Hybrid DCXO and RF PLL AFC for Gap-Free Reference Frequency
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Solution Overview
Problem
Wireless mobile devices face frequency drift issues in crystal oscillators, leading to increased bit error rates and reduced throughput, especially in high data rate applications like HSPA+, due to limitations in frequency resolution and accuracy of digitally-controlled crystal oscillators (DCXOs).
Innovation Solution
A hybrid automatic frequency control technique is implemented, combining coarse and fine adjustments in capacitor arrays of DCXOs with compensation signals applied to both the DCXO and RF Phase-Locked Loop (PLL) to maintain precise reference frequency, addressing static and dynamic errors and preventing frequency gaps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If DCXO with capacitor arrays is used to adjust reference frequency, then cost and integration are improved, but frequency resolution and accuracy deteriorate due to fabrication process variations
Solution Approach 1:
The patent segments the frequency adjustment function into two independent parts: a DCXO with capacitor arrays for coarse adjustment, and a VC-TCXO for fine adjustment. This segmentation allows each component to be optimized independently - the DCXO can use inexpensive fabrication processes while the VC-TCXO provides the necessary frequency precision through voltage and temperature control mechanisms.
Solution Approach 2:
The patent merges two different oscillator technologies (DCXO and VC-TCXO) into a hybrid AFC system that combines their advantages. The DCXO provides cost-effective integration and coarse frequency control, while the VC-TCXO adds precision and stability. The automatic frequency control mechanism coordinates both components to achieve overall system performance that neither oscillator could provide alone.
2Device complexity
If DCXO with capacitor arrays is used for frequency adjustment, then device complexity is reduced, but frequency accuracy deteriorates causing frequency gaps
Solution Approach 1:
The frequency control function is segmented between the DCXO (coarse control via capacitor arrays) and VC-TCXO (fine control via voltage/temperature adjustment). This segmentation resolves the contradiction by assigning the simple, low-complexity DCXO to handle bulk frequency adjustments while the VC-TCXO handles precision tuning, eliminating frequency gaps without requiring the DCXO to be overly complex.
Solution Approach 2:
The VC-TCXO acts as an intermediary component that bridges the frequency gaps created by the discrete capacitor array steps of the DCXO. By providing continuous fine-adjustment capability, the VC-TCXO mediates between the coarse DCXO steps to ensure continuous, gap-free frequency coverage across the operating range.
3Reliability
If VC-TCXO is used to maintain target oscillation frequency, then frequency stability is improved, but cost and integration increase
Solution Approach 1:
The system segments the frequency control workload: the DCXO with capacitor arrays handles the majority of frequency adjustment requirements at low cost, while the VC-TCXO is engaged only for fine-tuning and stability maintenance. This segmentation allows the system to achieve VC-TCXO level frequency stability without incurring VC-TCXO costs across the entire frequency control range.
Solution Approach 2:
Instead of using a full VC-TCXO for all frequency control, the system applies partial action by using the VC-TCXO only for the fine-adjustment portion of frequency control. The DCXO handles the bulk of frequency setting, and the VC-TCXO provides supplemental precision, achieving excessive frequency stability relative to the cost invested.
Data Source
AI summary
A technique to provide hybrid compensation to correct for drifts in a reference frequency output from a digitally-controlled crystal oscillator (DCXO). A first compensation is provided to the DCXO to adjust for overlap or discontinuity of the reference frequency caused by switching capacitors in the capacitor array that controls drift of the reference frequency output. The second compensation is obtained at a phase-locked loop (PLL) that receives the reference frequency signal from the DCXO. The second compensation adjusts the PLL to adjust for variations of the reference frequency that remain after performing compensation in the DCXO.