Oscilloscope Tutorial

Transcription

1 Oscilloscope Tutorial The oscilloscope is basically a graphdisplaying device It draws a graph of an electrical signal. In most applications the graph shows how signals change over time: the vertical (Y) axis represents voltage the horizontal (X) axis represents time.

2 Oscilloscopes Horizontal sweeps at a constant rate. Vertical plates are attached to an external voltage, the signal you attach to the scope.

3 Cathode Ray Tubes Variation in potential difference (voltage) placed on plates causes electron beam to bend different amounts. Sweep refers to refreshing repeatedly at a fixed rate.

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5 Scope (Con t) This simple graph can tell you many things about a signal: You can determine the time and voltage values of a signal. You can calculate the frequency of an oscillating signal. You can see the "moving parts" of a circuit represented by the signal. You can tell if a malfunctioning component is distorting the signal. You can find out how much of a signal is direct current (DC) or alternating current (AC). You can tell how much of the signal is noise and whether the noise is changing with time.

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7 How does an Analog Scope work?

8 How does a Digital Scope work?

9 Triggering Stabilizes a Repeating Waveform

10 Waveform shapes tell you a great deal about a signal

11 If a signal repeats, it has a frequency. The frequency is measured in Hertz (Hz) and equals the number of times the signal repeats itself in one second

12 Voltage, Current, & Phase

13 Performance Terms Bandwidth The bandwidth specification tells you the frequency range the oscilloscope accurately measures. Rise Time Rise time may be a more appropriate performance consideration when you expect to measure pulses and steps. An oscilloscope cannot accurately display pulses with rise times faster than the specified rise time of the oscilloscope. Vertical Sensitivity The vertical sensitivity indicates how much the vertical amplifier can amplify a weak signal. Vertical sensitivity is usually given in millivolts (mv) per division. Sweep Speed For analog oscilloscopes, this specification indicates how fast the trace can sweep across the screen, allowing you to see fine details. The fastest sweep speed of an oscilloscope is usually given in nanoseconds/div. Gain Accuracy The gain accuracy indicates how accurately the vertical system attenuates or amplifies a signal. Time Base or Horizontal Accuracy The time base or horizontal accuracy indicates how accurately the horizontal system displays the timing of a signal. Sample Rate On digital oscilloscopes, the sampling rate indicates how many samples per second the ADC can acquire. Maximum sample rates are usually given in megasamples per second (MS/s). The faster the oscilloscope can sample, the more accurately it can represent fine details in a fast signal.. ADC Resolution (Or Vertical Resolution) The resolution, in bits, of the ADC indicates how precisely it can turn input voltages into digital values. Record Length The record length of a digital oscilloscope indicates how many waveform points the oscilloscope is able to acquire for one waveform record.

14 Grounding Proper grounding is an important step when setting up to take measurements. Properly grounding the oscilloscope protects you from a hazardous shock and protects your circuits from damage. Grounding the oscilloscope is necessary for safety. If a high voltage contacts the case of an ungrounded oscilloscope, any part of the case, including knobs that appear insulated, it can give you a shock. However, with a properly grounded oscilloscope, the current travels through the grounding path to earth ground rather than through you to earth ground. To ground the oscilloscope means to connect it to an electrically neutral reference point (such as earth ground). Ground your oscilloscope by plugging its three-pronged power cord into an outlet grounded to earth ground. Grounding is also necessary for taking accurate measurements with your oscilloscope. The oscilloscope needs to share the same ground as any circuits you are testing. Some oscilloscopes do not require the separate connection to earth ground. These oscilloscopes have insulated cases and controls, which keeps any possible shock hazard away from the user.

15 Scope Probes Most passive probes have some degree of attenuation factor, such as 10X, 100X, and so on. By convention, attenuation factors, such as for the 10X attenuator probe, have the X after the factor. In contrast, magnification factors like X10 have the X first

16 Vertical Controls Position and Volts per Division The vertical position control lets you move the waveform up or down to exactly where you want it on the screen. The volts per division (usually written volts/div) setting varies the size of the waveform on the screen. A good general purpose oscilloscope can accurately display signal levels from about 4 millivolts to 40 volts. Often the volts/div scale has either a variable gain or a fine gain control for scaling a displayed signal to a certain number of divisions.

17 Input Coupling Coupling means the method used to connect an electrical signal from one circuit to another.

18 Horizontal Controls Position and Seconds per Division The horizontal position control moves the waveform from left and right to exactly where you want it on the screen. The seconds per division (usually written as sec/div) setting lets you select the rate at which the waveform is drawn across the screen (also known as the time base setting or sweep speed). This setting is a scale factor. For example, if the setting is 1 ms, each horizontal division represents 1 ms and the total screen width represents 10 ms (ten divisions). Changing the sec/div setting lets you look at longer or shorter time intervals of the input signal.

19 Trigger Position The trigger position control may be located in the horizontal control section of your oscilloscope. It actually represents "the horizontal position of the trigger in the waveform record." Horizontal trigger position control is only available on digital oscilloscopes. Varying the horizontal trigger position allows you to capture what a signal did before a trigger event (called pretrigger viewing). Digital oscilloscopes can provide pretrigger viewing because they constantly process the input signal whether a trigger has been received or not. A steady stream of data flows through the oscilloscope; the trigger merely tells the oscilloscope to save the present data in memory. I n contrast, analog oscilloscopes only display the signal after receiving the trigger.

20 Trigger Controls (con t)

21 Pulse and Rise Time Measurements

22 Multimeter tutorial A meter is a measuring instrument. An ammeter measures current, a voltmeter measures the potential difference (voltage) between two points, and an ohmmeter measures resistance. A multimeter combines these functions, and possibly some additional ones as well, into a single instrument.

23 To measure current, the circuit must be broken to allow the ammeter to be connected in series Ammeters must have a LOW resistance

24 To measure potential difference (voltage), the circuit is not changed: the voltmeter is connected in parallel Voltmeters must have a HIGH resistance

25 To measure resistance, the component must be removed from the circuit altogether Ohmmeters work by passing a current through the component being tested

26 Digital Multimeters Digital meters give an output in numbers, usually on a liquid crystal display. Most modern multimeters are digital and traditional analogue types are destined to become obsolete. Digital multimeters come in a wide range of sizes and capability. Everything from simple 3 ½ digit auto ranging pocket meters to larger 8 ½ digit bench model with operator or computer (IEEE488 compatible) settable range selection

27 Function Generator An electronic instrument that generates various waveforms such as Sine wave Square wave Pulse trains Sawtooth The amplitude, DC offset, frequency are adjustable.

28 Function Generators (con t) Like multimeters there is a wide variety of device offering various Amplitude characteristics Bandwidth Adjustments of rise and fall times Modulation capability (AM, FM, Pulse, etc.)

29 Power Supply This is the device that transfers electric power from a source to a load using electronic circuits. Typical application of power supplies is to convert utility's AC input power to a regulated voltage(s) required for electronic equipment. Depending on the mode of operation of power semiconductors PS can be linear or switching. In a switched-mode power supply, or SMPS power handling electronic components are continuously switching on and off with high frequency in order to provide the transfer of electric energy. By varying duty cycle, frequency or a phase of these transitions an output parameter (such as output voltage) is controlled. Typical frequency range of SMPS is from 20 khz to several MHz.

30 Power Supply (con t) Power supplies like many of the other electronic instruments, come in many varieties with a wide range of capabilities: Parameters that are Power Supply specific include: Voltage levels Current Regulation Protection Output impedance Noise (ripple) It s the designer (or researcher) responsibility to identify the characteristics required.

31 Oscilloscope

32 Oscilloscope(continue) DEMO.Lab3a