How to make basic voltage vs. time measurements using an oscilloscope?Follow article
Oscilloscopes are widely used for electrical designing as well as for testing and debugging almost anything that runs electricity. They are primarily used to measure and display voltage versus time for periodic or repeating waveforms. Modern oscilloscopes, however, can also easily display and hold non-periodic waveforms. Besides, they have several other functions such as the automatic measurement of parameters like peak-to-peak voltage or frequency, ability to look at serial buses and mixed-signal analysis and to do frequency domain analysis for signals – similar to a spectrum analyzer.
To explain the operating principle of an oscilloscope, typically four basic elements are used. In the following, these four elements will be explained in detail.
The vertical system
The vertical section controls the magnitude (amplitude) of the signal under test. A very common task when using an oscilloscope is scaling the displayed waveform using the “volts/div” control. This setting controls the amplification or attenuation of the input signal. If the “volts per division” control is being increased, the waveform shrinks and if it is being decreased, the waveform grows. The “offset” control is used to change the vertical position of the signal, i.e. to move the waveform up or down on the screen.
The most important thing to keep in mind when configuring the vertical system of a digital oscilloscope is to use the volts per division control in such a way that the waveform is maximized and fills the complete display vertically. In other words, the positive and negative peaks should be as close to the top and bottom as possible, without clipping the waveform. This is necessary to use all available bits of the oscilloscope’s ADC (analog-to-digital converter) which results in an optimized quantization of the signal.
The horizontal system
The horizontal section of an oscilloscope controls the time base of the measured signal, whereas the time corresponds to the horizontal axis of the signal display. The control is used to scale and position the waveform horizontally - similar to the vertical system. The “seconds per division” changes the horizontal scaling of the signal and the control “horizontal position” moves the signal to left or right.
The sampling rate is one important parameter to analyze a signal using an oscilloscope. The oscilloscope digitizes the input signal at a given sample rate – usually noted as “samples per second”. These samples are stored in memory and together these make up a so-called waveform record. As the sample rate is being increased, the horizontal (time) resolution of the signal increases which helps to capture fast signal changes with more details. This also increases the probability of seeing or catching infrequent events. Higher sample rates however, create greater storage requirements.
The trigger system
This system is very important since triggering is needed for almost all oscilloscope measurements. Essentially, a trigger defines the conditions that have to be met before the oscilloscope begins an acquisition or begins taking samples. The trigger condition could be as simple as a defined threshold level that the signal amplitude crosses, or more complex like a specific frame in a communication signal. The correct trigger can stabilize a repeating or periodic signal, such as a sign wave, by causing each sweep to start at a given point on the signal. A trigger can also be used to capture non-periodic single events like a single pulse or a burst, etc. Depending on the signal to be measured, it is crucial to set the trigger properly.
There are many different trigger types. They can be both, analog or digital. Modern scopes can trigger on things like pulse widths, runts, glitches, etc. A commonly used trigger, however, is the so-called edge triggering. In edge triggering, the user defines a voltage value and the trigger occurs when that threshold is crossed, either on the rising edge or on the falling edge on a waveform.
The display system
Once the signal is recorded as described before, it is shown on the display section of the oscilloscope. Modern digital oscilloscopes have touchscreens that allow straightforward access to helpful measurement functions and signal settings, such as zooming in and out of a signal and using cursors or markers to make manual measurements. Besides, there is also a large number of automated functions for analyzing or measuring the acquired waveforms like peak or peak-to-peak voltage, frequency, rise and fall times, slew rate, crest factor or pulse counts etc.
From everyday measurements to specialized tasks
Oscilloscopes enable their users to perform an extremely broad variety of measurements, ranging from general purpose tests to compliance tests of specific industry standards. Rohde & Schwarz oscilloscopes have outstanding features like e.g. digital trigger, deep memory, frequency response analysis (Bode plot), real-time de-embedding, fast update rates, and unique low noise.
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