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Old CRT type oscilloscopes were nothing but analog devices. They were filled with potentiometers. All this circuitry would drift over time. When they were calibrated, known waveforms were fed to the scope, the displayed wave form was then compared to the known and if the differences was within a given margin the pots would be adjusted to bring it back to the original spec. This procedure depends on that the scope has a predictable drift. If the difference between the known waveform and the displayed waveform was significantly greater than the expected drift, something was wrong with the scope and it needs repaired.

New digital oscilloscopes don't have potentiometers like the old CRT scopes. The verification process, I assume, is exactly the same. Feed the scope known waveforms and compare them with the displayed waveforms.

With no potentimeters how is the scope adjusted for drift over time?

Is the scope trimmed in software or if the scope is out of tolerance (but still within the excepted drift) it needs repaired?

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    \$\begingroup\$ Lots and lots of software coefficients that warp data and bias voltages. \$\endgroup\$ – PlasmaHH Sep 15 '16 at 13:07
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The verification process, I assume, is exactly the same

That is a wrong assumption, to have a person sit down and calibrate a scope like this is too expensive. It can be automated ! I bet the manufacturers have an automated bench for this, just connect signals to the input and control the scope using a PC. Feed it +1.000 V DC, measure the "raw number" which the scope's ADC measures. Then feed it -1.000 VDC, measure again. Then program the scope such that +1.000 V gives +1.000V on the screen. After the ADC it is just numbers so the calibration can be done in software.

The same is done in a modern digital multimeter. All the pots are gone, calibration is done in software and stored in a flash ROM.

Time calibration is also not needed on digital scopes as they use a quartz crystal which will always be miles ahead regarding accuracy compared to analog scopes using an RC sawtooth generator circuit (for the horizontal deflection).

On an analog CRT based scope, the spot where the electron beam hits the screen must be calibrated (X, Y position). On a digital scope, there's a an LCD display, each pixel can be adressed individually by the application processor. No need to calibrate.

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    \$\begingroup\$ I would add that a lot of scopes can self-calibrate. A reliable high-precision voltage source is cheap enough to include in the design. By internally connecting this to the input and then measuring the resulting ADC reading the system can calibrate out errors in its input buffers/amplifiers. While this isn't as good as a true external calibration it is at least a good sanity check that nothing is significantly off. \$\endgroup\$ – Andrew Sep 15 '16 at 14:43

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