Oscilloscopes measure parameters of time-dependent waveforms (periodic, transients).
Setting up an oscilloscope is slightly more complicated than a multimeter, but allows for far greater analysis of signal characteristics.
PSpice integration offers an easy route to cross-probe designs for circuit simulations.
For beginner oscilloscope users, this image may appear daunting at first glance
Every electronics student or hobbyist will eventually need to bring their theoretical lessons into the realm of practicality. Historically, this meant translating simple circuits to breadboards with through-hole components, power supplies, and multimeters. For DC measurements (and even some AC measurements via RMS), a multimeter is plenty capable. However, for periodic wave sources or those that contain transient components, an oscilloscope is necessary to see the true character of the signal as it travels through the circuit. Beginner oscilloscope study involves learning the operation of the machine, its uses in circuit analysis, and some simple examples that a reader can follow along with.
Why Can’t I Just Use a Multimeter?
What to Know as a Beginner Oscilloscope User
Beginner oscilloscope users will most likely be introduced to this instrument when first encountering alternating current signals. Broadly, the oscilloscope will function similarly to a multimeter by probing two or more points of a circuit for voltage readings. As periodic and other time-dependent signals have velocity readings that fluctuate with time, a multimeter is unlikely to provide insight into the actual function of the circuit. By probing at a point of interest and a reference voltage (typically ground), a visualization of the signal can be relayed to a monitor for physical oscilloscopes or represented digitally in apps. This is far more useful than a single voltage reading - the proper time division on the horizontal axis and voltage division on the vertical axis (and trigger event, if necessary) will display the growth, decay, or periodicity of the waveform.
From here, a multitude of analyses can be performed on the waveform, such as rise/fall time, frequency, and amplitude. An oscilloscope is a great tool for not only modeling circuit behavior in simulation tools but also for diagnosing and isolating errors in physical circuits.
How to Measure Voltage With an Oscilloscope
Unlike a multimeter, an oscilloscope doesn’t simply give a numerical output. Operators can either check the height of the waveform visually or calculate the difference in y-values between two points on the curve. Begin by ensuring the waveform is appropriately sized on the screen: with the peaks and troughs of the waveform in view, maximize the y-axis magnification of the waveform using the appropriate dial. This ensures no values are cut off while optimizing fidelity. Measuring points along the curve will prove most accurate, but users can also glance at the screen for a ballpark check. Some oscilloscopes may output the peak-to-peak voltage on a bottom readout of the screen without any further action necessary.
How to Measure Frequency With an Oscilloscope
The frequency can not be read directly from the graph of the waveform, but there are many ways to obtain this value. The horizontal axis of the oscilloscope shows time, with users able to adjust the period up (compresses the waveform horizontally) or down (stretches the waveform horizontally). For periodic signals, there is no way to capture the entire waveform horizontally due to its repetition; instead, users should capture some periods (2 is a good start, although this will depend on the size of the screen) of the waveform. A periodic waveform repeats every period, and operators can calculate the frequency as the reciprocal of the period. Much like with the voltage, there also exist methods for measurement and readout directly from the oscilloscope.
Setting Up and Sampling With an Oscilloscope
In general, a purely digital oscilloscope function or application will feature much less setup time, calibration, and testing than a physical unit. As a simulation, many of the guidelines are used to help analyze why an oscilloscope reading may look significantly different than expected in simulation software. The steps below are geared towards a physical interface, but the discussion of the “under the hood” functionality is still useful to those running simulations:
- Check/match attenuation on probes and scope - Standard probes will feature a 10x or 1x attenuation factor, which is the factor by which the amplitude of the original voltage signal is reduced. Unless the signal of interest is of extremely low voltage, 10x attenuation will be the preferred choice.
- Ballpark the size intervals of axes - If there is an expected value at the node being probed, set up the vertical voltage intervals such that the signal takes up approximately ½ - ⅔ of the screen. Much like a multimeter, the signal can only read values below its max - don’t forget the attenuation factor. Assuming a signal appears on screen, adjust the horizontal time intervals to capture one or two periods of the function. At this point, the waveform can be shifted horizontally and vertically. Shifting the waveform may make reading or calculating slightly easier, but any shifts should be notated.
- Set the trigger - The trigger can be used to tell the oscilloscope when to start scanning. By picking a maxima or minima value that ideally only appears once per period, the scope should grab a perfect snapshot of the waveform during this time frame and avoid a jittery image.
By this point, the waveform should be visible and ready for analysis. Readouts such as frequency, gain, amplitude, and more are available and may either be presented automatically, cycled between, or toggled on and off.
With practice and proper setup, generating waveforms will be as simple as probing
Oscilloscope Practice With PSpice
It is worth noting that oscilloscopes are referred to as wave analyzers in PSpice documentation, and will therefore be mentioned as such for the remainder of this section. PSpice’s wave analyzer features a split-screen display with a digital signal on top and an analog signal on bottom (when both are present). One of the best features of PSpice is the ability to cross-probe at the schematic level in OrCAD Capture’s PSpice menu integration. Choosing a marker from the PSpice menu and placing it at the node or nodes of interest will generate a waveform signal in PSpice of the selected marker category, such as voltage, current, phase, and gain. The probe window results of a circuit simulation can also be saved using the display control feature.
An example circuit for simulation practice is included in the installation directory. From the OrCAD Capture menu, click Open > Project and find “EXAMPLE.OPJ” in the installation path. The circuit has already been set up to run and generate a waveform in the probe window; the parameters of the wave analyzer can be adjusted by altering the variables and flags or by placing markers at the schematic level in OrCAD Capture as described above.
OrCAD’s PSpice integration allows for cross-probing at the schematic level for waveform analysis
Whether you are a beginner oscilloscope user or a seasoned designer, Cadence offers a robust tool suite to carry your project from schematic to final assembly. Circuit simulation is indispensable at the early stages of network analysis; try a full access free trial of PSpice to see how it can aid your workflow.