Analog filter with adjustable filter frequency

An analog filter with adjustable filter frequency, comprising:

• an oscillatory circuit:
• whose resonance frequency equals the filter frequency of the filter;
• which has a first circuit branch, in which a first frequency determining element is arranged;
• which has a second circuit branch connected in parallel or in series with said first circuit branch, wherein a second frequency determining element is arranged in said second circuit branch, and wherein one of the frequency determining elements is a capacitance and the other an inductance;
• and,
• an amplifier installed in one of said two circuit branches, with adjustable amplification:
• whose output is connected with its inverting input via said frequency determining element arranged in such circuit branch;
• and
• which, in filter operation, amplifies, according to said adjusted amplification a voltage applied across said frequency determining element arranged in such circuit branch and thereby effects a corresponding change of an electrical current flowing through such frequency determining element.