An oscilloscope is a necessary piece of equipment for any laboratory that does electronics work. Oscilloscopes are important instruments for viewing and monitoring electronic signals and are especially used for troubleshooting malfunctioning electronic equipment. Oscilloscopes allow the technician to view a variety of aspects of a signal, such as oscillations, ripple, transients, rise and fall times and other features that a typical Digital Multimeter cannot show.
Oscilloscope Self Calibration
Most newer digital oscilloscopes have an internal ‘Self Calibration’ program routine. This routine is used to optimize the oscilloscope’s signal path and adjusts for temperature variations and component changes over time to ensure the best performance and accuracy of the unit. The self-calibration routine should be run if the unit is being used in an environment with an ambient temperature difference over 5°C or more from the last time it was run or at least every 30 days during use.
Most oscilloscopes also have a built-in calibration or reference test point that uses a precision square-wave test signal that can be used to quickly check the vertical and horizontal settings of the oscilloscope. Simply connect the test signal to the oscilloscope’s inputs and adjust the volts/div and time/div settings to match the reference signal output. The appropriate vertical and horizontal graticule lines should line up with the test signal for a properly aligned oscilloscope. The reference test signal can also be used to adjust the probe compensation to provide a sharp signal trace when using the probe.
The use of the self-calibration routine or the reference test point does not replace the annual calibration of the oscilloscope. The annual calibration will use precise external signals to calibrate many important functions not checked during the above tests.
While there are both analog and digital types of oscilloscopes and different calibration steps required for them, there are four major functions that are necessary to be calibrated on every type and model of oscilloscope to ensure the accuracy and proper operation of the oscilloscope.
Vertical Deflection Accuracy
– The Vertical Deflection calibration tests the vertical offset, sensitivity and gain accuracy of the oscilloscope’s vertical amplifiers for each of the channel inputs. These parameters are critical for the oscilloscope to display an accurate representation and the correct amplitude of the incoming signal.
To calibrate the vertical deflection, a DC voltage signal is input into each oscilloscope input and the channel’s displayed signal is measured against the graticule divisions or cursor readings and compared to the input voltage level. An AC square wave signal should also be used to test the AC coupling signals for the channel inputs.
Bandwidth or Frequency Response
– Bandwidth determines an oscilloscope’s ability to measure a signal throughout the full frequency range of the unit. As the frequency increases towards the full frequency limit of the oscilloscope, the signal strength will ‘roll off’ and cause the signal amplitude to curve downward. Due to this, the oscilloscope will not be able to measure the correct amplitudes or any show any details at the highest frequencies.
To calibrate the bandwidth a ‘leveled sine wave’ signal is input into each channel at a reference frequency (usually 50kHz), and is either measured on the oscilloscope’s display or set to a nominal voltage value. The signal is then increased to the full bandwidth of the oscilloscope, for example, 100MHz. The measured signal at that frequency should be above 70.7%, or -3dB, of the reference amplitude for the bandwidth reading to be within specification.
Horizontal (Time Base) Accuracy
– The Horizontal accuracy indicates how accurately the horizontal system measures and displays the timing of the input signal.
This is calibrated by using a ‘time marker’ to input a pulse equal to one graticule division of the oscilloscope time/div setting to ensure that the display graticules line up with the signal. Any deviation can be measured by adjusting the time marker control to align the markers exactly on the vertical graticule lines, and reading the deviation from the time marker control.
For analog units, this test should be repeated for all the time/division settings on the oscilloscope, however, digital oscilloscopes use a quartz crystal which is significantly more accurate than the RC sawtooth generator circuits used in analog oscilloscopes, so calibration of every time/division settings is typically not required for digital oscilloscopes.
– An oscilloscope’s trigger function is important to help capture single waveforms and to stabilize periodic signals by aligning the waveforms so that they appear stationary on the oscilloscope display.
To calibrate the triggering system, a low amplitude signal, typically one half or one division at the highest bandwidth frequency is input into the appropriate channel to test that the system can trigger on the signal. The triggering is checked for both a rising edge signal and a falling edge signal.