An overhaul of the Radiometer CMB12 Capacitance Comparator
The level of the 1 MHz test signal from the CMB12 changes quite much after power on, and very irregularly.
After replacing trimmer R128 with a fixed resistor, the level stability improved dramatically. Note the different scale on the y-axis. The change of 2.1 % is reduced to 0.12 %. Yet another demonstration of why using trimmers can be a bad idea, especially if the quality of the trimmer is poor.
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Oscillator unstability issues
The oscillator in the CMB12 is a common base LC oscillator where the level and frequency are determined only by the passive components. There's no level stabiliser, and no crystal to tame the frequency. Though attempts were made by the designers to match the temperature coefficients of the coil and capacitors of the oscillator, there's still much room for improvement. When the two capacitors being compared are very similar, the difference is small and the frequency and level variations are less important. However, as the difference between the two capacitors increases, the frequency and level variations will play a role. When only one capacitor is measured, the frequency and level variations carry full weight on the measurement result. The measurement on the left shows an example of how the level changes over time after power on. The measurement was made by a Rohde & Schwarz URE RMS voltmeter connected to the CMB12 between the Guard terminal and the CN hot terminal. The jumps are particularly unpleasant, and I suspected an unstable level trimmer potentiometer R128, a 200 Ohm carbon trimmer. I replaced it with a 110 Ohm metal film resistor, which definitely improved matters. From power on and over a 4 hour period the frequency changed from 1.000430 MHz to 1.000612 MHz, a shift of less than 200 ppm, and quite well for a free-running LC oscillator. The overall picture is, however, that the oscillator could do with an overhaul to make it more stable if the goal is to use the CMB12 for capacitance measurements, and not just for comparing two capacitors close in value. |