This simply-constructed voltage calibrator can be built into practically any existing oscilloscope.
THIS simple calibrator enables 50 Hz square waves of exact amplitude to be displayed on an oscilloscope.
The calibrator can be added to existing oscilloscopes, or built as an external accessory. It eliminates measuring errors due to gain controls, or probe dividers, as a calibration signal is obtainable by inserting the probe tip directly into the calibration output socket and checking the displayed calibration signal against the calibration control switch setting.
The oscilloscope time-base accuracy can also be checked with this calibrator - the 50 Hz square-wave signal is derived from the mains thus providing a stable 20 millisecond period.
The calibration voltage is derived from a 22 volt zener diode; this voltage is chopped at 50 Hz by the BC 108 transistor, trimmed to exactly 20 volts by the calibration potentiometer and applied across a chain of precision resistors.
The consumption of the unit is negligible and is energized by the power supplies of the oscilloscope to which the unit is fitted.
It is obviously impossible to give installation instructions for each individual make and type of oscilloscope -- however all that is required is to locate an HT rail carrying between 250 and 350 volts for the main divider supply, and the filament transformer for the chopper supply. The diode connected between the BC 108's base and emitter protects against excess voltage at this point.
The divide' earth point and the emitter of the BC' 108 chopper transistor should be taken to the same point. All wiring should be shielded or located away from existing oscilloscope wiring to avoid cross talk.
The 50 Hz calibrator output should be taken to a binding post mounted on the front panel of the scope. The rotary calibrator switch should also be mounted on the front panel. If a miniature switch is used for this function no problem should be found finding a suitable location - the existing panel lamp mounting can often be utilized for the purpose.
The calibration potentiometer is best mounted internally. Once set it rarely needs further adjustment.
Initial calibration is performed by disconnecting the collector of the BC108 transistor and then using an accurate dc voltmeter to set the top of the divider chain at exactly 20 volts.
--------- Circuit of complete oscilloscope calibrator - note
that some resistor values in the divider chain are obtained by parallel
resistors of higher value.
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