US 6,646,497 B2Granted

Filter apparatus

AssigneeSony Corporation

InventorShinichiro Tsuda

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Abstract

A subordinate low pass filter 21 having characteristics similar to those of a main low pass filter 11 is provided. A signal of a frequency corresponding to a cut-off frequency is supplied from a signal generating circuit 22 to the subordinate LPF 21. A phase difference detecting circuit 23 detects a phase shift amount which is caused when the signal of the cut-off frequency is supplied to the subordinate LPF 21. An error detecting circuit 24 compares a value corresponding to the phase shift amount caused in the subordinate LPF 21 when the signal of the cut-off frequency is supplied with a reference value corresponding to the phase shift amount to be caused in the subordinate LPF 21 at the time of the cut-off frequency on phase characteristics. A comparison output is used as a control signal and the subordinate LPF 21 and main LPF 11 are controlled to have a desired cut-off frequency. By using multipliers of similar constructions as a phase difference detecting circuit 23 and a reference value generating circuit 25, an influence of temperature characteristics and a variation on processes can be cancelled.

Claims7

Dependent claims

A filter apparatus comprising:

a main filter which has an input terminal and an output terminal and whose characteristics is set by an external control signal;
a subordinate filter having substantially the same construction as that of said main filter;
a signal generator for generating a signal of a frequency equal to a cut-off frequency of each of said main filter and said subordinate filter and supplying it to said subordinate filter;
a phase difference detector for detecting a phase difference between the signal generated from said signal generator and an output signal of said subordinate filter and outputting a phase difference signal;
a reference signal generator for generating a reference signal corresponding to an ideal value of the phase difference detected by said phase difference detector;
and
an error detector for detecting an error between said phase difference signal and said reference signal and supplying an error signal as said external control signal to said main filter and said subordinate filter;
whereby said reference signal is set to a value based on an amount of phase shift that is caused when a signal having a frequency equal to the cut-off frequency of said subordinate filter is supplied to said subordinate filter.

5 more elements

Dependencies are inferred from the claim text.