I/Q-Interleaving RF-DAC for Wideband Linear Direct-Digital Modulation
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Solution Overview
Problem
Conventional digitally-intensive RF transmitters face issues with sampling replicas, out-of-band emissions, third-order counter-intermodulation distortion, and I-path/Q-path mismatch, which affect spectral compliance and energy efficiency, especially in compact devices like cellular handsets.
Innovation Solution
A digitally-intensive RF transmitter with a 2nd-order-hold interpolation filter and an in-phase/quadrature-interleaving RF digital-to-analog converter, which suppresses sampling replicas and cancels LO harmonics, reducing the need for bulky filters and improving linearity and image rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional digitally-intensive RF transmitters use standard RF-DAC architectures, then the transmitter can be manufactured with digital circuits, but sampling replicas appear in the output spectrum affecting spectral compliance
Solution Approach 1:
The patent applies preliminary action by performing digital filtering and sampling replica suppression before the RF signal is converted to analog and transmitted. The digital filter processes the digital baseband signal prior to DAC conversion, preventing sampling replicas from appearing in the final RF output spectrum, thus maintaining spectral compliance while keeping the transmitter digitally manufacturable
2Adaptability or versatility
If I-path and Q-path RF-DACs are used separately, then direct-digital RF modulation can be implemented, but I-path/Q-path mismatch occurs degrading performance
Solution Approach 1:
The patent merges the I-path and Q-path RF-DAC operations into a single interleaved RF-DAC architecture. By interleaving the digital I and Q samples in the time domain and using a single DAC structure, the invention eliminates I-path/Q-path mismatch while maintaining direct-digital RF modulation capability. This unified approach ensures identical signal paths for both I and Q components
3Device complexity
If third-order counter-intermodulation distortion is not suppressed, then the transmitter structure remains simple, but distortion products fall within the transmission band affecting signal quality
Solution Approach 1:
The patent applies preliminary action by implementing third-order counter-intermodulation distortion suppression through digital signal processing before RF conversion. The digital filter processes the baseband signal to pre-compensate and suppress potential distortion products, preventing them from appearing in the transmission band after RF amplification, thus maintaining simple transmitter structure while improving signal quality
Data Source
AI summary
A wideband, linear, direct-digital RF modulator (DDRM) for a digitally-intensive transmitter (DTX) includes an interpolation filter and an in-phase/quadrature (I/Q)-interleaving RF digital-to-analog converter (RF-DAC). The interpolation filter suppresses sampling replicas in the DDRM's output RF spectrum. I/Q interleaving performed by the interleaving RF-DAC avoids problems associated with using two separate I- and Q-path RF-DACs. Each unit cell of the interleaving RF-DAC is capable of producing four unique non-overlapping waveforms covering all four quadrants of the I/Q signal plane. In one embodiment of the invention, the interleaving RF-DAC includes three parallel-connected RF-DACs operating in accordance with a multi-phase set of LO clocks to both cancel 3rd-order and 5th-order LO harmonics generated by the RF-DAC unit cells' interleaving logic and prevent 3rd-order intermodulation from occurring in the DTX's final stage RF power amplifier.