Oscilloscopes

Objectives –After completing this chapter, the student should be able to: Explain the function of an oscilloscope. Identify the basic parts of an oscilloscope and explain their functions. Demonstrate the proper setup of an oscilloscope. Describe how to use an oscilloscope to make a measurement.

Oscilloscopes –Most versatile piece of test equipment available for working on electronic equipment and circuits. –Provides a visual display of what is occurring in the circuit.

Oscilloscopes provide: –The frequency of a signal. –The duration of a signal. –The phase relationship between signal waveforms. –The shape of a signal’s waveform. –The amplitude of a signal.

The basic parts of an oscilloscope are: –A cathode ray tube (CRT). –A sweep generator. –Horizontal and vertical deflection amplifiers. –Power supplies.

Cathode-ray tube (CRT) –A phosphor screen. –Deflection plates. –An electron gun.

Faceplate –Marked in centimeters along the vertical and horizontal axis. –Can be calibrated with a known voltage before testing an unknown signal. –Called a graticule, and is separate from the oscilloscope. –Mounted in front of the CRT.

Power switch –Usually on the front panel. –May be a toggle, push-button or rotary switch. –Mounted separately or with another switch. –Used to apply line voltage to operate the oscilloscope.

Intensity switch –Also called brightness. –Controls the electron beam within the CRT. –It is a rotary control. –Too much intensity for too long can burn a hole or etch a line in the phosphor screen.

Focus and astigmatism controls –Connected to the electron gun. –Used to adjust the electron beam size and shape. –Rotary controls.

Horizontal and vertical position controls –Rotary controls. –Allows the electron beam to be positioned anywhere on the face of the CRT.

Horizontal block –Consists of: A vertical input jack. An AC/DC switch. A volts/cm rotary switch. –The oscilloscope probe is connected to the input jack. –The probe is then connected to the circuit to be tested.

Horizontal block –Also called the time base. –Consists of: A time/cm rotary switch. A trigger-control switch. A triggering level control.

Level control –Sets the amplitude that the triggering signal must exceed before the sweep generator starts. Initial oscilloscope control settings: –Intensity: set to the center of range. –Focus: set to the center of range. –Astigmatism: set to the center of range. –Position: set to the center of range.

–Triggering: INT + –Level: AUTO –Tine/cm: 1 msec –Volts/cm: 0.02 –Power: ON

In Summary –An oscilloscope provides: Frequency of the signal. Duration of the signal. Phase relationships between signal waveforms. Shape of the signal’s waveform. Amplitude of the signal

–The basic parts of an oscilloscope are: Cathode-ray tube (CRT). Sweep generator. Horizontal deflection amplifier. Vertical deflection amplifier. Power supply.

19 Introduction  The Oscilloscope is nothing more than just an instrument for displaying electrical signals in the TIME domain. Meaning that, you can view waveforms on it.  Some of the uses of the scope may not be obvious but if you already possessed one, then you will most probably regard it as the most useful tool on your workbench.  So, what is actually an oscilloscope?

20 Basic Oscilloscope  A basic Oscilloscope is as shown below.

21 The basic controls are:  Bright – To adjust the intensity of display.  Focus – To adjust the focusing of display.  Grat (Graticule) – To illuminate a scale used to measure a trace.  Trigger – To select a trigger source.  Trace – To select which trace is to be displayed. Basic Oscilloscope

22  Timebase – To select the speed which the trace moves across the tube face.  Input Level- To adjust the input level.  Pos (Position) – To set the position of the trace on the display. Basic Oscilloscope

23 Brightness  This is self-explanatory, it controls the intensity of the display. It is well worth remembering that one cannot execute screen-saver programs on an oscilloscope, so leaving the scope with a high brilliance will certainly burn the trace into the tube face.  If you must leave your scope switched ON for any length of time then turn the brilliance control down.

24 Focus  The focus control is also quite self-explanatory but most "scopes" seem to adjust a single spot from a short horizontal to a vertical line.  A single horizontal line is therefore not the best waveform to adjust the focusing, this is best done whilst viewing a waveform.

25 Trace  Selects which trace is to be displayed.  There are usually two or more selections possible: A - Selects trace A only (single channel). B - Selects trace B only (single channel). A+B - Selects both trace A and trace B (dual channel). ADD - Both channel inputs are added and displayed as a single trace.

26 Timebase  The speed the spot travels across the screen of the tube may be varied by means of the Timebase selector switch.  This is calibrated in Seconds (S), MilliSeconds (mS = seconds) or MicroSeconds (uS = seconds).

27 Conclusion  We have learned at brief about the control functions of a basic oscilloscope.  Various control buttons on the oscilloscope serve their own purpose in control.