Low power oscillator with digital amplitude control

A voltage controlled oscillator (VCO) comprising:

• a first active negative transconductance circuit comprising:
• first cross-coupled transistor pair having drain terminals connected to a first end of a resonant tank circuit;
• a first programmable tail resistor coupled to source terminals of said first cross-coupled transistor pair at a first common node, said first programmable tail resistor sinking a bias current through the resonant tank circuit for tuning an amplitude of a VCO output oscillating signal;
• a first switch for coupling said first common node to an analog to digital converter (ADC) element, said ADC element sensing a voltage at the first common node when said switch is activated and providing a first digital control signal;
• a first signal bus for carrying said first digital control signal to said first programmable tail resistor, the first programmable tail resistor programmed to a certain resistance according to bit values of said first digital control signal for controlling the amplitude of said VCO output oscillating signal;
• and
• a second active negative transconductance circuit comprising:
• second cross-coupled transistor pair having drain terminals connected to a second end of the resonant tank circuit;
• a second programmable tail resistor coupled to source terminals of said second cross-coupled transistor pair at a second common node, said second programmable tail resistor sourcing a bias current through the resonant tank circuit for alternatively or additionally tuning the amplitude of said VCO output oscillating signal;
• a second switch for coupling said second common node to the ADC element, said ADC element alternatively or additionally sensing a voltage at the second common node when said second switch is activated and alternatively or additionally providing a second digital control signal;
• and
• a second signal bus for carrying said second digital control signal to said second programmable tail resistor, the second programmable tail resistor programmed to a certain resistance according to bit values of said second digital control signal for alternatively or additionally controlling the amplitude of said VCO output oscillating signal.

A method of operating a voltage controlled oscillator (VCO) circuit, the method comprising:

• coupling a first common mode voltage node of an active negative transconductance circuit of the VCO to an analog to digital converter (ADC) element through a first programmable tail resistor coupled between the ADC element and the first common mode voltage node, said active negative transconductance circuit including a cross-coupled transistor pair having coupled source terminals defining said first common mode voltage node and having drain terminals connected to a first end of a resonant tank circuit;
• upon initiating a circuit VCO oscillation, controlling said ADC element to sense a voltage at a second common mode voltage node at said resonant tank circuit of the VCO, said voltage sensing through a shared test bus of an on-chip system;
• and
• controlling, a resistance value of the first programmable tail resistor coupled between said ADC element and the first common mode voltage node, said controlling comprising:
• outputting by said ADC element, in response to said sensed voltage, a digital control signal;
• and
• providing said digital control signal to said first programmable tail resistor and using bit values of said digital control signal to control the resistance value of said first programmable tail resistor for controlling an amplitude of a VCO circuit oscillator signal output.