PLL Oscillator Input Switching to Suppress Frequency Fluctuation
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Solution Overview
Problem
Conventional PLL circuits experience fluctuations in output frequency due to weak leakage currents from high-impedance states, leading to inaccurate control of oscillation characteristics in voltage-controlled oscillators.
Innovation Solution
The PLL circuit design includes two voltage-current conversion circuits with a selection circuit that eliminates the need for a selection switch in the high-impedance state of the loop filter, allowing for precise control of the voltage controlled oscillator's input terminals and reducing time-varying fluctuations in output frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a selection switch is connected to the output portion of the loop filter in the high-impedance state to enable evaluation of VCO oscillation characteristics, then the ability to evaluate oscillation characteristics is improved, but the output frequency stability deteriorates due to leakage current causing fluctuations
Solution Approach 1:
The voltage-controlled oscillator is divided into two separate voltage-current conversion circuits: one for normal PLL operation connected to the loop filter, and another for evaluation mode connected to the VCO input control terminal. This segmentation allows independent optimization of each path, eliminating the need for a selection switch in the high-impedance state that caused leakage current issues.
Solution Approach 2:
The patent introduces a test control signal as an intermediary to switch between operation modes. This signal controls switches in the voltage-current conversion circuits to connect either the loop filter or the VCO input control terminal to the evaluation path, avoiding direct connection of selection switches to the high-impedance loop filter output.
2Ease of operation
If the loop filter output is in high-impedance state during evaluation mode, then the VCO can be controlled independently for oscillation characteristic evaluation, but leakage current causes voltage fluctuations at the loop filter output
Solution Approach 1:
The voltage-controlled oscillator is segmented into two independent voltage-current conversion circuits with separate input paths. One path receives signals from the loop filter for normal operation, while the other receives signals from the VCO input control terminal for evaluation mode, eliminating the high-impedance state problem.
Solution Approach 2:
Instead of switching the loop filter output connection as in conventional designs, the patent inverts the approach by switching the input source to the voltage-current conversion circuit. This ensures the loop filter remains in a stable low-impedance state while still enabling independent VCO control through the alternative input path.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design effectively suppresses time-varying fluctuations in output frequency, enabling accurate evaluation and control of oscillation characteristics without adding new circuits, and allows for monitoring of loop filter voltage during operation.
Implementation Method 1
two voltage-current conversion circuits for converting respective voltages at the two input terminals to currents
Data Source
AI summary
In a PLL circuit, a voltage controlled oscillator 4 has two voltage-current conversion circuits 40 and 41 and a selection circuit 42 for selecting an output of either one of the voltage-current conversion circuits 40 and 41. The output of the voltage-current conversion circuit selected by the selection circuit 42 is inputted to a current controlled oscillator 45. The one voltage-current conversion circuit 41 has an input thereof connected to an output of a loop filter 3, while the other voltage-current conversion circuit 40 has an input thereof connected to an input terminal 8 for evaluating the oscillation characteristics of the voltage controlled oscillator 4. As a result, time-varying fluctuations in the voltage of the loop filter resulting from a structure in which the input terminal for evaluating the oscillation characteristics of the voltage controlled oscillator is connected to the loop filter via a switch and time-varying fluctuations in the output frequency of the PLL circuit are effectively suppressed.