Efficient cancelling of AC line interference in electronic instrumentation
First Claim
1. In an electrical instrument having a source of a low level input signal which is amplified, converted to digital data and presented in a digital signal processor to provide an output signal, and wherein the input signal is contaminated by periodic noise components of substantially constant frequency, the improvement comprising:
- (a) means in the digital signal processor for estimating a noise component ni (k) at the sample k in the sampled input signal x(k) at the sample k in accordance with the expression;
space="preserve" listing-type="equation">n.sub.i (k)=a.sub.i cos[2τ
(f.sub.0 /f.sub.s)k]+b.sub.i sin[2τ
(f.sub.0 /f.sub.s)k] where ni (k) is the estimator of the noise component ni (i), f0 is the frequency of the interference noise component, fs is the sampling frequency at which the input signal is converted to digital data, and the coefficients ai and bi are recalculated for the sinusoidal component in the periodic noise at each time point j using the recursion relationships;
##EQU3## where ai (0)=bi (0)=0; and
(b) means in the digital signal processor for subtracting the noise estimator ni (k) at the sample k from the input signal x(k) to yield an output signal from which the noise component has been substantially cancelled.
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Abstract
Efficient cancellation of AC power line interference from a signal received by electronic instruments, such as biomedical monitors, is obtained by efficiently estimating the magnitude and phase of the noise signal from the contaminated input signal in a digital signal processor. Each interfering noise component of known frequency is represented as a sinusoid composed of cosine and sine components having estimated amplitude parameters. These amplitude parameters are recalculated at discrete points in time in the digital signal processor to carry out substantially a Fourier integration on the input signal at the known frequency f0 of the interfering signal, with the integration being updated to the last sample. The integration can be carried out to allow the estimates to adapt to time varying amplitudes and/or phases of the interfering signals. Cancellation of the estimated noise sinusoid from the input signal results in an output signal substantially uncontaminated by the noise.
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Citations
14 Claims
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1. In an electrical instrument having a source of a low level input signal which is amplified, converted to digital data and presented in a digital signal processor to provide an output signal, and wherein the input signal is contaminated by periodic noise components of substantially constant frequency, the improvement comprising:
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(a) means in the digital signal processor for estimating a noise component ni (k) at the sample k in the sampled input signal x(k) at the sample k in accordance with the expression;
space="preserve" listing-type="equation">n.sub.i (k)=a.sub.i cos[2τ
(f.sub.0 /f.sub.s)k]+b.sub.i sin[2τ
(f.sub.0 /f.sub.s)k]where ni (k) is the estimator of the noise component ni (i), f0 is the frequency of the interference noise component, fs is the sampling frequency at which the input signal is converted to digital data, and the coefficients ai and bi are recalculated for the sinusoidal component in the periodic noise at each time point j using the recursion relationships;
##EQU3## where ai (0)=bi (0)=0; and
(b) means in the digital signal processor for subtracting the noise estimator ni (k) at the sample k from the input signal x(k) to yield an output signal from which the noise component has been substantially cancelled. - View Dependent Claims (2, 3, 4)
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5. In an electrical instrument having a source of a low level input signal which is amplified, converted to digital data and processed in a digital signal processor to provide an output signal, and wherein the input signal is contaminated by periodic noise components of substantially constant frequency which may vary over time in amplitude, frequency, or phase, the improvement comprising:
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(a) means in the digital signal processor for estimating a noise component ni (k) at a sample k in the sampled input signal x(k) at the sample k in accordance with the expression;
space="preserve" listing-type="equation">n.sub.i (k)=a.sub.i cos[2τ
(f.sub.0 /f.sub.s)k]+b.sub.i sin[2τ
(f.sub.0 /f.sub.s)k]where ni (k) is the estimator of the noise component ni (k), f0 is the frequency of the interference noise signal and fs is the sampling frequency, and the amplitude estimates ai and bi are recalculated at each time point j using the following recursion relationships;
space="preserve" listing-type="equation">a.sub.i (j)=α
a.sub.i (j-1)+2(1-α
)×
(j)cos(j)
space="preserve" listing-type="equation">b.sub.i (j)=α
b.sub.i (j-1)+2(1-α
)×
(j)sin(j),where α
is a scalar between 0 and 1; and(b) means in the digital signal processor for subtracting the noise estimate ni (k) at the sample k from the input signal x(k) to provide an output signal from which the noise component has been substantially cancelled. - View Dependent Claims (6, 7, 8)
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9. A method of cancelling noise of substantially known discrete frequencies from an electrical input signal in an electronic instrument which includes a digital signal processor, comprising the steps of:
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(a) receiving the input signal containing the desired signal of interest and at least one noise component whose frequency is substantially known but whose phase and magnitude is not known; (b) sampling the input signal at a selected sample rate fs to provide digital data x(i) corresponding thereto, where k is the sample number; (c) estimating the magnitude and phase of the noise component ni within the input signal data x(i) in accordance with the expression;
space="preserve" listing-type="equation">n.sub.i (k)=a.sub.i cos[2τ
(f.sub.0 /f.sub.s)k]+b.sub.i sin[2τ
(f.sub.0 /f.sub.s)k]where ni (k) is the estimator of the noise component ni (i) and f0 is a known noise frequency, and calculating the component estimators ai and bi as Fourier integrals of the cosine and sine components, respectively, over a time interval up to the time of the sample k; and (d) subtracting the noise component estimate ni (k) from the digital signal x(k) to provide an output signal from which the noise is substantially canceled. - View Dependent Claims (10, 11, 12, 13, 14)
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Specification