PLL Divider Dithering in Voltage-Controlled Oscillators
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Solution Overview
Problem
Voltage controlled oscillators (VCXOs) face challenges in providing frequency variability with lower cost components and increased flexibility, while maintaining accuracy, due to issues like spurs that affect their performance.
Innovation Solution
A method and apparatus that utilize a one-bit analog-to-digital converter (ADC) to convert a voltage control signal into digital data, which is then upsampled and dithered to generate a divide ratio for a phase-locked loop circuit, effectively reducing spurs and noise without increasing power consumption or complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a voltage controlled crystal oscillator (VCXO) is used to provide frequency variability, then frequency adjustment capability is improved, but spurs are generated that affect performance
Solution Approach 1:
A dither signal is introduced as an intermediary element between the voltage control signal and the oscillator core. This dither signal modulates the divide ratio control, effectively spreading the spur energy across a wider frequency spectrum and reducing the amplitude of discrete spurs. The dither acts as a mediator that transforms concentrated harmful spectral lines into distributed noise-like characteristics.
Solution Approach 2:
The dither signal is applied periodically to the divide ratio control mechanism. By periodically varying the divide ratio around its nominal value according to a dither waveform, the system achieves frequency modulation that spreads spectral energy. This periodic action converts static spur generation into dynamic spectral distribution, reducing peak spur levels.
2Ease of manufacture
If lower cost components are used to replace VCXO, then cost is reduced, but accuracy and performance are compromised
Solution Approach 1:
The invention changes the operational parameters of the oscillator system by introducing digital dithering to the divide ratio control. This parameter change allows the use of lower-cost components while maintaining accuracy through digital signal processing techniques. The dithered divide ratio control compensates for component imperfections by dynamically adjusting the frequency division behavior.
Solution Approach 2:
The invention replaces traditional analog VCXO mechanisms with a digital control approach using a dithered divide ratio system. Instead of relying on analog voltage-controlled crystal oscillation, the system uses digital dithering of the feedback divider to achieve frequency control. This substitution enables the use of lower-cost components while maintaining or improving accuracy through digital signal processing.
3Object-generated harmful factors
If dithering is applied to reduce spurs, then spur levels are reduced, but system complexity increases
Solution Approach 1:
The dithering function is segmented into a separate, dedicated circuit module that operates independently from the main oscillator core. This segmentation allows the dithering functionality to be added without significantly complicating the overall system architecture. The dither signal generation and injection are handled by discrete blocks that interface cleanly with existing components.
Solution Approach 2:
The dithered divide ratio control mechanism serves multiple functions simultaneously: it reduces spur levels, enables frequency modulation, and provides a interface for digital control. By making the divide ratio control multi-functional, the system achieves spur reduction without adding proportionally complex dedicated hardware for each function.
Data Source
AI summary
A voltage control signal at a voltage control signal input terminal is used to adjust an output frequency of an oscillator circuit. The voltage level of the voltage control signal is converted in a one-bit analog-to-digital converter (ADC) to a digital output indicative of the voltage level. Successive digital representations of the voltage level of the voltage control signal are upsampled to generate upsampled blocks of data. A dither circuit inserts a digital dither in the upsampled blocks of data to generate dithered upsampled data, which is used to generate a control signal for a feedback divider of a phase-locked loop circuit and thereby adjust the output frequency.