DIGITAL CONTROLLED OSCILLATOR AND SWITCHABLE VARACTOR FOR HIGH FREQUENCY LOW NOISE OPERATION

摘要

Low noise switchable varactors and digital controlled oscillator (DCO) circuitry are presented for creating alternating signals at controlled frequencies, including a first transistor for selectively coupling two capacitors between varactor output nodes when a control signal is in a first state, second and third transistors for selectively coupling first and second internal nodes between the respective capacitors and the first transistor with a third internal node when the control signal is in the first state, and an inverter disconnected from the first and second internal nodes to mitigate phase noise and operable to control the voltage of the third internal node according to the control signal.

主权项

1. A digital controlled oscillator (DCO) circuit for generating an alternating differential signal at a controlled frequency, comprising:
an inductance coupled between first and second oscillator output nodes; a switchable capacitance circuit coupled in parallel with the inductance between the first and second oscillator output nodes, the switchable capacitance circuit including at least one capacitor cell, the at least one capacitor cell comprising:
a control input terminal for receiving a switching control input signal having a first state to turn the at least one capacitor cell on to provide a first capacitance between the first and second oscillator output nodes, and a different second state to turn the at least one capacitor cell off to provide a second lower capacitance between the first and second oscillator output nodes,a first capacitance including a first terminal coupled with the first oscillator output node, and a second terminal coupled with a first internal node,a second capacitance including a first terminal coupled with the second oscillator output node, and a second terminal coupled with a second internal node,a first transistor including a source terminal coupled with the first internal node, a drain terminal coupled with the second internal node, and a gate terminal coupled with the control input terminal to turn the first transistor on when the switching control input signal is in the first state and to turn the first transistor off when the switching control signal is in the second state,a second transistor including a drain terminal coupled with the first internal node, a source terminal coupled with a third internal node, and a gate terminal coupled with the control input terminal to turn the second transistor on when the switching control input signal is in the first state and to turn the second transistor off when the switching control signal is in the second state,a third transistor including a drain terminal coupled with the second internal node, a source terminal coupled with the third internal node, and a gate terminal coupled with the control input terminal to turn the third transistor on when the switching control input signal is in the first state and to turn the third transistor off when the switching control signal is in the second state, andan inverter circuit with an inverter input node coupled with the control input terminal, and an inverter output node controlling a voltage at the third internal node according to the switching control input signal; and a cross-coupled transistor pair circuit coupled in parallel with the inductance and the switchable capacitance circuit and operable to alternately couple the first oscillator output node with a constant voltage node while disconnecting the second oscillator output node from the constant voltage node, and then to couple the second oscillator output node with the constant voltage node while disconnecting the first oscillator output node from the constant voltage node, at the controlled frequency.