Ring Oscillator Frequency Control Using Relative Current Sources
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Solution Overview
Problem
Current controlled ring oscillators have a difficult-to-predict frequency due to complex relationships between charging and discharging currents and transistor sizes, leading to slow transients.
Innovation Solution
The introduction of multiple current sources that control the magnitudes of charging and discharging currents independently of the oscillating signal phase, with relative magnitudes determining the oscillator frequency, and the use of series-connected hysteresis circuits and cross-coupled inverters to simplify frequency prediction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If current sources are included in current controlled ring oscillators, then frequency control capability is improved, but frequency prediction becomes difficult and transient response becomes slow
Solution Approach 1:
The patent segments the current control function into separate dedicated current sources for charging and discharging operations. Instead of using a single current source that requires complex transistor size relationships, the invention uses multiple independent current sources (e.g., I_charge and I_discharge) that directly control the charging and discharging currents respectively. This segmentation simplifies the frequency prediction because the oscillation frequency can be directly calculated from the known current magnitudes and node capacitances without needing to account for complex transistor characteristics.
Solution Approach 2:
The current sources in the patent serve multiple functions simultaneously: they provide frequency control, determine oscillation frequency through their magnitude relationships, and enable fast transient response. By making the current sources independent of the oscillating signal phase and directly controlling both charging and discharging currents, the invention achieves universal control over the oscillator's frequency and transient characteristics without requiring separate control mechanisms.
2Adaptability or versatility
If transistor sizes are adjusted to control frequency, then frequency control is achieved, but transient response becomes slow and frequency prediction becomes complex
Solution Approach 1:
The patent changes the control parameter from transistor physical dimensions (sizes) to electrical current magnitudes. Instead of adjusting frequency by changing transistor sizes—which affects both frequency and transient response in complex ways—the invention controls frequency by adjusting the magnitudes of dedicated current sources. This parameter change allows independent optimization: the current magnitudes can be set to achieve the desired frequency while being sufficiently large to enable fast transient response, decoupling these two requirements from the transistor size relationships.
3Device complexity
If simple current source relationships are used, then frequency prediction is simplified, but frequency control precision is reduced
Solution Approach 1:
The patent implements feedback mechanisms through cross-coupled inverters and hysteresis circuits that sense the oscillation state and automatically adjust the charging and discharging current flows. The cross-coupled configuration creates a feedback loop where the output of each inverter feeds back to the input of the other, ensuring that the current sources operate in the correct phase relationship. This feedback ensures precise frequency control by maintaining the intended current magnitude relationships, while the hysteresis provides additional stability that prevents spurious oscillations and ensures clean switching transitions.
Data Source
AI summary
An oscillator circuit includes a circuit loop and multiple current sources. The circuit loop includes an output having the oscillating signal. The multiple current sources are turned on independently of a phase of the oscillating signal. The current sources control magnitudes of both charging current and discharging current at nodes of the circuit loop, including the output. Relative magnitudes of different current sources determine a frequency of the oscillating signal.