly, the invention relates to a method for minimizing cut-off frequency variation of such filters by using external reference components.
As is known in the art, an integrated circuit low pass filter using commonly available active and passive components will experience a large variation in the actual cut-off frequency due to the component dependencies on fabrication processes and temperature and can be controlled to a small variation (i.e., tolerance) in its value. Additionally, all stages in a multi-stage filter will vary independently. Such variations are undesirable in many applications.
SUMMARY OF THE INVENTION
In accordance with the present invention, a filter is provided having a low pass filter section on a semiconductor integrated circuit chip. The low pass filter section includes an on-chip component. The low pass filter section has a cut-off frequency related to the on-chip component. A control circuit is provided. The control circuit includes an on-chip component and an off-chip component. The control circuit on-chip component and the filter on-chip component are subject to the same integrated circuit fabrication process, and/or temperature, and are therefore matched. The control circuit off-chip component, being off-chip, is independent of the chip fabrication process, and/or temperature. The control circuit off-chip component is established in accordance with a desired low pass filter section cut-off frequency. In order to compensate for process, and/or temperature, induced variations in the desired cut-off frequency of the low pass filter section, a control loop is provided. The control loop is responsive to a control signal which is a proportional to the difference between the control circuit off-chip component and the control circuit on-chip component. The control loop is responsive to this difference and automatically adjusts the on-chip component in a proportional relationship with the off-chip component. The control loop is also used to adjust the filter on-chip component thereby automatically providing the desired cutoff frequency for the low pass filter section independent of process, and/or temperature, induced variations.
In accordance with the another feature of the invention, a filter is provided having a semiconductor integrated circuit chip and a low pass filter section on the chip. The low pass filter section includes a filter transconductor and a filter capacitor connected to the filter transconductor. The low pass filter section has a cut-off frequency related to the transconductance, gm
F
, of the filter transconductor and the capacitance of the filter capacitor, C
F
. A gm control circuit is provided. The gm control circuit includes a control circuit transconductor, a gm control circuit on-chip capacitor C
INT
, an off-chip resistor R
EXT
and an off-chip capacitor C
EXT
. The control circuit transconductor and the filter transconductor are subject to the same integrated circuit fabrication process and are therefore matched. The resistor R
EXT
and capacitor C
EXT
, being off-chip, are independent of the fabrication process. The product R
EXT
C
EXT
is established in accordance with a desired low pass filter section cut-off frequency. The on-chip capacitor, C
INT
, is matched to the filter capacitor, C
F
, and is therefore both subject to the same IC chip fabrication process. In order to compensate for process and/or temperature, induced variations in the desired cut-off frequency of the low pass filter section, the gm
F
of the filter transconductor is corresponding varied by a control loop to maintain R
EXT
C
EXT
proportional to gm
F
−1
C
INT
. More particularly, the transconductance, gm
C
, of the control circuit transconductor is automatically adjusted by a control signal produced by the control circuit so that its gm
C
−1
times C
INT
product is in proper proportion to R
EXT
C
EXT
. Simultaneously, the control signal is also used to adjust the gm
F
of the filter transconductor thereby automatically providing the desired cut-off frequency for the low pass filter section independent of process and/or temperature, induced variations.
In accordance another aspect of the invention, a filter is provided having a semiconductor integrated circuit chip and a low pass filter section on the chip. The low pass filter section includes a filter transconductor and a filter capacitor connected to the filter transconductor. The low pass filter section has a cut-off frequency related to the transconductance, gm
F
, of the filter transconductor and the capacitance of the filter capacitor. A gm control circuit having a control circuit transconductor is provided. The gm control circuit includes a first oscillator for producing a reference frequency. The first oscillator has a portion thereof on-chip, an off-chip capacitor, and an off-chip resistor. The reference frequency is inversely proportional to the product of the capacitance of the off-chip capacitor and the resistance of the off-chip resistor. A second oscillator is on the chip and produces a variable frequency. The second oscillator has an on-chip capacitor. The variable frequency is proportional to the transconductance gm
C
of the control circuit transconductor divided by the capacitance of the on-chip capacitor. The variable frequency is controlled by a feedback control signal. A control loop is provided. The control loop is responsive to a difference between the reference frequency and the variable frequency and such loop produces the control circuit transconductor control signal which feeds the control loop. The control loop, in response to the transconductor control signal, produces the feedback signal to drive the variable frequency towards the reference frequency. The control circuit includes the control circuit transconductor. The control circuit transconductor control signal varies the transconductance of the control circuit transconductor in accordance with the control circuit transconductor control signal, correspondingly changing the transconductance of the filter transconductor, and correspondingly changing the cut-off frequency of the low pass filter section.
With such an arrangement, by using a phase locked loop in the control loop and a pair of on-chip oscillators, the cut-off frequency of the low pass filter section can be controlled and variations thereof minimized using off-chip components to control the reference frequency.
In accordance with another aspect of the invention, the filter includes a plurality of serially coupled filter sections, the transconductance of the filter transconductor thereof being varied in accordance with the control circuit transconductor control signal produced by the gm control circuit.
With such an arrangement, the cut-off frequency can be controlled so that all of the serially coupled low pass filters (i.e., stages) have a fixed frequency relationship to one another.
In accordance with one embodiment of the invention, the gm control circuit includes a frequency/phase comparator, fed by the reference frequency and the variable frequency, for producing an output signal related to the difference between such fed signals. A variable current source is provided that is controlled by the output signal. An output of the current source provides the control circuit transconductor control signal for the control circuit transconductor.
In accordance with another aspect of the invention, a common mode reference voltage is fed to the control circuit transconductor. A common mode voltage control circuit is fed by the common mode reference voltage and an output of the control circuit transconductor for providing a common mode control signal related to the common mode reference voltage. A second current source is controlled by the common mode control signal for supplying current to the control circuit transconductor in accordance with the common mode c
Goldfarb Marc E.
Palmer Wyn T.
Analog Devices Inc.
Fish & Richardson P.C.
Nu Ton My-Trang
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