Wave transmission lines and networks – Automatically controlled systems
Reexamination Certificate
1999-05-17
2001-10-23
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Automatically controlled systems
C333S174000, C333S217000, C333S172000
Reexamination Certificate
active
06307442
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electronic filters and more particularly pertains to a new enhanced LC filter capable of allowing the tuning of a frequency and the calibration of a quality factor of the LC filter.
BACKGROUND ART
Filters are commonly used in various electronic applications to filter out certain components of an electric signal. Prior Art
FIG. 1
shows one example of a commonly known filter referred to as an LC filter. An LC filter basically includes an inductor element L and a capacitive element C combined in parallel.
During use, the LC filter substantially removes, i.e. filters, components of an input electric signal that reside out of a predetermined range, or in other words, are excessively distanced from a desired predetermined frequency, or center frequency f
c
. This center frequency f
c
is shown in the graph of Prior Art FIG.
2
. As shown in the graph, the LC filter suppresses an amplitude, or magnitude, of components of the electrical signal that are either to the right or left of the center frequency f
c
. Given this feature, the center frequency f
c
of the LC filter may be changed by altering either the inductor element L or the capacitive element C, thereby letting only selected components of the electrical signal to pass.
Yet another aspect of LC filters that serves as a measure of how well an LC filters accomplishes its intended function is the quality factor Q of the LC filter. An LC filter that has a high quality factor Q tends to only pass components of an electrical signal that are very close to the center frequency f
c
. With reference to Prior Art
FIG. 2
, a thinner curve which tightly encompasses the center frequency f
c
is indicative of a high quality factor Q. When designing an LC filter, the quality factor Q may be varied by introducing a parallel coupled resistive element R, as shown in Prior Art FIG.
1
.
The foregoing LC filter is often referred to as a “bandpass” filter. It should be noted that other types of filters exist which operate under similar principles. For example, “lowpass” and “highpass” filters operate to pass only components of an electrical signal that are less than or greater than a cutoff frequency, respectively.
It thus becomes apparent that two very important features associated with all types of filters may each be controlled by manipulating the various elements of the filter. Prior art methods of manipulating such filter elements, however, are very primitive in that manipulation tends to be done manually. Further, the control of the center frequency f
c
and quality factor Q is usually carried out independently.
There is thus a need for an enhanced filter that allows the tuning of a frequency and further the calibration of a quality factor of the filter in an effective manner that may be carried out while the filter is in use.
DISCLOSURE OF THE INVENTION
The present invention comprises a tunable electronic filter circuit including an input terminal and an output terminal connected to the input terminal with a node therebetween. An inductor, a variable capacitor, and a variable resistor are connected between the node and ground. Coupled to the variable capacitor and the variable resistor is a feedback control circuit.
The feedback control circuit is operable to tune the variable capacitor in order to set a predetermined frequency, or center frequency, of the electronic filter circuit. The feedback control circuit is further operable to tune the variable resistor in order to calibrate a quality factor of the electronic filter circuit. In use when an input signal is applied to the input terminal, the electronic filter circuit passes an output signal to the output terminal. Such output signal includes components of the input signal within a predetermined frequency bandwidth set by the center frequency and filters out other components of the input signal. By this design, the present invention allows effective control of both the center frequency f
c
and quality factor Q of the filter circuit.
In the event that there is a need for controlling the center frequency f
c
and quality factor Q while the electronic filter circuit is being used, a “slave” tunable electronic filter circuit may be provided including a variable capacitor and a variable resistor which are also controlled by the feedback circuit. This allows the “slave” filter to be tuned while being used.
REFERENCES:
patent: 3079571 (1963-02-01), Elliott et al.
patent: 4600903 (1986-07-01), Temer
patent: 5550520 (1996-08-01), Kobayashi
Silva-Martinez, J., Steyaert, M., Sansen W., “A 10.7-MHz 68-dB SNR CMOS Continuous-Time Filter with On-Chip Automatic Tuning,” IEEE Journal of Solid-State Circuits, vol. 27, pp. 1843-1853, Dec., 1992.
Park, C., Schaumann, R., “Design of a 4-MHz Analog Integrated CMOS Transconductance-C Bandpass Filter,” IEEE Journal of Solid-State Circuits, vol. 23, pp. 987-995, Aug. 1988.
Karni, S., Gengsheng Z., “The Analysis of the Continuous-Time LMS Algorithm”, IEEE Transactions on Acoustics, Speech, and Signal Processing, vol. 37, pp. 595-597, Apr. 1989.
Stevenson, J., Sanchez-Sinencio, E., “A Practical Quality Factor Tuning Scheme for IF and High-Q Continuous-Time Filters,” IEEE International Solid-State Circuits Conference, Feb. 1998, pp. 218-219.
Avasarala Madhu
Meyer Robert G.
Bettendorf Justin P.
Maxim Integrated Products
Oppenheimer Wolff & Donnelly LLP
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