 Oscilloscopes provide a graphical view of the waveform of a circuit and are an essential part of the design, repair, and maintenance of electronic equipment. Compared to digital multimeters, they are complicated devices with a wide range of specifications, which make it more difficult to choose the right scope for your application at the right price. Oscilloscope manufactures can add to this complication by promoting a scope's best specifications while leaving the rest for you to find in the product literature. That is why it is important to know what the important specifications are for an oscilloscope and to what extent the scope suits your application. This article will describe the types of oscilloscopes and some of the major elements to their specifications.
Oscilloscope Types
While analog devices make use of continually varying voltages, digital devices take several samples of waveforms. The samples are stored, accumulating until enough are taken in order to describe the waveform, which are then reassembled for display. Digital technology allows the information to be displayed with brightness, clarity, and stability. There are, however, limitations as with the performance of any oscilloscope. The highest frequency at which the oscilloscope can operate is determined by the analog bandwidth of the front-end components of the instrument and the sampling rate. Digital oscilloscopes can be classified into three primary categories: digital storage oscilloscopes, digital phosphor oscilloscopes, and digital sampling oscilloscopes. [1]
- Analogue oscilloscope: The analogue oscilloscope is the traditional form of oscilloscope that has been used in laboratories for many years. It relies on analogue techniques and takes in the vertical and sometimes horizontal signals, amplifying them in an analogue format and displaying them on a cathode ray tube.
- Digital storage oscilloscope (DSO): The digital storage oscilloscope (DSO) is the conventional form of digital oscilloscope. It uses a raster type screen like that used on a computer monitor or television and in this way displays an image that fills the screen and may include other elements in addition to the waveform. These additional items may include text on the screen and the like.
- Digital phosphor oscilloscope (DPO): The digital phosphor oscilloscope (DPO) is a highly versatile form of oscilloscope that uses a parallel processing architecture to enable it to capture and display signals under circumstances that may not be possible using a standard DSO. The key element of a DPO is that it uses a dedicated processor to acquire waveform images. In this way it is possible to capture transient events that occur in digital systems more easily. These may include spurious pulses, glitches and transition errors. It also emulates the display attributes of an analogue oscilloscope, displaying the signal in three dimensions: time, amplitude and the distribution of amplitude over time, all in real time.
- Sampling oscilloscope: These oscilloscopes are used for analyzing very high frequency signals. They are used for looking at repetitive signals which are higher than the sample rate of the scope. They collect the samples by assembling samples from several successive waveforms, and by assembling them during the processing, they are able to build up a picture of the waveform. The oscilloscope specifications for these items may detail a frequency capability or bandwidth sometimes as high as 50 GHz. However these scopes are very expensive. [2]
Bandwidth
One of the first features to consider after the type of oscilloscope is the bandwidth, which is the range of frequencies an oscilloscope can usefully display. “The way in which this is specified can be seen in IEEE 1057 which defines electrical bandwidth as the point at which the amplitude of a sine wave input is reduced by 3 dB (i.e. attenuated to 70.7% of the true value of the signal - a fall of approximately 30%) relative to its level at a lower reference frequency.” [2] Some scopes have an even narrower tolerance range than that. “For a digital oscilloscope, a rule of thumb is that the continuous sampling rate should be ten times the highest frequency desired to resolve; for example a 20 megasample/second rate would be applicable for measuring signals up to about 2 megahertz.” [1]
Sampling
The faster the oscilloscope samples the waveform, the greater the resolution of the detail on the waveform and with greater sample rates the less the likelihood that any critical information will be lost. Most scopes have two different sampling rates (modes) depending on the signal being measured: real-time and equivalent-time sampling (ETS) - often called repetitive sampling. However, ETS only works if the signal you are measuring is stable and repetitive, since this mode works by building up the waveform from successive acquisitions. For example, a scope that has a 50 MS/s real-time sampling rate will not show a square wave as well as a ETS sampling rate of 5 GS/s, but if the wave form is not repetitive, the ETS sampling will not be reliable at all. Some scopes have different sampling rates depending on the number of channels in use, so it is important to check the specifications carefully.
Memory Depth
DSOs capture samples and store them in memory, so the relationship between sampling rate and memory is important. To make sense of the relationship between bandwidth, sampling rate and memory depth, it makes sense to consider a real world example. Consider trying to capture one frame of USB (1.1) data. A frame of data lasts 1 ms and has serial data transmitted at 12 Mbps. To simplify our analysis, we can assume that we have to capture a 12 MHz square wave for 1 ms.
- Bandwidth - to measure the 12 MHz signal, we need an absolute minimum of 12 MHz, however this will give a very distorted signal so a scope with at least 50 MHz bandwidth would be sensible.
- Sampling rate - to reconstruct the 12 MHz signal, we need around 5 points per waveform, so a minimum sampling rate of 60 MS/s is required.
- Memory depth - to capture data at 60 MS/s for 1 ms requires a minimum memory depth of 60,000 samples.
For applications such as audio, noise, vibration and monitoring sensors (temperature, current, pressure) an 8 bit oscilloscope is often not suitable and you should consider 12 or 16 bit alternatives. [3]
Multimeter or Oscilloscope
 “It’s a common adage that the difference between a digital multimeter (DMM) and a digital storage oscilloscope (oscilloscope) is like the difference between numbers and pictures.” [4] Oscilloscopes are designed for engineering work or for troubleshooting systems that might contain complex signals. They make it easy to identify the waveform of the voltage; whether it’s a sine wave, a square wave, sawtooth wave, and the like. Scopes have much faster measurement engines and much wider measurement bandwidths than DMMs. Although scopes can display complex signals, they typically do not have the accuracy and resolution of a high-accuracy multimeter. Besides the ability of a DMM to measure voltages, it can also measure currents, resistances, and some can even check if diodes and transistors work. The bottom line is that both are essential for an electronics enthusiast.
- http://en.wikipedia.org/wiki/Oscilloscope_types
- http://www.radio-electronics.com/info/t_and_m/oscilloscope/oscilloscope-specs-specifications.php
- http://www.picotech.com/applications/oscilloscope_tutorial.html
- http://www.fluke.com/fluke/caen/community/fluke-news-plus/ArticleCategories/Electronics-News/Tech-Tips.htm
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