Dynamically calibrated trigger for oscilloscopes

Abstract

An oscilloscope provides adjustable control of the reference voltage level to the trigger comparator to calibrate the trigger for both gain and offset. An internal computer in the oscilloscope provides a digital input value to a gain digital-to-analog converter (DAC) the analog output of which is provided to the reference input of a level DAC. The level DAC also receives digital input from the control computer and provides its analog output to the reference input of the trigger channel comparator. The digital data value provided to the gain DAC determines the relative gain of the output signal from the level DAC. A calibration signal can be applied to the input of the trigger signal channel, preferably having a triangular waveform which varies between minimum and maximum values, allowing the computer controller to execute a calibration routine in which the voltages provided from the level DAC to the comparator are changed to find the digital data values corresponding to the minimum and maximum reference voltages at which the comparator stops changing states. This information can then be used by the computer controller to determine the relative gain of the trigger signal channel and to adjust the input to the gain DAC to provide a proper trigger reference voltage level. A further procedure is carried out to determine the offset which occurs in the trigger signal channel and to provide a correction count to the data value provided to the level DAC to properly accommodate the offset. Periodic recalibration in the field can be carried out utilizing an internal calibration signal generator which provides a calibration signal which can be correlated with the initial external calibration signal.