Nearly all consumer products today have electronic circuits. Whether a product is simple or complex, if it includes electronic components, the design, verification, and debugging process requires an oscilloscope to analyze the numerous electrical signals that make the product come to life. Understanding oscilloscope basics is critical to almost all product design.
Once you have an oscilloscope, there are basic things you need to do to set it up and begin using it.
- Proper grounding for safety.
- Setting oscilloscope controls.
- Calibrating the oscilloscope.
- Connecting probes.
- Compensating the probes.
Proper grounding is an important step when you set up to take measurements or work on a circuit, as it protects you from a hazardous shock. To ground the oscilloscope means to connect it to an electrically neutral reference point, such as earth ground. Ground your oscilloscope by plugging its three-pronged power cord into an outlet grounded to earth ground. Grounding the oscilloscope is necessary for safety. If a high voltage contacts the case of an ungrounded oscilloscope—any part of the case, including knobs that appear insulated—it can give you a shock. However, with a properly grounded oscilloscope, the current travels through the grounding path to earth ground rather than through you to earth ground. Grounding is also necessary for taking accurate measurements with your oscilloscope. The oscilloscope needs to share the same ground as any circuits you are testing.
Setting the Controls
After plugging in the oscilloscope, look at the front panel. The front panel is typically divided into three main sections: vertical, horizontal, and trigger. Your oscilloscope may have other sections, depending on the model and type. Notice the input connectors on your oscilloscope—this is where you attach the probes. Most oscilloscopes have at least two input channels and each channel can display a waveform on the screen. Multiple channels are useful for comparing waveforms. The front panel of a Mixed Signal Oscilloscope (MSO) will also have digital inputs.\
Some oscilloscopes have AUTOSET and/or DEFAULT buttons that can set up the controls in one step to accommodate a signal. If your oscilloscope does not have this capability, it is helpful to set the controls to standard positions before taking measurements.
Calibrating the Instrument
In addition to proper oscilloscope setup, periodic instrument self-calibration is recommended for accurate measurements. Oscilloscope calibration is needed if the ambient temperature has changed more than 5° C (9° F) since the last self-calibration or once per week. In the oscilloscope menu, this can sometimes be initiated as Signal Path Compensation. Refer to the manual that accompanied your oscilloscope for more detailed instructions.
Connecting the Probes
Once you have properly grounded the oscilloscope and set up the oscilloscope in standard positions, you are ready to connect a probe to your oscilloscope. A probe, if well-matched to the oscilloscope, enables you to access all the power and performance in the oscilloscope, and will ensure the integrity of the signal you are measuring. Measuring a signal requires two connections: The probe tip connection and the ground connection. Probes often come with a clip attachment for grounding the probe to the circuit under test. In practice, you attach the grounding clip to a known ground in the circuit, such as the metal chassis of a product you are repairing, and touch the probe tip to a test point in the circuit.
Compensating the Probes
Passive attenuation voltage probes (typically shipped with every oscilloscope) must be compensated to the oscilloscope. Before using a passive probe, you need to compensate it to balance its electrical properties to an oscilloscope. You should get into the habit of compensating the probe every time you set up your oscilloscope. A poorly adjusted probe can make your measurements less accurate.
What can you do with an oscilloscope?
The two most basic measurements you can make are: Voltage measurements and Time measurements. Just about every other measurement is based on one of these two fundamental techniques.
But, you may be asking yourself, “How do you measure current with an oscilloscope?” if the basic measurements are voltage and time. Ohm’s law states that voltage between two points in a circuit equals the current times the resistance. From any two of these quantities you can calculate the third. So, if you know the resistance at the test points in your circuit, and measure the voltage, you can calculate the current. Another method is to use a current probe. A current probe connects to an oscilloscope just like a standard probe. Once connected, it will provide automatic unit scaling and readout on the oscilloscope's display.
Voltage is the amount of electric potential, expressed in volts, between two points in a circuit. Usually one of these points is ground (zero volts), but not always. Voltages can also be measured from peak-to-peak. That is, from the maximum point of a signal to its minimum point. You must be careful to specify which voltage you mean. The oscilloscope is primarily a voltage-measuring device. Once you have measured the voltage, other quantities are just a calculation away.
The most basic method of taking voltage measurements is to count the number of divisions a waveform spans on the oscilloscope’s vertical scale. Adjusting the signal to cover most of the display vertically makes for the best voltage measurements, as shown in the figure below. The more display area you use, the more accurately you can read the measurement. You should always measure voltage on the center vertical graticule line.
Many oscilloscopes have cursors that let you make waveform measurements automatically, without having to count graticule marks. A cursor is simply a line that you can move across the display. Two horizontal cursor lines can be moved up and down to bracket a waveform’s amplitude for voltage measurements, and two vertical lines move right and left for time measurements. A readout shows the voltage or time at their positions.
Time and Frequency Measurements
You can make time measurements using the horizontal scale of the oscilloscope. Time measurements include measuring the period and pulse width of pulses. Frequency is the reciprocal of the period, so once you know the period, the frequency is one divided by the period. Like voltage measurements, time measurements are more accurate when you adjust the portion of the signal to be measured to cover a large area of the display, as illustrated in the figure below.
Pulse Width and Rise Time Measurements
In many applications, the details of a pulse’s shape are important. Pulses can become distorted and cause a digital circuit to malfunction, and the timing of pulses in a pulse train is often significant.
Standard pulse measurements are pulse rise time and pulse width. Rise time is the amount of time a pulse takes to go from a low to high voltage. By convention, the rise time is measured from 10% to 90% of the full voltage of the pulse. This eliminates any irregularities at the pulse’s transition corners.
Pulse width is the amount of time the pulse takes to go from low to high and back to low again. By convention, the pulse width is measured at 50% of full voltage. The figure below illustrates these measurement points.
How does an oscilloscope work?
A basic oscilloscope consists of three different systems – the vertical system, horizontal system, and trigger system. Each system contributes to the oscilloscope’s ability to accurately reconstruct a signal. The front panel of an oscilloscope is divided into three sections labeled Vertical, Horizontal, and Trigger. Your oscilloscope may have other sections, depending on the model and type. When using an oscilloscope, you adjust settings in these areas to accommodate an incoming signal:
- Vertical: This is the attenuation or amplification of the signal. Use the volts/div control to adjust the amplitude of the signal to the desired measurement range.
- Horizontal: This is the time base. Use the sec/div control to set the amount of time per division represented horizontally across the screen.
- Trigger: This is the triggering of the oscilloscope. Use the trigger level to stabilize a repeating signal, or to trigger on a single event.
Vertical System and Controls
Vertical controls are used to position and scale the waveform vertically, set the input coupling, and adjust other signal conditioning. Common vertical controls include:
- Coupling: DC, AC, and GND
- Bandwidth: Limit and Enhancement
- Termination: 1M ohm and 50 ohm
- Invert: On/Off
Scale: Fixed Steps and Variable
Horizontal System and Controls
An oscilloscope’s horizontal system is most closely associated with its acquisition of an input signal. Sample rate and record length are among the considerations here. Horizontal controls are used to position and scale the waveform horizontally. Common horizontal controls include:
- Sample Rate
- Position and Seconds per Division
- Time Base
- XY Mode
- Z Axis
- XYZ Mode
- Trigger Position
- Trace Separation
- Record Length
Trigger System and Controls
An oscilloscope’s trigger function synchronizes the horizontal sweep at the correct point of the signal. This is essential for clear signal characterization. Trigger controls allow you to stabilize repetitive waveforms and capture single-shot waveforms. The trigger makes repetitive waveforms appear static on the oscilloscope display by repeatedly displaying the same portion of the input signal.
Edge triggering, available in analog and digital oscilloscopes, is the basic and most common type. In addition to threshold triggering offered by both analog and digital oscilloscopes, many digital oscilloscopes offer numerous specialized trigger settings not offered by analog instruments. These triggers respond to specific conditions in the incoming signal, making it easy to detect, for example, a pulse that is narrower than it should be. Such a condition is impossible to detect with a voltage threshold trigger alone.
Advanced trigger controls enable you to isolate specific events of interest to optimize the oscilloscope’s sample rate and record length. Advanced triggering capabilities in some oscilloscopes give you highly selective control. You can trigger on pulses defined by amplitude (such as runt pulses), qualified by time (pulse width, glitch, slew rate, setup-and-hold, and time-out), and delineated by logic state or pattern (logic triggering).
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