VCO Cascode Transistors for Gain and Phase Noise
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Solution Overview
Problem
Existing voltage-controlled oscillators (VCOs) face challenges in achieving high startup gain while maintaining low distortion and improved phase noise performance due to the sizing of cross-coupled transistors, which can lead to degraded performance in radio frequency synthesizers.
Innovation Solution
The implementation of cascode transistors in the VCO design, where the gates of these transistors are coupled to a common-mode voltage (Vcm) or a reference potential node, along with the use of filters to suppress harmonics, reduces non-linearity and improves the quality factor (Q) and phase noise performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If cross-coupled transistors are sized to achieve high startup gain, then startup gain is improved, but distortion increases and phase noise performance degrades
Solution Approach 1:
Cascode transistors are introduced as intermediary elements between the cross-coupled transistors and the output nodes. These cascode transistors act as mediators that isolate the cross-coupled transistors from direct interaction with the output, thereby reducing the distortion generated by the cross-coupled transistors while preserving the startup gain benefit. The cascode configuration provides a buffer that prevents the harmful distortion effects from propagating to the output signal.
Solution Approach 2:
The invention changes the electrical parameters of the VCO by introducing cascode transistors that modify the impedance characteristics and operating points of the cross-coupled transistors. This parameter change allows the cross-coupled transistors to operate in a regime that provides high startup gain while the cascode transistors simultaneously suppress the distortion by altering the signal waveforms and reducing harmonic content.
2Reliability
If cross-coupled transistors are sized to achieve high startup gain, then startup gain is improved, but phase noise performance degrades
Solution Approach 1:
The cascode transistors serve as intermediary elements that decouple the high-gain cross-coupled transistors from the output nodes, preventing the phase noise degradation caused by direct coupling. The cascode configuration acts as a buffer that filters out the phase noise components while allowing the beneficial startup gain to be maintained.
3Measurement precision
If cascode transistors are added to reduce distortion and improve quality factor, then large signal quality factor is improved, but device complexity increases
Solution Approach 1:
The VCO circuit is segmented into distinct functional blocks: the cross-coupled transistor pair for gain generation, the cascode transistor pair for distortion reduction, and the output nodes. This segmentation allows each component to be optimized for its specific function, achieving high quality factor while maintaining a relatively simple overall structure that can be easily fabricated in standard CMOS processes.
Data Source
AI summary
Certain aspects of the present disclosure are directed towards a voltage-controlled oscillator (VCO). The VCO may include: a first n-type metal-oxide-semiconductor (NMOS) transistor having a gate coupled to a first output of the VCO; a second NMOS transistor having a gate coupled to a second output of the VCO; a first NMOS cascode transistor coupled in cascode between the first NMOS transistor and the second output; and a second NMOS cascode transistor coupled in cascode between the second NMOS transistor and the first output, wherein gates of the first NMOS cascode transistor and the second NMOS cascode transistor are coupled to a common-mode voltage (Vcm) node of the VCO.