I. Basic Parameter Preset Strategy
A two-stage cascaded RC filter structure is adopted: the first stage targets low-frequency interference at the 50Hz power frequency, and the second stage targets high-frequency spike interference at the kHz level, covering the entire frequency band of variable interference from tens of Hz to several MHz.
First-stage low-pass parameters: R1=15kΩ, C1=1μF, cutoff frequency approximately 10Hz, specifically filtering out low-frequency drift interference at and near the power frequency.
Second-stage low-pass parameters: R2=1kΩ, C2=100nF, cutoff frequency approximately 1.6kHz, specifically filtering out high-frequency spike interference generated by motors and frequency converters.
The overall two-stage filter's total time constant is controlled within 160ms, and the temperature measurement signal delay is far below the safe threshold of 200ms, preventing overshoot due to temperature rise.
II. Dynamic Adjustment Steps
First, continuously acquire the input interference signal using an oscilloscope for 10 minutes, statistically analyze the frequency distribution range of the interference, and mark the 2-3 main interference frequency points with the highest proportion.
For the lowest frequency interference, which accounts for the largest share, set the cutoff frequency of the first-stage RC filter to 1/5 of this frequency and adjust the values of R1 or C1 to ensure attenuation of ≥20dB for this frequency band.
For the highest frequency interference, which accounts for the largest share, set the cutoff frequency of the second-stage RC filter to 1/10 of this frequency and adjust the values of R2 or C2 to ensure attenuation of ≥30dB for high-frequency spike interference.
After a 2-hour trial run, if occasional temperature reading jumps still occur, add a 10μH inductor between the two filter stages to form an LC-R composite filter structure, further expanding the interference suppression bandwidth.
III. Boundary Constraint Rules
The total time constant τ = R1×C1 + R2×C2 of the two-stage RC filters must be ≤0.2s to avoid signal lag introduced by the filter leading to overshoot and runaway temperature control.
Setting the cutoff frequency of a single-stage RC filter below 5Hz is prohibited; otherwise, normal temperature change signals will be mistakenly filtered out, resulting in severe sampling lag in the temperature controller.
If the interference cannot be completely eliminated after adjustment, prioritize checking the grounding loop and the grounding status of the temperature measuring line shield to solve the problem from the source of the interference and avoid excessively sacrificing the temperature measurement response speed by simply relying on RC filtering.
