Digital Oscilloscopes

Digital Oscilloscopes

An oscilloscope is a test and measurement instrument used primarily to measure voltage over time. A Digital Storage Oscilloscope (also known as a DSO) takes the input signal and converts it from an analog wave to a series of digital signals. Once it is digitized, the digital storage oscilloscope can then store the information in memory and display it on the screen. The faster the signal is processed, the better the display will be.

The digital oscilloscope uses graphical grid called a graticule to display a signal. The Y-axis (vertical) is usually the voltage (though it could also be current, pressure, or another type of signal that is then converted into voltage). The X-axis (horizontal) is usually time (though it could also be frequency). Some digital storage oscilloscopes also use signal brightness as their Z-axis.

To make it easier to read the graticule, it is typically broken into 8 squares (or divisions) going vertically and 10 squares (or divisions) going horizontally. This can change from manufacturer to manufacturer, but it is fairly standard. The reason why it is done this way is that as long as you know what each division is, it makes it easier to read the values on screen. Luckily, digital storage oscilloscopes can also display the exact value at a given point on the screen. You have to move your marker, or cursor, to the spot in question and you should be able to see the specific value. A scope with multiple cursors will allow you to measure the difference between to spots, which can be very handy.

Some common measurements digital storage oscilloscopes are used for include

• Looking at the shape of a signal (also known as a waveform)
• Checking the amplitude (strength) of a signal
• Checking the frequency (timing) of a signal
• Checking the amount of time between events
• Looking for problems (noise) with a signal

What are the key specifications in selecting a digital storage oscilloscope?

There are typically four parameters that should be considered when choosing your instrument:

• Bandwidth
• Sample Rate
• Rise Time
• Recording Length

How much scope bandwidth do I need?

The amount of times a signal repeats itself in one second (Hertz or Hz) is its frequency. Oscilloscopes can view signals occurring anywhere from 1Hz (or less) up to 1GHz (1,000,000,000 (that's one billion) times per second) or more. Select an digital storage oscilloscope that can see more than the fastest signal you want to measure. In theory, you want your signal to be no faster than 71% of your maximum. The rule of thumb is that the bandwidth be five times (5x) greater than the maximum signal. So, if your signals to be observed are a maximum of 100MHz (100,000,000 times per second) then choose a model with a 500 MHz bandwidth.

How much sampling do I need?

As mentioned, a digital storage oscilloscope converts an analog signal into a digital one. This is done through a process known as sampling. The faster the sample rate, the more information about the original signal is captured and converted. This a common specification you see on data sheets for these types of scopes. It is measured in Samples per Second (S/s). For high-speed samples, you will often see it measured in MS/s (Mega Samples per second) or GS/s (Giga Samples per second).

There is something called Nyquist's Theorem, which states that in order to properly slice up an analog signal (so you have enough information to recreate it back again) you need to have a sample rate at least twice the fastest signal you are looking at. That is of course, a minimum amount. In practice, most scopes are built to sample at least 5 times the highest speed it can capture. So, for example, a 200MHz signal would be best sampled at a rate of at least 1GS/s.

How fast does a scope need to be?

The speed at which a signal goes from 10% of its level (in amplitude) to 90% of its top value is called the rise time. In order to see the maximum amount of each signal edge (vertical) both the scope and the probe must have a fast enough rise time. This is especially true for when the signal changes. Once again, the practical scenario calls for the rise time on the instrument to be five times faster than the signal. If your fastest signal has a rise time of 5 usec (micro second), then you want a scope/probe combo to have a rise time of 1 usec.

How deep should the memory be?

The last major specification to be considered is memory depth or record length. The major benefit of the digital storage oscilloscope is the storage part. This gives you the ability to recall, compare, and perform math functions on a captured signal. The record length is measured in samples or points. The total amount of time you can record for is determined by the number of points available and the sample rate (each sample being a point). You would simply divide the number points by the sample rate to get your acquistion time. If you have a total memory depth of 1 Mpoints and a sample rate of 250 MS/second, then you can record a signal that is 4 msec (millisecond) long.

What other factors should be considered when purchasing a digital storage oscilloscope?

Beyond the basic four specifications, it is common to consider:

• Number of channels (typically two or four). If you need to record multiple high-speed signals beyond four, you might want to look at a dedicated recorder.
• Size of the display is often a consideration. Larger, clearer screens make it easier to see multiple signals at once. Luckily today's digital storage oscilloscope also has different color lines for each signal.
• How you capture a signal is also important. This is where triggers come into play. It is often important to see only signals with specific characteristics among the many captured. With most digital storage oscilloscopes, a variety of different trigger types are available to find particular events that happen during signal analysis.
• If you are looking at packets of serial data, you may also find it useful to decode the signal to make sure that the correct instructions are being sent. Protocols such as I2C, SPI, CAN, LIN, and RS232 are commonly used to communicate between devices. It is important to make sure that the right commands are communicated when a specific event happens.

What is an Oscilloscope?

An Oscilloscope is an instrument that is used as a graph displaying device of an electrical signal. The graph will show how signals change over time. The vertical (Y) axis represents voltage and the horizontal (X) axis represents time. The horizontal sweeps at a constant rate. The (Z) axis, although not that common, can display brightness or intensity of the display. With a proper transducer, an oscilloscope can measure just about anything. A transducer is a device that creates an electrical signal in response to physical stimuli such as, sound, pressure, light, heat, etc.
 

When graphing a signal, what do you want to find out?

• The time and voltage value of a signal
• The frequency of an oscillating signal
• How much of a signal is direct current (DC) or alternating current (AC)
• How much of the signal is noise and if the noise is changing over time
• To see the “moving parts” of a circuit represented by the signal
• To tell if a malfunctioning component is distracting the signal

Oscilloscopes come in many different versions (visit the respective sections of the TEquipment website)

• Analog
• Digital
• Mixed signal
• Portable
• PC based versions

If the recording of a waveform is required, a digital scope will be applicable. If you need to see the waveform in real time, or to see the original intensity an analog scope would better suit that requirement.  The higher the input signal frequency is, the higher the bandwidth that will be required. If you do not have the appropriate amount of bandwidth, you risk the possibility of not getting accurate results.

If there is doubt about the amount of bandwidth that is required, then you should go the next step up. The bandwidth can usually be calculated by this formula: BANDWIDTH = (0.35 / rise time of the signal)

The higher the sampling rate, the more accurate and precise the captured waveform is. As the sampling rate increases, it allows for more samples a captured waveform has, for any given period of time.

In almost every electric application, including lab use, research and development, and product development there is a need for an oscilloscope to provide waveform analysis.

 

Here is the best guide we can recommend on getting to know everything about oscilloscopes.  This is the XYZs of oscilloscopes by Tektronix.  It has 64 pages packed with information.  Click the picture below to download.