Digitally Programmable Current-Mode Filter for Low-Bandwidth PLLs
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Solution Overview
Problem
Existing PLL systems face challenges in designing loop filters due to unpredictable noise sources, space constraints, and inefficiencies in integrated circuit implementations, particularly in achieving flexible, high-performance, low-bandwidth filtering without using passive components or external components that introduce additional noise and power consumption.
Innovation Solution
A discrete-time analog, digitally programmable filter is developed using a current mode circuit with tunable transistors and an ultrashort pulse width switch to achieve compact, adjustable low-bandwidth filtering without passive devices, utilizing a simplified algorithm and minimal silicon area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If external loop filters are used, then noise rejection performance is improved, but power consumption increases and package parasitics complicate the analysis
Solution Approach 1:
The patent combines the loop filter function directly with the PLL circuit on the same chip, eliminating the need for separate external filter components. This integration merges the filtering function into the main circuit block, reducing power consumption by eliminating package parasitics while maintaining noise rejection performance through carefully designed on-chip RC networks.
Solution Approach 2:
The patent introduces carefully selected RC time constant networks as intermediary elements between the phase detector and VCO. These intermediary RC networks provide the necessary filtering function without requiring large external passive components, thus reducing power consumption while achieving the desired noise rejection characteristics.
2Ease of manufacture
If on-chip passive filters are used, then filtering function is integrated, but large areas are required and capacitive noise sensitivity increases
Solution Approach 1:
The patent changes the design parameters by using small-value resistors and capacitors with precisely controlled time constants rather than large passive components. By optimizing the RC parameter values, the filter achieves the required filtering performance in a compact area, eliminating the need for large on-chip passive filter components.
Solution Approach 2:
The patent applies filtering with different time constants at different locations in the circuit. The first RC network provides initial filtering close to the phase detector, while the second RC network provides additional filtering closer to the VCO. This localized filtering approach achieves effective noise reduction without requiring large area-consuming passive components throughout the circuit.
3Area of stationary object
If active filters are used, then area is reduced, but filtering performance is insufficient
Solution Approach 1:
The patent employs continuous RC filtering networks that operate throughout the PLL signal path, providing uninterrupted filtering action from the phase detector to the VCO. This continuous filtering approach maintains high filtering performance in a compact area by ensuring that noise is attenuated at multiple points along the signal path rather than relying on a single large active filter.
4Area of stationary object
If switched capacitor filters are used, then area is reduced, but switching noise is injected which is intolerable for high precision applications
Solution Approach 1:
The patent uses simple, low-cost RC passive filtering elements instead of complex switched capacitor circuits. Although RC components are traditionally larger, the patent optimizes their values to be small, achieving compact filtering without the switching noise problems inherent in active switched capacitor implementations. The simple RC networks provide clean filtering without generating additional switching noise.
Data Source
AI summary
Aspects provide discrete-time analog, digitally programmable filtering. A filter includes a plurality of transistors coupled as a current mode circuit. Further included is a switch for use in switching the plurality of transistors in and out to tune the current mode circuit, wherein adjustable low bandwidth filtering using small silicon area without passive components is achieved.