A guide to Digital Storage Oscilloscopes (DSOs)

Updated 20 October 2016

oscilloscope-desktopAn Oscilloscope is a particularly versatile piece of test and measuring equipment. They will (very accurately) display a graph of voltage change over time, allowing us to observe how voltage changes across a circuit in real time! We call these observed signals waveforms and they have their own lexicon of terms to describe their different characteristics.

waveform-oscilloscope-screenshot

The main display of the Oscilloscope resembles a Cartesian plane. As usual, this plane can be broken up into a vertical (voltage) and horizontal (time) axis. We call the fixed, equally-spaced gridlines along each axis, divisions and the control knobs on your Oscilloscope front panel can be used to manipulate the size each division represents.

The graph area is where your waveform will be displayed, as a function of voltage and time. You will require 1 channel per waveform you wish to capture. Luckily, each channel's waveform will then be displayed using a unique and fixed color; this makes it extremely easy to differentiate multiple signals. This is a true godsend when examining the relationship between signals in your circuit.  

See along the bottom of the screenshot to see the vertical division values for each channel, and see the top for the value of the horizontal divisions.

This oscilloscope screenshot was taken using our Rigol 1074z DSO connected up with our Rigol DG1022 Waveform Generator. If you are interested in Waveform Generators we've got a great rundown here. In any case, try to apply some of the following concepts to the depicted real-world example.

Down the center of each axis, you can see the trigger points are set. Trigger modes and methods are quite a conversation, if you want to learn about them we have a great little tutorial that will get you up to speed.

When a waveforms shape is repeated over some time period, we say that it has a frequency. This frequency is measured in Hertz (Hz) which is the SI unit for cycles per second.

Interestingly we can infer the frequency of a signal from its period. Period refers to the amount of time (in seconds) it takes for the signal to transmit once. The mathematical relationship between frequency and period is 1/Frequency(Hz) = Period. If we do some simple rearranging we find that 1/Period(s) = Frequency(Hz) also. We can see along the top of the scope's display that the time divisions are currently at 200us (200 microseconds) it repeats itself every 5 divisions. Doing the calculation 1/0.001s we get 1,000Hz or 1KHz as the displayed signals frequency.

The peak amplitude of the signal refers to the vertical height of the signal from zero and in our case, it refers to the voltage level of the signal. The vertical divisions are set to 1.00V so we can see that the amplitude of the signal is 2.5V.

Peak to Peak Amplitude is different to the peak amplitude, it refers to the voltage between the highest and lowest point in the waveform. As the signal depicted is oscillating around the 0V mark, we see that our signal is going from -2.5V to 2.5V meaning the peak to peak voltage is 5.0V. Vpp is how you will see Peak to Peak Voltage defined.

We find that the signal being measured in the diagram is a 1.0KHz Sine Wave at 5V peak to peak voltage!


What specs matter when buying your own Oscilloscope

When it comes to Digital Storage Oscilloscopes, there are a couple of specifications to be on the lookout for. Most of these specs will directly affect the price. When purchasing an Oscilloscope, knowing how you intend to use your scope will be essential in helping you purchase a sensibly priced scope. We have done our best to cover off on these specifications below; hopefully, this will ensure you know what's what when you are looking for your next piece of testing and measurement equipment!

Bandwidth - The bandwidth of your Oscilloscope refers to the frequency range your scope is able to accurately measure. In our example, we were measuring a 5kHz signal on a 70mHz scope, so it was well within the scopes bandwidth. The upper limit on your scopes bandwidth refers to the frequency at which your measured signal attenuates by -3dB, meaning the signal becomes unreliable. It's easy to see that wider bandwidth ranges will cover more applications, however; this specification increases alongside price so knowing what you intend to measure will help a great deal in selecting a bandwidth that will cover all your purposes. Usually, anything around 100MHz will be sufficient. When measuring digital signal's , it's important to understand that the digital signal is composed of a fundamental signal and its harmonics. You can research the actual theory behind this, and we recommend it to get a good grasp of what that means in practice. But for now, it's relevant to bandwidth as you will need to be able to accurately capture those harmonics, not just the fundamental waveform. The rule of thumb here is the 5x rule, you want a scope rated to measure signals 5x faster than that of the digital signal you wish to form. 

Max Real-time Sample Rate- This specification refers to how many samples your scope can take per second. The sample rate is usually given is MS/s and the more samples the better. Again, the price will usually increase as the sample rate increases. Our scope has a single channel sample rate of 1GSa/s but that rate is halved when using 2 channels and quartered when using 4 channels.

Max Memory Depth- Memory is where all the sample points of your waveform will be stored within your scope, depth refers to a number of points your scope can store. More memory allows your scope to acquire more points with which to plot your signal, great for high resolution, low-frequency signals. Bigger is not always better when it comes to your scope's memory, though, you want a scope with at least 1Mpts if you are forking out the cash for a scope in the first place. Our Rigol 1074Z has a standard memory depth of 12Mpts.

Channels- This refers to the number of Analog signals that can be read by your scope, our example scope has 4 Channels which seems to be a decent amount for measuring most SPI/I2C buses in today's circuits. If you wanted to read a bit more about how and why the channel count matters when buying your scope, take a look at this tutorial.

Minimum Vertical Sensitivity- This refers to the smallest value that can be assigned per vertical division. It essentially gives you the smallest measurement you can accurately measure in mV/division. The smaller the sensitivity of a scope becomes, the more expensive the scope will become.

Max Waveform Capture Rate - When the oscilloscope captures a waveform and logs all the sample points into its memory, by design, there is a part of the signal that is not captured by the scope. This is referred to as the dead time. The Waveform capture rate is a measurement of the rate at which your scope will trigger, acquire and display waveforms, the greater this number the more likely it is that your scope will capture random events within your tested circuit; as the dead zone is lessened as the capture rate is increased. It is measured in waveforms per second and you can expect that price increases alongside it.


rigol-oscilloscope-passive-probe

Probing into Probes

Oscilloscope probes are much more than just alligator clips that connect your scope to your waveform. They are actually extremely well-designed connectors that are made for the purpose of measuring a signal without influencing the behavior of the signal.

The influence your probe can have on your circuit is generally known as circuit loading and to combat this effect, probes come with attenuators. An attenuator is a device that is able to reduce the amplitude of a signal without affecting the waveforms shape, this process is the opposite of amplification.

There are a few types of probe available to us in testing and measuring, Passive and Active Probes being the main two.

A Passive probe works to minimize the effect it will have on a circuit. It does this by having a (sometimes multiple!) attenuation factor built-in, however, this makes measuring smaller (~10mV) signals difficult. Your scope will require you to select the type of probe and attenuation factor you are using; this enables it to compensate the readings taken accordingly.

Conversely, Active Probes provide amplification (or some other function) to the signal before processing it to the oscilloscope. In other words, they work to compensate for the effect they have on the circuit where passive probes work to minimize it. 

Either way, you should refer to the manufacturer's instructions when connecting probes to your scope as the probes they recommend are designed to work with your model!

That pretty much covers the essentials when it comes to DSOs. If you want a more directed guide on what to look our for when buying your first scope, we've got you covered with this straightforward discussion. Check out our range of Rigol Oscilloscopes if you are looking for really great prices on top notch oscilloscopes. Kick off the conversation in the forum with us if you think there is anything we should add or change in our tutorial, we're open to making great content that the community enjoys reading!

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