eit exp 5 dsoscilloscope

PUNE INSTITUTE OF COMPUTER TECHNOLOGY, PUNE - 411043

Department of Electronics & Telecommunication

CLASS : S.E. E &TC SUBJECT : EIT

EXPT. NO. : 5 DATE :

TITLE : STUDY OF DIGITAL STORAGE OSCILLOSCOPE

OBJECTIVE : 1. To study the front panel controls of digital storage oscilloscope.

2. To study measurement techniques of various functions of DSO & to measure parameters.

APPARATUS : Digital storagel oscilloscope and probes.

THEORY :

Oscilloscope is electronic instrument used to observe, measure, or record transient physical phenomena and present the results in graphic form. The oscilloscope produces a two-dimensional graph with the voltage presented at an input terminal plotted on the vertical axis and time plotted on the horizontal axis.

Usually the graph appears as an illuminated trace on the screen of a cathode-ray tube (CRT) and is used to construct a useful model or representation of how the instantaneous magnitude of some quantity varies during a particular time interval. The "quantity" measured is often a changing voltage in an electronic circuit.

An electric current probe is designed specifically for use with an oscilloscope. General-purpose oscilloscopes are classified as analog oscilloscopes or digital oscilloscopes.

P:F:-LTL-UG/03/R1 EIT5.1



Analog and digital oscilloscope basics

The classic oscilloscope is the analog form, characterized by the use of a CRT as a direct display device. A beam of electrons (the "cathode rays") is formed, accelerated, and focused in an electron gun and strikes a phosphor screen, causing visible light to be emitted from the point of impact.

The voltage transients to be displayed are amplified and applied directly to vertical deflection plates inside the CRT, resulting in an angular deflection of the electron beam in the vertical direction. This amplifier system is conventionally referred to as the "vertical amplifier." The linear vertical deflection of the point at which the electron beam strikes the screen is thus proportional to the instantaneous amplitude of the voltage transient. Another voltage transient, generated inside the oscilloscope and increasing at a uniform rate, is applied directly to the horizontal deflection plates of the CRT, resulting in a simultaneous, uniform, left-to-right horizontal motion of the point at which the electron beam strikes the phosphor screen. The electronic module that generates the signals that sweep the beam horizontally and control the rate and synchronization of those signals is called the "time base." Thus the point on the phosphor screen illuminated by the electron beam moves in response to those voltages, and the glowing phosphor traces out the desired graph of voltage versus time.

The digital oscilloscope has been made practical and useful by recent advances in the state of the art of the digitizing devices called "analog-to-digital converters" (ADC). An ADC is a device, which at suitable regular intervals measures ("samples") the instantaneous value of the voltage at




the oscilloscope input and converts it into a digital value (a number) representing that instantaneous value.

The oscilloscope function of recording a voltage transient is achieved by storing in a digital memory a series of samples taken by the ADC. At a later time, the series of numbers can be retrieved from memory, and the desired graph of volts versus time can be constructed. The graphing or display process, since it is distinct from the recording process, can be performed in several different ways. The display device can be a CRT using direct beam deflection methods (in effect using an analog oscilloscope as the display device in a digital oscilloscope). Alternatively, a "raster scan display," similar to that used in a conventional television receiver or a computer monitor, can be used. Or the samples could be plotted on paper using a printer with graphics capability or a plotter.

The digital oscilloscope is usually configured to resemble the traditional analog instrument in the arrangement and labeling of its controls, the features included in the vertical amplifier, and the labeling and presentation of the display. In addition, the system that controls the sample rate and timing of the data acquisition cycle is configured to emulate the functions of the time base in the analog instrument.

Control panelThe control panel of the oscilloscope contains three important groups:

(1) the display screen and its associated controls, such as focus and intensity adjustments, (2) the vertical amplifier input signal connectors, sensitivity adjustments, and other input signal conditioning controls, and (3) the horizontal or time-base controls, which set the speed and timing of the signal capture.These controls are provided so that the operator can adjust the oscilloscope to frame a precise window in voltage and time to capture and display the desired voltage transient.




The Digital OscilloscopeIn the analog oscilloscope and the operation of various parts of the

oscilloscope that are essentially the same in the analog and digital models.

Block diagramA block diagram of a two-channel digital oscilloscope is shown in Fig.

Signal acquisition is by means of an analog-to-digital converter (ADC) or digitizer, which at uniformly spaced time intervals measures (samples) the instantaneous amplitude of the signal appearing at its input and converts it to a digital value (a number), which in turn is stored in a digital memory. When the trigger condition is satisfied, the sampling process is interrupted, the stored samples are read from the acquisition memory, and the volts versus time waveform is constructed and graphed on the display screen. The time interval between samples ts is called the "sample time," and its reciprocal is called the "sample frequency" fs. The signal that regulates the sampling process in the ADCs is called the "sample clock," and it is generated and controlled by the time-base circuit. A crystal oscillator is used as a reference for the time base to ensure the accuracy of the sample interval and ultimately of the time measurements made using the digital oscilloscope.




Resolution Voltage resolution is determined by the total number of individual

codes that can be produced. A larger number permits a smoother and more accurate reproduction of the input waveform but increases both the cost of the oscilloscope and the difficulty in achieving a high sample frequency. ADCs are usually designed to produce a total code count that is an integer power of 2, and a unit capable of 2n levels of resolution is called an "n-bit digitizer."

Digital oscilloscopes are available in resolutions from 6 up to 12 bits, with the resolution varying generally in an inverse relationship to sample rate. Eight bits is the most frequently used resolution.

Acquisition memoryEach sample code produced must be stored immediately in the

acquisition memory, and so it must be capable of accepting data from the digitizer continuously at the sample frequency.

Acquisition methodsThe digital signal capture is performed in three distinct modes

depending on the intended application. The three acquisition methods are called "real-time sampling," "sequential repetitive sampling," and "random repetitive sampling."

Automatic measurementsEmbedded controllers are beneficial and widely used in analog

oscilloscopes, e.g., in implementing a user interface that is powerful yet easily understood. However, in the digital configuration, the waveform data are accessible to the computer, and this opens up a large array of useful features.

CalibrationThe ability to take measurements and analyze the results

automatically is used to streamline the process of calibration of the digital oscilloscope. Previously, calibration was done by a trained technician who viewed the response to various standard signals and adjusted internal controls to bring the instrument responses into conformance with the manufacturer's specifications, but this is a costly and time-consuming process potentially involving many tens of measurements and adjustments.




The automated process in a digital oscilloscope uses the system-embedded controller to measure and analyze the response to standard signals, and the results are stored in calibration tables in nonvolatile memory.

Auto scale. Automatic measurement is also used to implement "auto scale."

Invoked by pressing a button on the control panel, auto scale causes the oscilloscope to scan all channels for active signals. If any are found, the control settings are automatically adjusted to display a few cycles in the display window. This helps the operator quickly get a picture on screen and is especially useful when the signal has an unknown amplitude, dc offset, or frequency.

MeasurementMeasurement of signal parameters, such as amplitude or period, is

easily done directly from the display screen, but in addition, a number of preprogrammed parametric measurements can be selected which are calculated directly from the sampled data record. In some cases, the automatic computation is merely a convenience for the operator. However, the measurement may be difficult to execute or may need to be repeated many times, and using the computing power built into the oscilloscope can be a real time saver. The following is a representative list of built in measurements: rise time, fall time, frequency, period, positive pulse width, negative pulse width, duty cycle, delta time, peak -to- peak voltage, maximum voltage, minimum voltage, average voltage, and rms voltage.Mathematical operations

Occasionally, it is desirable to do a mathematical operation on the entire sampled data record captured on a single channel, with the result plotted on the display screen. Examples of this type of analysis are inversion, integration, differentiation, or fast Fourier transform. Additionally, it may be of interest to combine data captured on two different channels. If the two records are labeled A and B, then the result of the following operations could be plotted on the display screen as a single waveform: A+B, A-B, A×B, and A versus B.

Data communicationCaptured waveforms can be stored in the embedded controller

memory and later recalled, as can the oscilloscope control settings needed to perform a particular measurement, called a "setup." But there is also a




need to transmit waveforms and setups to and from an external computer or computer peripheral device.

Information can be copied to a flexible disk memory in the oscilloscope for storage or transfer, or it can be transmitted over an I/O bus. RS-232 and IEEE-488 (HPIB or GP-IB) are two bus protocols commonly used for this purpose.

A printer or plotter produces a permanent copy of a waveform when connected to a digital oscilloscope using an I/O bus. Connection to an external computer brings even more power and flexibility. Waveforms and setups can be shared.




Comparing Analog and Digital Oscilloscopes

Type Advantages DisadvantagesAnalog Responsive display

Direct access controls Easily understood Low cost

No negative time No multi-channel

simultaneous capture Dim display (low repetition

rate signals) Camera hard copy No waveform I/O

Real time Digital

Multi-channel simultaneous capture

Negative time Easy single transient

capture Stored display image Easy calibration Printer or plotter hard copy Automatic measurement

support Digital waveform I/O

Low throughput (Waveforms / second)

Possibility of aliased displays

High cost




Operating Basics: Model No. TDS 1000/2000Make: TektronixTDS 1002: Two channel, 60 MHz, 1 GF/S

The figure shows the front panels for 2-channel and 4-channel models.

To use your oscilloscope effectively, it is necessary to learn about the following oscilloscope functions:




_ Setting up the oscilloscope_ Triggering_ Acquiring signals (waveforms)_ Scaling and positioning waveforms_ Measuring waveforms

The figure below shows a block diagram of the various functions of the oscilloscope and their relationship to each other.

Setting Up the OscilloscopeYou should become familiar with three functions that you may use

often when operating your oscilloscope: Autoset, saving a setup, and recalling a setup.

Using AutosetThe Autoset function obtains a stable waveform display for you. It

automatically adjusts the vertical scale, horizontal scale and trigger settings. Autoset also displays several automatic measurements in the graticule area, depending on the signal type.

Saving a Setup




The oscilloscope saves the current setup if you wait five seconds after the last change before you power off the oscilloscope. The oscilloscope recalls this setup the next time you apply power. You can use the SAVE/RECALL Menu to permanently save up to ten different setups.

Recalling a SetupThe oscilloscope can recall the last setup before power off, any of your

saved setups or the default setup.

Default SetupThe oscilloscope is set up for normal operation when it is shipped from

the factory. This is the default setup. To recall this setup, push the DEFAULT SETUP button.

TriggeringThe trigger determines when the oscilloscope starts to acquire

data and display a waveform. When a trigger is set up properly, the oscilloscope converts unstable displays or blank screens into meaningful waveforms.

When you push the RUN/STOP or SINGLE SEQ buttons to start an acquisition, the oscilloscope goes through the following steps:1. Acquires enough data to fill the portion of the waveform record to the left

of the trigger point. This is also called the pre-trigger.2. Continues to acquire data while waiting for the trigger condition to occur.3. Detects the trigger condition.4. Continues to acquire data until the waveform record is full.5. Displays the newly-acquired waveform.

Source:




You can use the Trigger Source options to select the signal that the oscilloscope uses as a trigger. The source can be any signal connected to a channel BNC, to the EXT TRIG BNC or the AC power line (available only with Edge triggers).

Types:The oscilloscope provides three types of triggers: Edge, Video, and

Pulse Width.Modes can select a Trigger Mode to define how the oscilloscope

acquires data when it does not detect a trigger condition. The modes are Auto and Normal.

To perform a single sequence acquisition, push the SINGLE SEQ button. Coupling you can use the Trigger Coupling option to determine which part of the signal will pass to the trigger circuit. This can help you attain a stable display of the waveform.

To use trigger coupling, push the TRIG MENU button, select an Edge or Pulse trigger, and select a Coupling option.

To view the conditioned signal being passed to the trigger circuit, push and hold down the TRIG VIEW button.

Position:The horizontal position control establishes the time between the

trigger and the screen center.

Acquiring SignalsWhen you acquire a signal, the oscilloscope converts it into a digital

form and displays a waveform. The acquisition mode defines how the signal is digitized and the time base setting affects the time span and level of detail in the acquisition.Acquisition Modes

There are three acquisition modes: Sample, Peak Detect, and Average. Sample. In this acquisition mode, the oscilloscope samples the signal in evenly spaced intervals to construct the waveform. This mode accurately represents signals most of the time. However, this mode does not acquire rapid variations in the signal that may occur between samples. This can result in aliasing and may cause narrow pulses to be missed. In these cases, you should use the Peak Detect mode to acquire data.




Peak Detect. In this acquisition mode, the oscilloscope finds the highest and lowest values of the input signal over each sample interval and uses these values to display the waveform. In this way, the oscilloscope can acquire and display narrow pulses, which may have otherwise been missed in Sample mode. Noise will appear to be higher in this mode.Average. In this acquisition mode, the oscilloscope acquires several waveforms, averages them, and displays the resulting waveform. You can use this mode to reduce random noise.

Time Base:The oscilloscope digitizes waveforms by acquiring the value of an input signal at discrete points. The time base allows you to control how often the values are digitized. To adjust the time base to a horizontal scale that suits your purpose, use the SEC/DIV knob.

Scaling and Positioning WaveformsYou can change the display of waveforms by adjusting their scale and position (Vertical and horizontal). When you change the scale, the waveform display will increase or decrease in size. When you change the position, the waveform will move up, down, right, or left. The channel reference indicator (located on the left of the graticule) identifies each waveform on the display. The indicator points to the ground level of the waveform record.

Taking MeasurementsThe oscilloscope displays graphs of voltage versus time and can help

you to measure the displayed waveform. There are several ways to take measurements. You can use the graticule, the cursors, or an automated measurement.

Graticule. This method allows you to make a quick, visual estimate. For

example, you might look at waveform amplitude and determine that it is a little more than 100 mV. You can take simple measurements by counting the major and minor graticule divisions involved and multiplying by the scale factor.




Cursors:This method allows you to take measurements by moving the cursors,

which always appear in pairs, and reading their numeric values from the display readouts. There are two types of cursors: Voltage and Time. To use cursors, push the CURSOR button.

Voltage Cursors: Voltage cursors appear as horizontal lines on the display and measure the vertical parameters.

Time Cursors: Time cursors appear as vertical lines on the display and measure the horizontal parameters.

Automatic: The MEASURE Menu can take up to five automatic measurements. When you take automatic measurements, the oscilloscope does all the calculating for you. Because the measurements use the waveform record points, they are more accurate than the graticule or cursor measurements.

OBSERVATION:1) Analog and Digital Storage Oscilloscope Difference.




2)Parameter measurement

Voltage applied

Observed voltage

Frequency applied

Frequencyobserved

3)Phase Measurements using XY mode:




4) Measure Rise time and Fall time:

Sr. No. Rise time Fall time

5) Measurement of Propagation delay:

Sr. No. Propagation Delay in data

sheet

Propagation Delay Measured

TTL

CMOS

References:1. Electronic Instrument Handbook - Coombs 3rd Edition2. DSO User Manual TDS1000-and TDS2000-Series Tektronix.


eit exp 5 dsoscilloscope

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