RF DAC Notch Filtering for Sigma-Delta Quantization Noise
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Solution Overview
Problem
Conventional notch filtering techniques for sigma-delta modulation in wireless devices, such as mobile phones, face limitations in achieving high clock frequencies and low power consumption while effectively reducing quantization noise in critical frequency bands like DCS, PCS, or W-CDMA, due to the need for high-order modulators and FIR filters that increase power consumption and complexity.
Innovation Solution
A radiofrequency digital to analog converter (DAC) array is used to place notches in the DAC transfer function, employing cascaded first-order sigma-delta modulator stages and multiple RF DAC blocks with programmable delays to reduce quantization noise, allowing for higher clock frequencies without excessive power consumption and optimizing signal amplitude by using positive coefficients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a high-order single loop multi-bit digital sigma-delta modulator is used to place notches in the noise transfer function, then quantization noise is reduced in critical frequency bands, but the clock frequency is limited to around 1 GHz and power consumption increases to 150 mW
Solution Approach 1:
The invention divides the single high-order modulator into multiple cascaded first-order modulator stages. Each stage operates independently at high clock frequencies, and their outputs are combined through a multi-bit DAC array. This segmentation allows each stage to run at higher frequencies with lower individual power consumption, while collectively achieving the noise suppression of a high-order system.
Solution Approach 2:
The invention introduces a multi-bit DAC array with programmable delay elements as an intermediary between the cascaded first-order modulators and the output. This intermediary structure enables flexible notch placement by adjusting delay values, allowing quantization noise suppression in specific frequency bands without requiring high-order modulation, thus reducing overall power consumption.
2Object-affected harmful factors
If a high-order single loop multi-bit digital sigma-delta modulator is used to place notches in the noise transfer function, then quantization noise is reduced in critical frequency bands, but device complexity increases
Solution Approach 1:
The invention segments the complex high-order modulator into multiple simple first-order modulator stages. Each stage has minimal complexity, but their cascaded connection combined with the multi-bit DAC array achieves the noise transfer function characteristics of a high-order system. This segmentation dramatically reduces the complexity of individual modulator blocks while maintaining overall performance.
Solution Approach 2:
The multi-bit DAC array serves multiple functions: it combines outputs from multiple first-order modulators, provides programmable delay elements for notch placement, and enables flexible filtering in different frequency bands. This universal component replaces the need for complex high-order modulator design, reducing overall device complexity.
3Object-affected harmful factors
If a FIR filter with many elements is used to achieve a narrow bandpass filter with good attenuation, then filtering performance is improved, but the number of coefficients increases and implementation becomes difficult
Solution Approach 1:
The invention extracts the filtering function from a complex FIR filter structure and implements it through a simpler multi-bit DAC array with programmable delay elements. By taking out the essential filtering capability and implementing it through delay-based notch filtering, the system achieves narrow bandpass filtering with good attenuation using far fewer elements than a traditional FIR filter would require.
Solution Approach 2:
The invention changes the filtering approach from using many FIR coefficients to using programmable delay values in the multi-bit DAC array. By adjusting the delay parameters, the system achieves flexible notch placement and narrow bandpass filtering characteristics without requiring a large number of FIR elements, thus reducing implementation complexity.
4Object-affected harmful factors
If negative coefficients are used in the FIR filter to achieve good bandpass filter characteristics, then filtering performance is improved, but radiofrequency power is cancelled and efficiency decreases
Solution Approach 1:
Instead of using negative coefficients that cause RF power cancellation, the invention inverts the approach by using only positive coefficients in the multi-bit DAC array. The desired filtering effect is achieved through constructive combination of delayed signals with positive weights, eliminating the RF power cancellation problem while maintaining filtering performance.
Solution Approach 2:
The invention converts the potential harm of signal cancellation into benefit by using positive coefficients only. The delayed signals from multiple first-order modulators are combined constructively through the multi-bit DAC array, transforming what would have been destructive interference into useful signal reinforcement, thereby improving RF power efficiency while achieving the desired notch filtering effect.
Data Source
AI summary
An electronic device, includes sigma-delta modulation circuit to operate with a clock signal and having output circuitry to deliver a digital data signal. First circuitry delivers a radiofrequency transposition signal. A notch filter includes radiofrequency digital-to-analog conversion blocks, having first input circuitry coupled to the output circuitry. Second input circuitry receives the radiofrequency transposition signal. Second output circuitry delivers a radiofrequency analog signal. Digital delay circuitry is controlled by the clock signal and includes a delay block between the two first input circuits. The frequency of a notch of the notch filter is related to the value of the delay from the delay block. Summation circuitry sums the radiofrequency signals.