An oscilloscope is a device that plots the relationships between two or more variables, with the horizontal axis normally being a function of time and the vertical axis usually a function of the voltage generated by an input signal. Oscilloscopes are powerful tools for designing and testing electronic devices, as they allow you to see what the electronic signals actually look like and how they change over time. Oscilloscopes are used in a wide range of fields, from engineering to telecommunications to research.
In this article, we will explore some of the basics of how oscilloscopes work, how to use them, and what they can measure. We will also look at some of the different types of oscilloscopes and their advantages and disadvantages.
How Oscilloscopes Work
The main component of an oscilloscope is the cathode-ray tube (CRT), which is a vacuum tube that contains an electron gun, a deflection system, and a phosphor-coated screen. The electron gun emits a beam of electrons that travels through the tube and hits the screen, creating a bright spot. The deflection system consists of two pairs of metal plates that can apply electric fields to the electron beam, causing it to move horizontally and vertically. By varying the voltage applied to the plates, the spot can be moved across the screen in any direction.
The input signal that we want to measure is connected to the vertical deflection plates, while a reference signal, usually a periodic waveform called a sweep, is connected to the horizontal deflection plates. The sweep signal controls the speed and direction of the horizontal movement of the spot, while the input signal controls the vertical movement of the spot. As a result, the spot traces out a graph of the input signal on the screen, where the horizontal axis represents time and the vertical axis represents voltage.
The screen is coated with a phosphor material that glows when hit by electrons, creating a visible trace of the signal. The phosphor has a property called persistence, which means that it continues to glow for a short time after the electrons stop hitting it. This allows us to see the trace even if the signal is changing very fast. However, if the signal is changing too fast or too slow, the trace may appear as a blur or a dot, respectively. To avoid this, we need to adjust the sweep frequency and amplitude to match the input signal frequency and amplitude, so that the trace appears as a clear and stable waveform.
How to Use Oscilloscopes
To use an oscilloscope, we need to connect the input signal to the input terminal of the oscilloscope, usually using a probe. A probe is a device that connects the signal source to the oscilloscope and provides some features such as attenuation, compensation, and isolation. Attenuation reduces the amplitude of the signal to prevent overloading the oscilloscope. Compensation adjusts the frequency response of the probe to match the oscilloscope. Isolation prevents the ground of the signal source from being shorted to the ground of the oscilloscope.
Once the signal is connected, we need to adjust some settings on the oscilloscope to display the signal properly. Some of the common settings are:
- Vertical scale: This controls the voltage sensitivity of the vertical deflection plates. It is usually expressed in volts per division, where a division is a unit of measurement on the screen. For example, if the vertical scale is set to 1 V/div, then each division on the screen represents 1 volt of the input signal. We can adjust the vertical scale to zoom in or out on the signal amplitude.
- Horizontal scale: This controls the time sensitivity of the horizontal deflection plates. It is usually expressed in seconds per division, where a division is a unit of measurement on the screen. For example, if the horizontal scale is set to 1 ms/div, then each division on the screen represents 1 millisecond of the input signal. We can adjust the horizontal scale to zoom in or out on the signal period.
- Trigger: This controls when the sweep signal starts and stops. It is usually set to a certain voltage level and slope of the input signal, so that the sweep signal is synchronized with the input signal. For example, if the trigger is set to 2 V and rising, then the sweep signal will start when the input signal crosses 2 V in the positive direction. This ensures that the trace is stable and repeatable on the screen.
- Coupling: This controls how the input signal is connected to the vertical deflection plates. It can be either AC or DC. AC coupling blocks the DC component of the input signal, so that only the AC component is displayed on the screen. This is useful for measuring small AC signals on top of large DC signals. DC coupling allows both the AC and DC components of the input signal to be displayed on the screen. This is useful for measuring the absolute voltage of the input signal.
By adjusting these settings, we can display the input signal on the screen in a clear and meaningful way. We can then use the oscilloscope to measure various characteristics of the signal, such as amplitude, frequency, phase, rise time, fall time, duty cycle, and more.
Types of Oscilloscopes
There are different types of oscilloscopes that have different features and capabilities. Some of the common types are:
- Analog oscilloscope: This is the traditional type of oscilloscope that uses a CRT to display the signal. It has a high bandwidth and fast response time, but it has a limited memory and resolution. It is also susceptible to noise and distortion. Analog oscilloscopes are becoming obsolete as digital oscilloscopes become more advanced and affordable.
- Digital oscilloscope: This is the modern type of oscilloscope that uses a digital signal processor (DSP) to convert the analog input signal into a digital signal, store it in a memory, and display it on a liquid crystal display (LCD) or a light-emitting diode (LED) screen. It has a high memory and resolution, but it has a limited bandwidth and sampling rate. It is also immune to noise and distortion. Digital oscilloscopes are the most widely used type of oscilloscopes today.
- Mixed-signal oscilloscope: This is a type of digital oscilloscope that can measure both analog and digital signals simultaneously. It has two sets of input channels: one for analog signals and one for digital signals. It can display both the analog and digital waveforms on the same screen, as well as the logic states and timing relationships of the digital signals. Mixed-signal oscilloscopes are useful for testing and debugging mixed-signal systems, such as microcontrollers, sensors, and communication devices.
- Digital storage oscilloscope: This is a type of digital oscilloscope that can store the digital signal in a memory and display it later. It can also perform various mathematical operations and analysis on the stored signal, such as filtering, averaging, Fourier transform, and more. Digital storage oscilloscopes are useful for capturing and analyzing transient or rare events, such as glitches, spikes, and bursts.
- Digital phosphor oscilloscope: This is a type of digital oscilloscope that can display the signal intensity and color on the screen, similar to an analog oscilloscope. It can also capture and display multiple signals at different time intervals, creating a three-dimensional view of the signal behavior. Digital phosphor oscilloscopes are useful for observing complex and dynamic signals, such as modulated, pulsed, and burst signals.
Advantages and Disadvantages of Oscilloscopes
Oscilloscopes have many advantages and disadvantages compared to other measuring instruments, such as multimeters, spectrum analyzers, and logic analyzers. Some of the advantages are:
- Oscilloscopes can measure both the amplitude and the shape of the signal, while multimeters can only measure the amplitude.
- Oscilloscopes can measure both the frequency and the phase of the signal, while spectrum analyzers can only measure the frequency.
- Oscilloscopes can measure both the analog and the digital signals, while logic analyzers can only measure the digital signals.
- Oscilloscopes can display the signal in real-time, while other instruments may have a delay or a lag.
Some of the disadvantages are:
- Oscilloscopes are more expensive and complex than other instruments, especially the high-end models.
- Oscilloscopes require more skill and knowledge to operate and interpret the results, especially for advanced measurements and analysis.
- Oscilloscopes have a limited bandwidth and sampling rate, which may affect the accuracy and resolution of the measurement, especially for high-frequency signals.
Conclusion
Oscilloscopes are versatile and powerful tools for measuring and analyzing electronic signals. They can display the signal in a graphical form, showing the relationship between the voltage and the time. They can also measure various characteristics of the signal, such as amplitude, frequency, phase, and more. They can be used in a wide range of fields, from engineering to telecommunications to research.
However, oscilloscopes also have some limitations and challenges, such as cost, complexity, and bandwidth. Therefore, it is important to understand how oscilloscopes work, how to use them, and what they can measure. By doing so, you can unravel the mysteries of oscilloscopes and use them effectively and efficiently.