Digital Storage Oscilloscope Working Principle

Keywords: working principle of oscilloscope

Oscilloscopes are used to measure electric signals to display it as waveforms. Oscilloscope is known as scope and its most common usage is to observe signal. It was invented by Karl Braun in 1897. There are several number oscilloscopes in the market such as digital, analogues, dual beam, mixed signal etc but the main focus on the report is digital storage oscilloscope. A digital oscilloscope is a measuring instrument that uses binary numbers which writes to samples of the voltage.  Digital oscilloscope is affected by bandwidth e.g. as the frequency is increasing the oscilloscope accuracy decreases. Its also affected by sample rate, e.g. The quicker it can sample, the accurate the results are displayed for fast signal. The digital storage oscilloscope (DSO) is of the three digital oscilloscopes but DSO is the conventional form of digital oscilloscope. Its screen is like a computer monitor or TV screen as it uses raster type screen. The operation of the digital storage oscilloscope is pretty simple, “The first stage the signal enters within the scope is the vertical amplifier where some analogue signal conditioning is undertaken to scale and position the waveform. Next this signal is applied to an analogue to digital converter (ADC).”(www.Radio-electronics.com). The DSO is easy to set up. The digital storage oscilloscope has many controls, e.g. Switch, TIME / DIV, Trigger controls, Intensity and focus, CH I and CH II inputs, VOLTS / DIV etc. The DSO in comparison with Digital Phosphor Oscilloscope has less bandwidth, less sample rate but same channels etc.

Aims:

  • To understand the basics about digital storage oscilloscope
  • To be able to operate an digital storage oscilloscope
  • To be able to set-up an oscilloscope
  • To be able understand  the designs of digital storage oscilloscope
  • To be able to understand advantages and disadvantages of different types of oscilloscope

Introduction:

Oscilloscopes are measuring equipment which displays electric waveforms on a screen like a small Television.  An oscilloscope is known as CRO, DSO, scope or an O-scope. Its common usage is to observe wave shapes of signal. There are four sections in an oscilloscope: the vertical and horizontal controls trigger controls and finally the display screen (the screen consists of cathode ray tube).  The research on oscilloscope has been done through books, internet, magazines etc. “Karl Braun was the inventor of oscilloscope in 1897” (http://inventors.about.com/od/bstartinventors/a/Karl_Braun.htm). The main sector oscilloscopes used in are engineering, medicine, telecommunications and science. “In October 2010 Tektronix Inc manufacturer of oscilloscopes discovered that Series of digital and mixed signal oscilloscopes that now deliver 100 GS/s sampling rate performance. This enables lower noise along with increased data points on 5x oversampled 20 GHz acquisitions” (http://www.prlog.org/11013155-tektronix-raises-bar-for-oscilloscope-sampling-rates-signal-integrity.html) There are several numbers of oscilloscope e.g. digital, analogues, dual beam, mixed signal etc.  This reports main focus will be the design and operating principle of digital storage oscilloscope.  Firstly the report will look into how the oscilloscope works, and then it will follow on to how it’s designed and finally how to set it up.

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A digital oscilloscope is a measuring instrument that uses binary numbers which writes to samples of the voltage.  ADC (analogue to digital converter) is used to change the analogue data into digital data then it makes the signal digitally.  For it to be displayed in the screen it’s then converted back to analogue. A digital oscilloscope has its limitation to performing just like an analogue oscilloscope.  There is a limit to the frequency which it can work up to. The limits of frequency are affected by analog bandwidth front-end section which is known as -3 dB point and sampling rate of the oscilloscope, the samples is taken in regular breaks.  When the sample rate is high, the frequency increases on screen.  

Factors affecting the frequency of digital oscilloscope:

  • Bandwidth specification
  • Oscilloscope sample rate

Bandwidth specification

The bandwidth specification determines the frequency range which the scope (oscilloscope) measures accurately in the display. As the frequency is increasing the oscilloscope accuracy decreases. The bandwidth is mainly defined as a drop of 3 decibels (dB) or sensitivity at lower frequency at 0.707. Bandwidth in Hz x rise time in seconds = 0.35. E.g. to resolve an oscilloscope pulses with the rise of 2 nanosecond would have a bandwidth of 700MHz. but for a digital oscilloscope the sampling rate would have to be ten times higher frequency to resolve. E.g. 10megasample/second would measure up to 1 megahertz of signals.

Oscilloscope sample Rate

The oscilloscope sampling rate indicates on digital oscilloscopes how many samples per second the analog to digital converter can gain. The quicker it can sample, the accurate the results are displayed for fast signal. The maximum sample rate is given by MS/s which is mega samples per second. The minimum sample rate might come in handy if you need to look at signals changing slowly. The sampling rate can be change by the controls (sec/div) on the oscilloscope.

Digital storage oscilloscope

The digital storage oscilloscope is of the three digital oscilloscopes but DSO is the conventional form of digital oscilloscope. Its screen is like a computer monitor or TV screen as it uses raster type screen. By using the raster screen its helps to display images that fill the whole screen and it may include text on the screen. (Ian P (2004) oscilloscope types [internet]. Available fromhttp://www.radio-electronics.com/info/t_and_m/oscilloscope/oscilloscope_types.php [date accessed 18/10/10]). First you have to store the waveform in the digital format to get the raster type display on screen. As a result of storing the waveform form digitally it can be processed by the oscilloscope or by connecting to a computer. “This enables a high degree of processing to be achieved, and the required display provided very easily and often with a very cheap processing platform. It also enables the waveform to be retained indefinitely, unlike the analogue scopes for which the waveform could only be stored for a very limited time.” (www.Radio-electronics.com).

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The operation of the digital storage oscilloscope is pretty simple, “The first stage the signal enters within the scope is the vertical amplifier where some analogue signal conditioning is undertaken to scale and position the waveform. Next this signal is applied to an analogue to digital converter (ADC).” (www.Radio-electronics.com). The samples are taken at regular intervals. The sampling rate is important because it determines the resolution of the signal. The samples are taken in per second or MS/s (mega sample rate). All the samples are stored within is the oscilloscope as waveform points, and several samples of waveform make up a single waveform point. “The overall waveform is stored as a waveform record and its start is governed by the trigger, its finish being determined by the horizontal time base time.” (www.Radio-electronics.com).

The digital storage oscilloscope is an in the digital format which means there is a signal processor. With having a signal processor it helps to process the signal in different ways, before it passes the display memory and the display.

Digital storage oscilloscope

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Controls of digital storage oscilloscope

Screen – this is where all the waveform signals are displayed by using the X axis and Y axis.

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Switch – to turn ON the oscilloscope and to turn OFF.

X-Y control – it’s used to display component characteristic curves. It’s mainly used for output position.

TV-separation – it allows the display to be shown on TV system so it can compare signals different points.

TIME / DIV – by using this control V/t graph horizontal scale can be changed.

Trigger controls – it allows the oscilloscope display to be coordinated with the signal you want to look into.

Intensity and focus – this control is used to change the brightness of the scope by adjusting it.

X-POS – by using this control the whole V/t graph can be moved side to side.

X-MAG – horizontal scale of the V/t graph is increased by 10 times in the IN position.

CAL outputs -the top terminal gives square wave at 0.2 V peak to peak but the lower terminal gives square wave of 2 V peak to peak at 50 Hz.

Component tester – A changing voltage is provided by the output socket to allow component characteristic curves to be displayed.

Y-POS I and Y-POS II – this control allows the resultant outcome to move up or down

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Invert – when its invert the waveform signal on the screen is turned upside down.

CH I and CH II inputs – using the BNC plugs the signals are connected to the BNC input socket. The smaller socket is called earth or ground.

VOLTS / DIV – independently the vertical scales for CH I and CH II can be adjusted.

DC/AC/GND slide switches – In the DC position, the signal input is connected directly to the Y-amplifier of the corresponding channel, CH I or CH II. In the AC position, a capacitor is connected into the signal pathway so that DC voltages are blocked and only changing AC signals are displayed (www.doctronics.co.uk)

Trace selection switches – settings of control switches for oscilloscope screen.

How to set up an oscilloscope

First you warm up the oscilloscope by switching it on. At this stage do not connect any input leads.

Select the DC/AC/GND switch to DC (the Y-input)

Select the X-Y/SWP to SWP (sweep)

Put the trigger level to AUTO

Select the trigger source to INT (the Y input, internal)

Put 5V/cm at the Y-AMPLIFIER

Put the TIMEBASE to 10ms/cm

Select 1 or Cal for the time base VRIABLE control.

to map out the middle of the screen adjust the Y-SHIFT (up/down) and X SHIFT (left/right)

to brighten up the focus adjust the INTENSITY and FOCUS

now the oscilloscope is ready to go

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Digital storage oscilloscope

Digital Phosphor Oscilloscope

100 MHz & 200 MHz bandwidth

500 MHz, 300 MHz, & 100 MHz bandwidth

2 GS/s Real time sample rate

5 GS/s sample rate

2 or 4 Channels

2 or 4 Channels

It rapidly documents and examines Measurement. Its results with OpenChoice® Software or Integrated CompactFlash® Mass Storage

21 Automatic Measurements

FFT Standard on All Models

FFT for Frequency and Harmonic Analysis

Advanced Triggers to Quickly Capture the Event of Interest

Advanced Triggers, such as, Glitch, Width, and Logic

Full VGA Color LCD on all Models

Multi-Language User Interface

Built-in Floppy Disk Drive for Easy Storage and Documentation

Quick Menu User Interface Mode for Quick, Easy Operation

Traditional, Analog-style Knobs and Multilanguage User Interface for Easy Operation

9-bit Vertical Resolution

Quick Setup and Operation with Auto set Menu, Auto range, Waveform and Setup Memories, and Built-in, Context-sensitive Help

Telecommunications Mask Testing (TMT)

Backlit Menu Buttons for High Visibility

Extended Video Application Module

11 of the Most Critical Automatic Waveform Measurements

Support for Active Probes, Differential Probes, and Current Probes that Provide Automatic Scaling and Units

8 Hours of Continuous Battery Operation with Two Batteries Installed, Hot Swappable for Virtually Unlimited Freedom from AC Line Power

Optional Power Application Software offers the Broadest Range of Power Measurements at its Price Point

Digital Phosphor Oscilloscope

http://infrared.als.lbl.gov/content/PDF/equipment/Tektronix_TDS3052_Data_Sheet.pdf

Digital storage oscilloscope

http://www2.tek.com/cmswpt/psdetails.lotr?ct=ps&cs=psu&ci=13304&lc=EN

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