Dynamically calibrated trigger for oscilloscopes

A method of calibrating a trigger signal channel of an oscilloscope which receives a trigger signal at an input and operates on it to provide a conditioned trigger signal having a gain and offset, the conditioned trigger signal being provided to one input of a comparator, with the other input of the comparator receiving a reference voltage against which the trigger signal is compared, wherein the comparator changes its output state as the trigger signal to the comparator goes through the reference voltage level, comprising the steps of:

• (a) providing the output voltage of a gain digital-to-analog converter which converts a digital input word to an analog output voltage to a level digital-to-analog converter which receives the analog voltage output of the gain digital-to-analog converter as its reference input, and providing the analog output of the level digital-to-analog converter to the reference input of the comparator, the analog output of the level digital-to-analog converter voltage determined by the digital input word provided to it and the reference voltage from the gain digital-to-analog converter;
• (b) applying a calibration signal to the input of the trigger signal channel which changes periodically and linearly between known high and low voltage values and which has zero offset to provide a conditioned calibration signal;
• (c) applying a digital input to the gain digital-to-analog converter to provide an upper limit output voltage from the gain digital-to-analog converter which is applied to the level digital-to-analog converter;
• (d) applying a digital input to the level digital-to-analog converter which is at a value at which the output voltage of the level digital-to-analog converter will be in about the middle of its output range such that the output of the comparator is changing states;
• (e) changing the value of the digital data provided to the level digital-to-analog converter until its output voltage is at a level at which the trigger comparator stops changing states at a voltage level which is at the maximum of the conditioned calibration signal and storing the digital input value at this maximum as a top digital data value;
• (f) changing the digital data value provided to the level digital-to-analog converter until its output voltage is at a level at which the comparator stops changing states at the minimum of the conditioned calibration signal and storing the digital input value at this minimum as a bottom digital data value;
• (g) subtracting the bottom digital data value from the top digital data value and comparing the difference with a full desired digital input range of the level digital-to-analog converter and determining whether the difference is equal to the full desired digital input range of the level digital-to-analog converter, and, if not, then
• (h) changing the value of the digital input to the gain digital-to-analog converter and repeating steps (b) through (g) above until the difference between the top and bottom input values is equal to the desired digital input range and then stopping and storing the value of the input to the gain digital-to-analog converter at that time as the gain correction count, and using that gain correction count as the input to the gain digital-to-analog converter for subsequent operation of the oscilloscope where the oscilloscope is triggering on a signal provided to the input of the trigger channel.

Trigger generating apparatus for an oscilloscope comprising:

• (a) a trigger signal channel receiving a trigger signal at an input and operating on it to provide a conditioned trigger signal;
• (b) a comparator which receives the conditioned trigger signal at one input with the other input of the comparator being the reference voltage against which the conditioned trigger signal is compared, wherein the comparator changes its output state as the trigger signal received by the comparator goes through the level of a reference voltage received by the comparator;
• (c) means for providing a selectable output reference voltage to the reference input of the comparator based on digital input data values provided to it, including a gain digital-to-analog converter which converts a digital input data value to an analog output voltage and a level digital-to-analog converter which receives the analog voltage from the gain digital-to-analog converter as its reference voltage input and also receives a digital input data value and provides an analog output voltage which is determined by its digital input data value and the reference voltage, the output voltage of the level digital-to-analog converter determining the output voltage provided to the reference input of the comparator;
• (d) control means for providing a digital input data value to the gain digital-to-analog converter and a digital input data value to the level digital-to-analog converter, the data value provided to the gain digital-to-analog converter determining the gain of the signal applied to the comparator and the data value provided to the level digital-to-analog converter determining the relative voltage level of the reference voltage provided to the comparator.

A method of calibrating a trigger signal channel of an oscilloscope which receives a trigger signal at an input and operates on it to provide a conditioned trigger signal having a gain and offset, the conditioned trigger signal being provided to one input of a comparator with the other input of the comparator being the reference voltage against which the conditioned trigger signal is compared, wherein the comparator changes its output state as the trigger signal to the comparator goes through the reference voltage level, comprising the steps of:

• (a) applying a calibration signal to the input of the trigger signal channel which varies linearly and periodically between known high and low voltage values which are equal to a nominal full scale trigger signal range, and which has zero offset;
• (b) applying a selected trigger level voltage to the reference input of the comparator while the calibration signal is being applied to the trigger signal channel input and then changing the trigger level voltage applied to the reference input of the comparator until the comparator stops changing state at a top voltage above a nominal level of zero volts and then changing the voltage applied to the reference input of the comparator until the comparator stops changing at a bottom voltage below a nominal level of zero volts;
• (c) determining an offset correction factor as one-half the difference between the absolute values of the top and bottom voltages;
• (d) adding the offset correction factor to nominal trigger level voltages applied to the reference input of the comparator when triggering on a trigger signal applied to the input of the trigger signal channel.

A method of calibrating a trigger signal channel of an oscilloscope which receives a trigger signal at an input and operates on it to provide a conditioned trigger signal having a gain and offset, the conditioned trigger signal being provided to one input of a comparator, with the other input of the comparator receiving a reference voltage against which the trigger signal is compared, wherein the comparator changes its output state as the trigger signal to the comparator goes through the reference voltage level, comprising the steps of:

• (a) providing the output voltage of a gain digital-to-analog converter, which converts a digital input value to an analog output voltage, to a level digital-to-analog converter which receives the analog voltage output of the gain digital-to-analog converter as its reference input, and providing the analog output of the level digital-to-analog converter to the reference input of the comparator, the analog output of the level digital-to-analog converter voltage determined by the digital input value provided to it and the reference voltage from the gain digital-to-analog converter;
• (b) applying a calibration signal to the input of the trigger signal channel which changes periodically and linearly between known high and low voltage values and which has zero offset;
• (c) changing the digital input value provided to the level digital-to-analog converter until the comparator stops changing state at the top of the conditioned calibration signal waveform and storing this digital input value to the level digital-to-analog converter at this point as the top value;
• (d) changing the digital input data value to the level digital-to-analog converter until the trigger comparator stops changing state at the bottom of the conditioned trigger signal and storing the digital input value to the level digital-to-analog converter at this point as the bottom value;
• (e) determining the upper digital count as the difference between the midpoint of a full desired digital input range to the level digital-to-analog converter and the top value and the lower digital count as the difference between the midpoint and the bottom value, and determining an offset correction factor as one-half of the upper digital count minus the absolute value of the lower digital count and storing this offset correction factor count;
• (f) applying a trigger signal to the input of the trigger signal channel during normal operation of the oscilloscope, providing a digital data value which corresponds to a desired nominal voltage level at which triggering is to occur, and correcting that digital input data value by adding the offset correction factor count to it to yield a corrected digital level count and then applying the corrected digital level count to the level digital-to-analog converter.