Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2000-12-18
2001-11-20
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S556000
Reexamination Certificate
active
06320459
ABSTRACT:
BACKGROUND
The present invention concerns design of filters for electrical circuits and pertains particularly to a notch filter implemented using analog sampling.
A filter rejects some frequencies within a signal and allows other frequencies to be transmitted. In a low-pass filter the frequencies that are transmitted extend from zero to some maximum frequency. In a high-pass filter the frequencies that are transmitted are greater than some minimum frequency. In a band-pass filter, the frequencies that are transmitted are between a minimum frequency and a maximum frequency. In a notch filter, a very narrow range of frequencies is removed from the signal.
There are many ways to construct filters. Generally, when designing filters it is desirable to very particularly pass only desired frequencies while as completely as possible rejecting other frequencies. Generally, filters that are very effective at passing desired frequencies and rejecting other frequencies are difficult to design and expensive to build. The present invention sets out filters that are elegant in design, inexpensive to build and extremely effective.
SUMMARY OF THE INVENTION
In accordance with the preferred embodiment of the present invention, a filter system is presented. The filter system includes a low pass filter. The first low pass filter includes an input, an output, a storage means, a switching means and a control means. An input signal is placed on the input. An output signal is generated on the output. The storage means provides storage of a signal sample over time. The switching means, when closed, electrically couples the input to the first end of the storage means. The switching means, when open electrically isolates the input from the first end of the storage means. The control means controls the switching means. The control means generates a switching control signal. The switching control signal has a sampling frequency. A maximum cutoff frequency of the low pass filter is dependent on a value of a capacitance provided by the storage means and a pulse width of the switching control signal.
The filter system also includes a first summer circuit, a second low pass filter and a second summer circuit. The first summer circuit subtracts the output signal from the input signal to produce a high pass output signal. The second low pass filter receives the high pass output signal and produces a bandpass filtered output. The second summer circuit subtracts the bandpass filtered output from the input signal to produce a notch filtered output.
For example, the first summer circuit is implemented using a differential amplifier. The second summer circuit also can be implemented using a differential amplifier.
The output varies based on the pulse width of the switching control signal. For example, the control means varies the pulse width of the switching control signal to change the maximum cut off frequency of the low pass filter. In a preferred embodiment of the present invention, the control means allows the pulse width of the switching control signal to be reduced so that the maximum cut off frequency of the low pass filter is less than the sampling frequency divided by two. Alternatively, or in addition, the control means allows the pulse width of the switching control signal to be set at a value that allows for a cutoff frequency of the low pass filter to be equal to the sampling frequency divided by two. Alternatively, or in addition, the control means also allows the pulse width of the switching control signal to be set at a value that allows for a cutoff frequency of the low pass filter to be greater than the sampling frequency divided by two. Alternatively, or in addition, the control means allows the pulse width of the switching control signal to be set at a value that allows for signal spikes at harmonics of the sampling frequency to be included in a pass band of the low pass filter.
The present invention provides for a cost effective way to design a notch filter with a center frequency that is determined by the sample frequency (Fsample) and the sample pulse width (Fsample width). The bandwidth is determined by the sample pulse width (Fsample width) and is inversely proportional to the sample pulse width. A notch bandwidth equal to 1 Hz is possible at almost any center frequency from DC to Fsample/2. At any notched frequency, with the bandwidth equal to 1 Hz, the output wave form of the notch filter will be a square wave. By adjusting the sample width of Fsample, from a minimum to a maximum of ¼ the period of Fsample as the frequency of Fsample is adjusted from a maximum to a minimum of two times the frequency of the input signal, the notch bandwidth of the filter approaches 1 Hz, and the peak output amplitude at resonance can be made equal to the peak input amplitude of that sampled signal. Hence, the filter has a gain of “1”. In notch filter designs, where a low pass analog sampling filter network is used in a negative feedback loop for improved filter performance, adjustment of Fsample width, from a minimum to a maximum of ¼ the period of Fsample as the frequency of Fsample is adjusted from a maximum to a minimum of two times the frequency of the input signal, will cause a gain of greater than 1. If the filter, through adjustment of Fsample and Fsample width, attempts to resolve a notch bandwidth less than 1 Hz at any center frequency, the filter will oscillate and lock on that frequency. These simple relationships allow extremely fine adjustment and selection of any desired notch frequency and bandwidth (hence filter quality “Q”) performance. The simplicity and versatility of design using analog sampling technology as set forth, are significant improvements over prior art notch filters.
REFERENCES:
patent: 4496859 (1985-01-01), Crooks
patent: 4939473 (1990-07-01), Eno
patent: 6072360 (2000-06-01), McCullock
patent: 6166567 (2000-12-01), McCullock
patent: 6194959 (2001-02-01), Kamoshida et al.
Callahan Timothy P.
Nguyen Linh
Weller Douglas L.
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