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I have an older digital power meter and there are 21 potentiometers inside and I'm guessing the calibration is made to the analog side of the circuit. Some newer instruments use EEPROM to store calibration offsets of other part and these have very few or no potentiometers. The stored values within the EEPROM is digital and it does not change over time and it is only changed to make an adjustment to drift of other components.
Do newer instruments using ASICs and electronic calibration offer better calibration stability in general compared to older ones made with lots of general purpose ICs, discrete components and dozens of adjustment potentiometers on the analog side?
Having a lot of discrete components and potentiometers seems like a lot more room for calibration drift.