Relaxation Oscillator Current Swapping for Offset Cancellation
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Solution Overview
Problem
Relaxation oscillators suffer from temperature-dependent oscillation frequency due to current offset and phase noise at low offset frequency, primarily caused by transistor mismatch and offset voltage, which are unpredictable and introduce timing differences in comparator triggering.
Innovation Solution
A relaxation oscillator circuit with a current swapping mechanism that alternates sourcing of currents between two outputs in response to switching signals, effectively canceling current and voltage offsets by periodically swapping the pair of currents, thereby reducing temperature sensitivity and phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional relaxation oscillator circuitry is used, then the circuit is simple in structure, but the oscillation frequency is strongly temperature dependent due to current offset
Solution Approach 1:
The patent applies periodic action by implementing a current swapping mechanism that alternates between two current sources (I1 and I2) at regular intervals. The switching control circuit periodically exchanges the currents flowing through the first and second capacitors, ensuring that temperature-dependent offset effects average out over time. This periodic swapping maintains stable oscillation frequency despite temperature variations, as the cumulative effect of alternating current paths cancels the temperature drift that would otherwise occur in a static configuration.
Solution Approach 2:
The patent changes the operational parameters of the circuit by dynamically switching between different current sources (I1 and I2) and their corresponding paths. Instead of using fixed current values, the system varies the current parameters periodically through controlled switching. This parameter change strategy allows the oscillator to compensate for temperature-dependent drift by alternating between current sources with complementary temperature characteristics, thereby stabilizing the overall oscillation frequency across temperature ranges.
2Device complexity
If traditional relaxation oscillator with comparator is used, then the circuit is simple, but phase noise at low offset frequency occurs due to offset voltage
Solution Approach 1:
The patent eliminates phase noise by implementing periodic swapping of current sources I1 and I2 along with their associated switches. This periodic action ensures that offset voltage effects are alternately applied in opposite directions, causing them to cancel out over complete cycles. The switching control circuit coordinates the periodic exchange of currents and switch states, ensuring that comparator offset errors do not accumulate and manifest as phase noise, thereby improving signal purity without requiring additional noise-filtering circuitry.
Solution Approach 2:
The patent converts the harmful effect of offset voltage into a beneficial cancellation mechanism. By deliberately alternating between two current sources with opposite polarity connections to the comparator, the system causes offset voltage errors to manifest in opposite directions during different phases of the oscillation cycle. This transforms the previously harmful offset voltage into a self-correcting mechanism where the error in one half-cycle is compensated by the opposite error in the next half-cycle, effectively eliminating phase noise.
3Productivity
If current sources are used in relaxation oscillator, then oscillation can be generated, but current offset causes frequency variation with temperature
Solution Approach 1:
The patent addresses transistor mismatch by implementing periodic swapping of the two current sources I1 and I2. Instead of requiring perfectly matched transistors (which is difficult to achieve in manufacturing), the system periodically exchanges the roles of the two current sources. This periodic action ensures that any static mismatch between transistors is averaged out over time, as each current source alternates between charging and discharging roles. The switching control circuit manages this periodic exchange, allowing the oscillator to achieve frequency stability despite manufacturing variations in transistor characteristics.
Data Source
Figure 1
Figure 2A~2F
Figure 3A
AI summary
In described examples, a relaxation oscillator (100) reduces temperature sensitivity and phase noise at low offset frequency by periodically swapping a first current (II) and a second current (12), so that after the first current (II) has been input to a first pair of circuits (120, 140) and the second current (12) has been input to a second pair of circuits (130, 140), the second current (12) is input to the first pair of circuits (120, 140), and the first current (II) is input to the second pair of circuits (130, 140).