From the entire pool of test equipment available at our disposal as electronics engineers and technicians, the most useful is undoubtedly the oscilloscope. This one device is very powerful and can help us capture voltage measurements over time, very quickly and accurately – something that can’t easily be done with any other device found in the laboratory. The oscilloscope is an essential tool used in manufacturing, design, troubleshooting, signal integrity, and if desired, in simply understanding how electronic circuits work.
Even though the modern oscilloscope looks complicated and scary with its entire set of buttons, knobs, probes and associated attachment points and colored display, it is, in reality, a very simple device to use. Don’t let the complicated looks of an oscilloscope intimidate you! The key to becoming an oscilloscope expert is first understanding the basics and then building upon that basic knowledge. In this regard, the following brief article will cover some key points and common pitfalls new users encounter surrounding the basic use of oscilloscopes. It will help point you in the right direction. As time progresses and more time is spent actually using the oscilloscope, you will eventually become more proficient and comfortable taking almost any measurement.
To keep things simple, this article only covers conventional digital oscilloscopes known as Digital Storage Oscilloscopes (DSOs), with a raster-type of display. The older analog scopes that utilized luminous phosphor to display information and newer specialized oscilloscopes such as Digital Phosphor Oscilloscopes (DPOs), Mixed Domain Oscilloscopes (MDOs), or Mixed Signal Oscilloscopes (MSOs) are not covered in this article.
Grounding and Safety
Before getting too deep into the basics of oscilloscopes, understanding proper grounding and safety will help to not blow up your DSO or its probes. Improper connection of the probe ground across the chassis/safety ground can create a path for current flow, resulting in damage to the probe. In brief, the issue is that the metal part of the connector the probe connects into on the oscilloscope is directly tied to safety earth ground through the power cord of the scope. You can vary this connection yourself with an Ohm meter. It’s a low impedance connection and when the circuit you are probing is also connected to safety earth ground a loop forms and the very low impedance allows the current from the circuit to get excessive. The current carrying capacity of the ground lead of the probe is quickly exceeded, and the lead wire abruptly opens and you will likely hear a loud POP! The best solution to this problem is to break the ground loop through isolation of the circuit under test or isolation of the oscilloscope ground. Since it’s a safety issue if the safety ground on the oscilloscope is defeated, the best option is to make sure the circuit you’re testing is floating (i.e. not tied to earth safety ground). Choose to either power up the test circuit with an isolated supply or with a battery. Be careful with applying power to the circuit under test with something like a USB connector as these types of devices are usually not isolated from ground and you will still have a ground loop problem.
What is an Oscilloscope?
An oscilloscope measures voltage waveforms from voltage sensors such as the oscilloscope voltage probes which come with the device or some other sensors such as a load cells, current probes, sound level meter or other sensor. The graph of the oscilloscope measures voltage in the vertical axis and time along the horizontal. From the waveform captured we can obtain information such as frequency, amplitude, period, phase, distortion, noise, DC, AC, duty cycle (on time versus off time), rise/fall time, etc.
Besides the display, there are three other important functional blocks that make up the common oscilloscope. These functional blocks are the trigger block, the volts-per-division block and the seconds-per-division block.
The trigger function is what is used to synchronize the horizontal sweep at the precise position of the signal, vital for unambiguous signal representation. The trigger makes recurring waveforms look stationary on the display by repetitively displaying the matching portion of the input signal. The most rudimentary and conventional form of triggering is called edge-type triggering. This is what you are most likely to use when first beginning to use the oscilloscope. There are many other specialized and sophisticated trigger types that respond to specific conditions and are what can really make the DSO a powerful tool. These triggers include slew-rate, glitch, pulse-width, time-out, runt-pulse, logic, setup-and-hold, and communication triggering just to name a few.
The volts-per-division (volts/div) control allows movement of the waveform up or down on the display based on a scaling factor. For instance, if the knob is set to 1 volt and the display is made up of 10 vertical divisions, then 10 volts can be displayed from top to bottom of the display. Note the reading could change based on the attenuation factor of the probe taking the readings. If a 10X (meaning 10 times) probe is used and the oscilloscope doesn’t automatically correct for it, then you must multiply the resultant waveform reading by 10 in order to obtain the correct amplitude of that reading. I don’t think you will have to worry about this issue if you are using a relatively modern oscilloscope.
Input coupling is another simply, yet commonly misinterpreted function found within the volts/div section of the oscilloscope. It refers to the method used to connect an electrical signal from one circuit to another, i.e. the connection from your test circuit to the oscilloscope. You can configure input coupling as DC, AC, or ground. AC coupling simply blocks the DC portion of a signal and you see the waveform centered around zero volts on the display. The ground setting disconnects the input signal from the vertical control thereby letting you see where zero volts is located on the display. The DC setting allows all of the input signal (DC and AC) to be displayed.
The seconds-per-division (sec/div) function is what establishes the rate at which the waveform is moved across the display. As with the volts/div control described above, the sec/div control setting is also a scale factor. If the setting is 10ms on the knob, then each horizontal division on the display represents 10ms and the total screen width (also assuming 10 divisions total on the display) is equal to 100ms. Observing longer and shorter time intervals of the input signal is easily accomplished by changing the sec/div setting control knob.
The rise times of the switching mechanisms in components we are using are getting faster and faster and the ability to effectively measure these rise times comes into question. You will often be asked if the oscilloscope has enough bandwidth. The typical formula used to determine adequate bandwidth of the oscilloscope is 0.35 divided by the rise time. For instance, needing to measure a pulse with a rise time of 1ns means the minimum bandwidth the oscilloscope should be around 350MHz. Of course, more bandwidth is always better.
Sample rate – specified in samples-per-second (S/s) is also another important oscilloscope consideration. Samples-per-second refers to how frequently a DSO takes a snapshot or sample of the input signal. The higher the sample rate, the greater the resolution and detail of the displayed waveform and the less likely that critical information will be lost. A good rule of thumb if you’re measuring a sinusoidal waveform is that your oscilloscope should have a sample rate at least 2.5 times the highest frequency component of the signal you intend to measure and 10 times the highest frequency component of the signal you intend to measure if you’re measuring square waves, pulses and other signal types.
A separate article could be written about oscilloscope probes. The most basic general-purpose type that you will encounter is the passive 1X or 10X probes. Be cautious of excessive capacitive loading of the circuit under test with these probes. For high-speed signal probing, active and differential probes are necessary. Logic probes are available when the capture of multiple channels of data is necessary.
The capability versus costs for oscilloscopes is getting better and better. A quick look on the internet reveals that you can obtain hundreds of megahertz of bandwidth and professional level functionality for under $500. This may be enough performance to get most of your probing done. As rise-times increase and measurements get more challenging you will have to pay for performance. When you budget for a new scope be sure to include costs to obtain the probes you need, calibration of the scope and probes, and shipping of the scope and probes back and forth to your calibration provider.
Oscilloscopes are the workhorses of product development and testing. They look complicated at first, but in reality, are pretty easy to use. Just remember the basics and you will soon be considered the resident oscilloscope expert at your firm. I hope you have fun working with oscilloscopes and you are able to enhance your ability to use them as you progress in your engineering career! Good luck.
- Tektronix, XYZ’s of Oscilloscopes – Primer