Automatically tuned tail filter

A method for minimizing a phase noise of an oscillating circuit arrangement including a resonating arrangement resonating at a first resonance frequency ω0 and a voltage controlled oscillator arrangement, comprising the steps of:

• connecting a tunable filter arrangement to the voltage controlled oscillator (VCO) arrangement;
• tuning the filter arrangement through applying a voltage or a current wherein the filter arrangement resonates at a second resonance frequency being substantially an integer multiple n, n=1 or 2, of said first resonance frequency and corresponding to a minimum number of functional switch transistors of the oscillating circuit arrangement which are required for oscillation at said first resonance frequency ω0;
• wherein the step of connecting the filter arrangement further comprises the step of connecting a tunable capacitor in Parallel with an inductor such that it resonates at said second resonance frequency ωf;
• wherein tuning the filter arrangement further comprises the step of tuning the filter arrangement particularly a tunable voltage, such that the voltage amplitude (A) at a source or emitter node of the voltage controlled oscillator arrangement comprising a differential pair structure comprising a cross-coupled differential pair VCO or a single ended structure with one or more switch transistors acting as a single functional switch transistor, is maximized;
• and
• using, in a control procedure, a control circuit for automatically finding the optimum varactor tuning voltage or current, the control procedure further comprising the steps of:
• establishing the amplitude of the second resonance frequency ωf, of the signal in a source or emitter node;
• establishing if the amplitude is increasing or decreasing;
• controlling the tuning voltage or current applied to the tunable capacitor such that the amplitude is increased, and
• wherein the step of establishing if the amplitude is increasing or decreasing further comprises:
• taking a derivative, using a differentiator, of the amplitude of the signal in a source or emitter node or in a common source or emitter node if it comprises a differential structure;
• and
• increasing or decreasing the tuning voltage or current if the derivative is negative such that the detected amplitude (A) will be maximized.