Variable Frequency Clock Circuit for Noise Rejection in Transmitters
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Process variable transmitters are susceptible to synchronous noise, particularly during electromagnetic compatibility testing, which causes measurement errors due to interference at or near the sampling frequency or its harmonics, leading to narrow band errors.
Innovation Solution
A harmonic energy detector and variable frequency clock circuit are implemented to sample interference at the same frequency as the sensor signal, comparing it to a threshold; if excessive, the clock signal is adjusted to change the sampling frequency, moving away from the interfering frequency to prevent errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the A/D converter samples the sensor signal at a fixed frequency, then the conversion process is simple and fast, but synchronous noise at the sampling frequency or its harmonics causes measurement errors
Solution Approach 1:
The patent applies dynamics by making the sampling frequency variable rather than fixed. The system continuously monitors for synchronous noise at the current sampling frequency or its harmonics, and when detected, dynamically adjusts the sampling frequency to a new value that avoids the interfering frequency. This dynamic adaptation resolves the contradiction by maintaining fast sampling while preventing measurement errors through frequency adjustment.
Solution Approach 2:
The patent changes the sampling frequency parameter in response to detected synchronous noise. The system monitors the sampling frequency and its harmonics for noise presence, and when noise exceeds a threshold, it modifies the frequency parameter to a different value. This parameter change allows the system to maintain high productivity while eliminating measurement precision degradation caused by synchronous noise.
2Measurement precision
If the sampling frequency is adjusted to avoid interference, then measurement precision is improved, but system complexity increases due to frequency monitoring and adjustment mechanisms
Solution Approach 1:
The patent implements feedback by continuously monitoring the sampling frequency and its harmonics for synchronous noise, comparing the noise level against a threshold, and adjusting the sampling frequency based on this feedback. The system uses a noise detector to monitor for interference, and when noise is detected above threshold, it triggers a frequency adjustment. This feedback mechanism improves measurement precision while keeping complexity manageable through automated closed-loop control.
Solution Approach 2:
The system performs self-service by automatically detecting synchronous noise and adjusting its own sampling frequency without external intervention. The transmitter autonomously monitors its sampling frequency for noise, determines when adjustment is needed, and modifies its operating parameters independently. This self-service approach improves measurement precision while minimizing added complexity by eliminating the need for external control systems.
3Productivity
If synchronous noise is present at the sampling frequency, then the A/D converter operates efficiently, but narrow band errors occur in the base band at very low frequencies
Solution Approach 1:
The patent applies preliminary anti-action by proactively detecting synchronous noise at the sampling frequency before it causes narrow band errors in the base band. The system continuously monitors for noise at the sampling frequency and its harmonics, and when detected, adjusts the frequency in advance to prevent aliasing errors from occurring. This preliminary detection and correction maintains conversion efficiency while preventing harmful narrow band errors.
Solution Approach 2:
The patent converts the harmful effect of synchronous noise into a beneficial detection mechanism. By monitoring for noise at the sampling frequency, the system uses the presence of interference as a signal to trigger frequency adjustment. The harmful noise presence becomes the trigger for the corrective action, transforming the problem into the solution mechanism that maintains both conversion efficiency and measurement accuracy.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
In a process variable transmitter, a sensor signal is sampled, using a clock signal, at a sensor sampling frequency. An interference signal from the device housing is also sampled at the same sensor sampling frequency. A comparison is made to determine whether the level of the interference signal at this frequency or harmonics of it exceed a threshold level. If so, the clock signal is changed to adjust the sensor sampling frequency away from the previous frequency where the level of noise is too high.