PLL Loop Filter Sampling for Bus Clock Jitter Reduction
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Solution Overview
Problem
Noise on the clock signal of synchronous buses, such as the SPI bus, degrades bus performance and increases the chance of bit errors due to thermal noise.
Innovation Solution
Implement a down-sampling function that folds thermal noise into a lower frequency band by using a capacitor to sample and store a voltage during bus inactivity, supplying it during bus activity to an operational amplifier, thereby isolating the voltage generator from thermal noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the voltage generator is continuously coupled to the operational amplifier, then the clock signal can be continuously supplied, but thermal noise degrades bus performance and increases bit errors
Solution Approach 1:
The capacitor samples and stores the voltage from the voltage generator during the first time period (when the bus is inactive) before the bus becomes active. This preliminary action prepares a clean voltage reference in advance, which is then supplied to the operational amplifier during the second time period (when the bus is active), avoiding the need for continuous coupling that would introduce thermal noise.
Solution Approach 2:
The system uses periodic sampling where the capacitor periodically couples to the voltage generator to sample the voltage, then periodically supplies this sampled voltage to the operational amplifier. This periodic action replaces continuous coupling, allowing the voltage generator to be isolated during critical periods when thermal noise would degrade performance, while still maintaining continuous clock signal supply through the stored voltage.
2Object-affected harmful factors
If the capacitor samples voltage during bus inactivity, then thermal noise is reduced, but the voltage generator must be isolated during bus activity
Solution Approach 1:
The capacitor acts as an intermediary between the voltage generator and the operational amplifier. It couples to the voltage generator during the first time period to sample the voltage, then supplies this sampled voltage to the operational amplifier during the second time period. This intermediary mechanism allows the voltage generator to be isolated from the operational amplifier during critical periods, reducing thermal noise while still enabling voltage supply through the capacitor mediator.
3Use of energy by moving object
If continuous coupling is used, then voltage supply is maintained, but noise folds into the bandwidth of interest
Solution Approach 1:
The capacitor performs preliminary sampling of the voltage from the voltage generator when the bus is inactive, storing a clean voltage reference before noise folding occurs during active operation. This preliminary action ensures that the operational amplifier receives a pre-sampled voltage that does not contain the thermal noise that would otherwise fold into the bandwidth of interest during bus activity.
Solution Approach 2:
The system implements periodic sampling where the capacitor couples to the voltage generator periodically (during inactive periods) rather than continuously. This periodic action maintains voltage supply continuity through the stored sampled voltage while preventing continuous noise folding into the bandwidth, as the coupling occurs only during periods when noise folding is not a concern.
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 approach reduces thermal noise on the bus clock signal, leading to improved bus performance with a lower bit error rate by moving noise out of the bandwidth of interest.
Implementation Method 1
A capacitor is coupled to a node between the voltage generator and the operational amplifier. The capacitor is coupled to the voltage generator during the first time period and configured to store a sampled voltage and the sampled voltage is supplied to the operational amplifier during a second time period.
Data Source
AI summary
A down-sampling function folds the thermal noise into a lower frequency band. A capacitor samples a voltage during a period of bus inactivity and supplies the sampled voltage to an input of operational amplifier in a loop filter of a phase-locked loop when the bus is active. The sampling frequency determines the reduction in thermal noise that can be achieved. The PLL generates a clock signal for a bus. A voltage generator charges the capacitor through a transistor when the bus is inactive. The transistor turns on responsive to the bus being inactive to allow the capacitor to charge and the transistor turns off responsive to the bus being active to isolate the capacitor and operational amplifier from the voltage generator. When the bus is active, the voltage across the capacitor is supplied to the operational amplifier.