Variable characteristic filters
AssigneeNational Research Development Corporation
InventorsAntonio J. Sainz, Victor C. Roberts
Life of this patent
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Abstract
A variable characteristic filter is described in which output signals are frequency analyzed by means of a frequency-to-voltage converter to derive a control signal. Variable impedance elements in the filter are controlled by the control signal to vary the filter characteristic automatically. The filter, including the frequency-to-voltage converter, may be such that the filter attenuation is reduced in the frequency region where the input or output signals mainly occur. The invention is particularly useful in Doppler blood-flow velocimeters to reduce noise.
Claims8
Signal processing apparatus, comprising:
- filter circuit means for filtering input signals within a fixed predetermined frequency range, said filter means having a variable low attenuation portion of said range delimited from a variable high attenuation portion of said range by a variable cut-off frequency, said cut-off frequency falling within said predetermined frequency range whenever signals of particular frequencies are applied to said filter means;
- frequency analysis means for analysing signals within said predetermined frequency range applied to said filter circuit means and for providing an output signal responsive to the periodic frequency content of said applied signals;
- and
- control means for automatically controlling said variable cut-off frequency in accordance with said output signal of said frequency analysis means to cause the filter to pass substantially all periodic filter input signals which occur within said predetermined frequency range.
Signal processing apparatus, comprising:
- filter circuit means for filtering signals within a fixed predetermined frequency range, said means having a variable low attenuation portion of said range delimited from a variable high attenuation portion of said range by a variable cut-off frequency, the cut-off frequency occurring, at least when signals at some particular frequencies are applied to the filter, within said predetermined frequency range;
- phase locked loop circuit means for responding, over substantially the whole of said predetermined frequency range, to signals appearing at terminals of said filter circuit means;
- and
- control means for automatically controlling said cut-off frequency in accordance with an output of the phase locked loop circuit means, to cause said filter means to pass all periodic input signals of significant amplitude which occur within said predetermined frequency range.
Dependencies are inferred from the claim text.