Transmission Control Unit for Spread-Spectrum DAC Image Suppression
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Solution Overview
Problem
Conventional RF transmitters generate spurious DAC images around multiples of the DAC clock frequency, which violate spectrum regulations and interfere with other communication systems, and existing solutions to mitigate these images are costly in terms of current and chip area.
Innovation Solution
A control unit comprising a sample rate converter, PRBS generator, and frequency synthesizer is used to spread DAC images with reduced amplitudes by clocking the DAC with a spread-spectrum clock, pre-distorting digital input data, and using a PLL with a divider element to modulate the clock cycle randomly, thereby reducing the energy spread of the fundamental component.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If spread spectrum technique is applied to DAC clock, then DAC images are spread over frequency range with reduced amplitudes, but fundamental component of DAC output signal is also spread which degrades signal quality
Solution Approach 1:
The patent segments the frequency spectrum treatment by applying different processing to different components: the fundamental component passes through unchanged while DAC images are spread. This is achieved by using a comb filter to extract and preserve the fundamental component separately from the images, then recombining them after image spreading.
Solution Approach 2:
The patent introduces intermediary components (comb filter, selective amplifier, summer) that mediate between the spread-spectrum clock and the DAC output. These intermediaries selectively process only the image frequencies while leaving the fundamental component untouched, acting as a bridge between conflicting requirements.
2Object-generated harmful factors
If reconstruction filter order is increased to reduce DAC images, then spectral compliance is improved, but current consumption and chip area increase
Solution Approach 1:
The patent converts the harmful DAC images into a beneficial form by spreading them over a wide frequency range with reduced peak amplitudes. This transformation turns the images from concentrated interference that requires heavy filtering into distributed noise-like signals that naturally comply with spectral regulations without requiring complex filters.
Solution Approach 2:
The patent changes the temporal parameter of the DAC clock from a fixed periodic signal to a spread-spectrum signal with randomly varying clock periods. This parameter change fundamentally alters the spectral characteristics of the DAC output, spreading image energy across frequency and reducing peak amplitudes without requiring increased filter order.
3Object-generated harmful factors
If DAC sample rate is increased to push images to higher frequencies, then spectral compliance is improved, but current consumption and device complexity increase
Solution Approach 1:
Instead of pushing images to higher frequencies (which increases sample rate requirements), the patent converts the images into spread-spectrum noise by randomizing the clock period. This transformation achieves spectral compliance by distributing image energy across the frequency range rather than concentrating it at specific harmonic frequencies, avoiding the need for higher sample rates.
Data Source
AI summary
Control unit for a transmission system, including: a sample rate converter, which is coupled to a digital-to-analog converting unit, wherein digital input data (data_in) are feedable to the sample rate converter; a PRBS generator, wherein an output signal (ss_div) of the PRBS generator is feedable to the sample rate converter and to a delay element, wherein an output signal (ss_div_del) of the delay element is feedable to a frequency synthesizer, wherein the frequency synthesizer is clockable by a reference clock (clk_ref) and wherein an output signal (clk_ss) of the frequency synthesizer is feedable to a clock input of the sample rate converter and to a clock input of the digital-to-analog converting unit.