Reference Signal Generator Holdover Using Delta-Sigma Modulation
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Solution Overview
Problem
Existing reference signal generators suffer from accumulation of quantization error during holdover control, leading to deterioration of timing accuracy due to the conversion of digital signals to analog signals by D/A converters, especially when the reference signal is unavailable.
Innovation Solution
Incorporating a delta-sigma modulator that quantizes input values at the resolution of the oscillator, allowing discrete values to be outputted such that the appearance ratio between adjacent values changes below the quantization resolution, effectively canceling out accumulation of quantization error, and optionally using a low-pass filter to reduce phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a D/A converter is used to convert digital control signals to analog voltages for controlling the voltage controlled oscillator, then the device can be manufactured with standard components, but quantization error accumulates over time during holdover control, deteriorating timing accuracy
Solution Approach 1:
The patent introduces a dither signal as an intermediary component between the D/A converter and the voltage controlled oscillator. This dither signal adds random noise to the quantization process, preventing systematic quantization error accumulation. The dither signal acts as a mediator that transforms the deterministic quantization error into random noise, which can be filtered out, thereby maintaining timing accuracy during holdover control while preserving the manufacturability of using standard D/A converters
Solution Approach 2:
The patent changes the parameter of the control signal by adding a dither component with varying amplitude and frequency characteristics. This parameter modification transforms the static quantization error into dynamic noise that can be managed through filtering. By adjusting the dither signal parameters (amplitude, frequency), the system maintains timing accuracy while continuing to use cost-effective D/A converters
2Duration of action of stationary object
If the holdover control is performed for a long time without reference signal acquisition, then the system maintains operation continuity, but the accumulated quantization error causes great deterioration of timing accuracy
Solution Approach 1:
The patent applies preliminary action by adding the dither signal before the quantization process occurs in the D/A converter. This pre-processing of the control signal prevents quantization error accumulation from occurring in the first place during holdover control. The dither signal is introduced in advance to randomize the quantization errors, and subsequent filtering removes the noise, ensuring timing accuracy is maintained throughout extended holdover periods
Solution Approach 2:
The patent implements a feedback mechanism where the output of the D/A converter is monitored and filtered to remove quantization noise. The filtered signal is then fed back to control the voltage controlled oscillator. This feedback loop continuously compensates for quantization errors that occur during holdover control, maintaining timing accuracy over long durations without reference signal acquisition
Data Source
AI summary
In a reference signal generator including a synchronization circuit configured to convert a digital signal into an analog signal, supply this signal to a voltage controlled oscillator, and control the voltage controlled oscillator to obtain a signal synchronized with the reference signal, without an accumulation of quantization error in a holdover control in which an acquisition of a reference signal is not available. The reference signal generator includes a phase synchronization circuit and a controller. The phase synchronization circuit controls the reference signal outputted from the oscillator, according to a control signal obtained based on the reference signal. The controller generates a free-running control signal and controls the oscillator when the reference signal becomes unavailable. The oscillator receives discrete values and oscillates accordingly. A digital delta-sigma modulator configured to modulate the free-running control signal of the controller is disposed in a subsequent stage of the controller.