DDS Phase Persistence for Coherent Agile Frequency Switching
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Solution Overview
Problem
Direct digital synthesizers (DDSs) face challenges in maintaining continuous phase when switching between output frequencies, as their phase accumulator retains a history of prior frequencies, leading to phase discontinuity when returning to a previously programmed frequency.
Innovation Solution
A phase persistent agile signal source method that includes a DDS clock rate, frequency tuning word, DDS update rate, equivalent frequency least significant bit, and current phase of the LSB accumulator, generating a coherent phase to ensure continuous phase tracking across frequency changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the DDS phase accumulator stores history of all prior programmed frequencies to enable rapid frequency switching, then the frequency switching speed is improved, but the phase continuity is degraded when returning to a previous frequency
Solution Approach 1:
The patent introduces a phase history table as an intermediary data structure that stores the phase state of the phase accumulator for each programmed frequency. When a frequency is reprogrammed, the system retrieves the stored phase state from the table and restores it to the phase accumulator, thereby maintaining phase continuity without interfering with the rapid frequency switching capability of the original DDS architecture
Solution Approach 2:
The patent changes the parameter being tracked from just frequency to include both frequency and phase state. By storing tuples of (frequency, phase_state) in the phase history table, the system can rapidly switch frequencies while simultaneously restoring the correct phase state, thus resolving the contradiction between switching speed and phase continuity
2Measurement precision
If the DDS phase accumulator accumulates phase values continuously to maintain frequency accuracy, then the frequency accuracy is improved, but the phase coherence is degraded when frequency is reprogrammed to a previous value
Solution Approach 1:
The patent performs preliminary action by storing the phase state in the phase history table before the phase accumulator accumulates values for a new frequency. When returning to a previous frequency, the stored phase state is restored first, ensuring that the phase coherence is maintained from the start of the frequency transition, rather than allowing the accumulator to drift and then correct
3Device complexity
If the DDS uses a simple phase accumulator for frequency synthesis, then the device complexity is reduced, but the ability to maintain phase persistence across frequency changes is degraded
Solution Approach 1:
The patent creates a copy of the phase state information in the phase history table for each programmed frequency. This copying mechanism allows the system to retrieve and restore phase states without modifying the original phase accumulator structure, thus maintaining phase persistence while adding minimal complexity - essentially creating a lookup table that mirrors the phase states at different frequencies
Data Source
AI summary
A phase persistent agile signal source method, apparatus, and/or computer program product provides a direct digital synthesizer (DDS) clock rate, provides a frequency tuning word (FTW) for a desired output frequency, provides a DDS update for a desired DDS update rate, provides an equivalent frequency least significant bit (LSB) for the desired DDS update rate, provides a current phase of an LSB accumulator, and generates a coherent phase of the desired output frequency based on the DDS clock rate, FTW, DDS update rate to the DDS, equivalent LSB for the desired DDS update rate, and current phase of the LSB accumulator. The coherent phase can be the fraction portion of the result obtained from the multiplication of the FTW and the current phase of the LSB accumulator.