DDS Spur Cancellation Using Pre-Distortion and Clock Alignment
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Solution Overview
Problem
Current direct digital synthesizer (DDS) waveform generators face challenges in wideband applications due to inadequate spurious performance, with spurious signals resulting from non-linearities in the DDS and frequency mixing, which are difficult to mitigate with existing components.
Innovation Solution
A waveform generator circuit incorporating a primary DDS and at least one cancellation DDS, which generates pre-distortion signals to cancel out spurious signals by adjusting the local oscillator clock frequency to align with the DAC clock frequency, allowing for in-system spur cancellation and minimizing spurious emissions through phase offset and amplitude matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If frequency mixing is used for wideband applications, then bandwidth is expanded, but spurious mixing products become a significant limitation for spurious performance
Solution Approach 1:
The patent applies preliminary anti-action by generating pre-distortion signals that are the inverse of expected spurious mixing products before they occur. The system predicts where spurious products will appear based on LO and IF frequency relationships, generates cancellation signals in advance, and combines them with the main signal to preemptively neutralize the harmful effects of frequency mixing in wideband applications.
Solution Approach 2:
The patent converts the harmful spurious mixing products into beneficial cancellation signals. By analyzing the predictable mathematical relationships of spurious products (mLO +/- nIF), the system generates precise inverse signals that, when combined with the original signal, transform the harmful spurious energy into constructive interference that eliminates the unwanted products while preserving the desired signal.
2Adaptability or versatility
If frequency multiplication is used to increase frequency, then bandwidth is expanded, but phase noise and spurious degrade by 20 log N
Solution Approach 1:
The patent applies preliminary action by pre-generating correction signals that compensate for the expected phase noise and spurious degradation before frequency multiplication occurs. The system calculates the anticipated degradation based on the multiplication factor N and prepares cancellation signals in advance, allowing the degraded signal to be corrected through constructive interference rather than attempting to prevent the degradation itself.
Solution Approach 2:
The patent introduces an intermediary cancellation signal path that mediates between the frequency multiplication process and the final output. This intermediary path generates and injects correction signals that act as a bridge, compensating for the phase noise and spurious degradation introduced by frequency multiplication without requiring changes to the multiplication process itself.
3Device complexity
If DDS spurious are directly in band, then signal generation is simplified, but spurious performance deteriorates as they cannot be filtered out
Solution Approach 1:
The patent applies the taking out principle by extracting and separately processing the spurious signal components from the main signal path. The system identifies spurious frequencies, generates dedicated cancellation signals for these extracted components, and removes them through destructive interference, effectively separating the harmful spurious elements from the desired signal without adding complex filtering stages.
Data Source
AI summary
A waveform generator circuit provides low spurious output signal and includes a primary DDS for generating a RF signal at a first frequency. A DAC receives an output signal from the primary DDS and converts the digital DDS output to an analog output. A spectrum analyzer identifies spurious signals in the DAC output determining the amplitude and frequency characteristics of the spurious signals. The waveform generator includes at least one cancellation DDS configured to generate a pre-distortion signal corresponding to frequencies where spurious signals are expected due to non-linearities in the DAC circuitry. The pre-distortion signals are phase offset from the determined spurious signals to cancel the spurious signals. The pre-distortion signals are combined with the output of the primary DDS. The combined signal contains the primary DDS output signal and pre-distortion signals to produce an analog output signal which cancels out the expected spurious signals.