Apparatus and method for digitally controlling capacitance

An apparatus for controlling an amount of capacitance in an oscillator circuit, comprising:

• a first capacitor coupled between a first node and a second node;
• a second capacitor coupled between a third node and a fourth node;
• a switch, having a control node, coupled between the second node and the fourth node;
• a first DC feed circuit, having a first end coupled to the second node and a second end configured to receive a control signal;
• and
• a second DC feed circuit, having a first end coupled to the fourth node and a second end configured to receive the control signal,
• wherein the control node of the switch is configured to be tied to a predetermined bias voltage, and when a first control signal is applied to the second ends of the first and second DC feed circuits, the first capacitor and the second capacitor are coupled between the first node and the third node via the switch, and when a second control signal is applied to the second ends of the DC feed circuits the first capacitor and the second capacitor are decoupled from between the first node and the third node.

A method for controlling an amount of capacitance in a circuit, comprising:

• applying a predetermined bias voltage to a control node of switch that couples together a first capacitor and a second capacitor;
• applying, through a first DC feed circuit, one of a first control signal or a second control signal to a node connecting the first DC feed circuit, the first capacitor and the switch;
• and
• applying, through a second DC feed circuit, the one of a first control signal or a second control signal to a node connecting the second DC feed circuit, the second capacitor and the switch,
• wherein when the first control signal is applied the first capacitor and the second capacitor are coupled together and in parallel with an LC tank circuit, and when the second control signal is applied the first capacitor and the second capacitor are decoupled from one another and the LC tank circuit.

A frequency synthesizer having an oscillator circuit having at least one output frequency, the output frequency changing as a capacitance in the oscillator circuit varies, the frequency synthesizer comprising:

• a discretely switchable capacitance module configured to control an amount of the capacitance in the oscillator circuit, the discretely switchable capacitance module having
• a capacitor coupled between a first node and a second node;
• a switch, having a control node, coupled between the second node and a third node;
• and
• a DC-feed circuit, having a first end coupled to the second node and a second end configured to receive a first control signal or a second control signal,
• wherein the control node of the switch is tied to a predetermined bias voltage, and when the first control signal is applied to the second end of the DC feed circuit an AC signal of the oscillator circuit passes through the capacitor and the switch, and when the second control signal is applied to the second end of the DC feed circuit the AC signal of the oscillator circuit does not pass through the capacitor and the switch.

A frequency synthesizer, comprising:

• a first capacitor coupled between a first node and a second node;
• a second capacitor coupled between a third node and a fourth node;
• a switch, having a control node, coupled between the second node and the fourth node;
• a first DC feed circuit, having a first end coupled to the second node and a second end configured to receive a control signal;
• and
• a second DC feed circuit, having a first end coupled to the fourth node and a second end configured to receive the control signal,
• wherein the control node of the switch is configured to be tied to a predetermined bias voltage, and when a first control signal is applied to the second ends of the first and second DC feed circuits, the first capacitor and the second capacitor are coupled between the first node and the third node via the switch, and when a second control signal is applied to the second ends of the DC feed circuits the first capacitor and the second capacitor are decoupled from between the first node and the third node.

A wireless communication system having an oscillator circuit having an output frequency, the output frequency changing as a capacitance in the oscillator circuit varies, the wireless communication system comprising:

• a discretely switchable capacitance module configured to control an amount of the capacitance in the oscillator circuit, the discretely switchable capacitance module having
• a capacitor coupled between a first node and a second node;
• a switch, having a control node, coupled between the second node and a third node;
• and
• a DC-feed circuit, having a first end coupled to the second node and a second end configured to receive a first control signal or a second control signal,
• wherein the control node of the switch is tied to a predetermined bias voltage, and when the first control signal is applied to the second end of the DC feed circuit an AC signal of the oscillator circuit passes through the capacitor and the switch, and when the second control signal is applied to the second end of the DC feed circuit the AC signal of the oscillator circuit does not pass through the capacitor and the switch.

A communication system, comprising:

• a first capacitor coupled between a first node and a second node;
• a second capacitor coupled between a third node and a fourth node;
• a switch, having a control node, coupled between the second node and the fourth node;
• a first DC feed circuit, having a first end coupled to the second node and a second end configured to receive a control signal;
• and
• a second DC feed circuit, having a first end coupled to the fourth node and a second end configured to receive the control signal,
• wherein the control node of the switch is configured to be tied to a predetermined bias voltage, and when a first control signal is applied to the second ends of the first and second DC feed circuits, the first capacitor and the second capacitor are coupled between the first node and the third node via the switch, and when a second control signal is applied to the second ends of the DC feed circuits the first capacitor and the second capacitor are decoupled from between the first node and the third node.

A circuit having an output frequency, the output frequency varying as a capacitance within a discretely switchable capacitance module varies, the discretely switchable capacitance module comprising:

• a capacitor coupled between a first node and a second node;
• a switch, having a control node, coupled between the second node and a third node;
• and
• a DC-feed circuit, having a first end coupled to the second node and a second end configured to receive a first control signal or a second control signal,
• wherein the control node of the switch is tied to a predetermined bias voltage, and when the first control signal is applied to the DC feed circuit an AC signal of the circuit passes through the capacitor and the switch, and when the second control signal is applied to the DC feed circuit the AC signal of the circuit does not pass through the capacitor and the switch.