Switched-Cap Free-Running Oscillator for Low Phase Noise
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Solution Overview
Problem
Existing low power silicon-based free running oscillators (FROs) face challenges in achieving high frequency accuracy and low phase noise, particularly in low power wireless systems where energy is scarce, and are affected by parasitic capacitance that reduces frequency accuracy and effective gain.
Innovation Solution
The implementation of a switched-cap feedback loop with a novel auto-zeroing solution and a voltage-controlled oscillator, combined with a pre-charging amplifier to address parasitic capacitance, ensuring high frequency stability and low phase noise through a robust comparison circuit and phase control signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a voltage controlled oscillator circuit is used to generate oscillation signals, then frequency tuning capability is achieved, but phase noise increases
Solution Approach 1:
The patent implements a feedback mechanism where the oscillation signal is fed back through a frequency-to-voltage conversion circuit and compared with a reference voltage. The comparison result adjusts the VCO control voltage to maintain stable frequency and reduce phase noise, resolving the contradiction between frequency tuning capability and phase noise performance.
Solution Approach 2:
The patent introduces an intermediary frequency-to-voltage conversion circuit and comparison circuit between the VCO and reference signal. This intermediary mechanism converts frequency variations into voltage adjustments, enabling precise frequency control while suppressing phase noise through the mediating feedback loop.
2Device complexity
If parasitic capacitance is present in the amplifier circuit, then circuit simplicity is maintained, but frequency accuracy deteriorates
Solution Approach 1:
The patent applies preliminary action by pre-charging the parasitic capacitance at the amplifier output to the integration capacitor voltage before the comparison phase. This pre-charging action neutralizes the parasitic capacitance effect, allowing the circuit to maintain simplicity while achieving high frequency accuracy by eliminating the parasitic capacitance error in advance.
3Use of energy by moving object
If low power consumption is achieved in the oscillator circuit, then energy efficiency is improved, but frequency stability deteriorates
Solution Approach 1:
The patent employs periodic action by operating the comparison circuit and switches in periodic phases (sampling phase and comparison phase). This periodic operation allows the circuit to maintain low power consumption by activating high-precision components only when needed, while the integration capacitor maintains frequency stability between cycles through continuous voltage holding.
4Measurement precision
If a robust comparison circuit is implemented to reduce phase noise, then phase noise performance is improved, but device complexity increases
Solution Approach 1:
The patent merges the comparison circuit functionality with the existing amplifier and integration capacitor in the frequency-to-voltage conversion path. By combining these functions into a unified feedback loop rather than adding separate independent circuits, the patent achieves robust phase noise performance while minimizing the increase in device complexity through functional integration.
Data Source
AI summary
Various embodiments relate to a free running oscillator, that includes a switch capacitor based frequency-to-voltage converter (F2V), a comparator, and a voltage controlled oscillator (VCO), which may be collectively configured to reduce amplifier offset and flicker noise while increasing effective gain of the amplifier of the comparator. The F2V may produce a feedback voltage Vfb corresponding to frequencies of output of the VCO. The comparator may be configured to sample a reference voltage Vref using a sampling capacitor, compare Vref to Vfb, and generate an output based on any difference between Vref and Vfb, where the output may be integrated using an integrating capacitor of the comparator. The comparator may compensate for parasitic capacitance at the output of the amplifier by using an amplifier having two outputs, with the sampling capacitor and integrating capacitor being coupled to respectively different outputs of the amplifier.