CATHODE RAY OSCILLOSCOPE

CATHODE RAY OSCILLOSCOPE

� Popular instrument to show time, voltage

both DC and AC. Shows Volts / Time.

� Display waveforms. Spectrum scope shows volts to Frequency

� Cathode (-ve ) is heated, emits electrons,

accelerated toward a (+ve) fluorescent accelerated toward a (+ve) fluorescent

screen. Intensity grid, Focus grid, Accelerating anode. (Electron gun)

� Horizontal deflection plates.

� Vertical deflection plates

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OSCILLOSCOPE: CATHODE-RAY TUBES (CRT)


� Electron gun


� Deflection system


� Fluorescent screen


� Electron gun


� Electron gun


� Electron gun


� Electron gun


� Electron gun


� Deflection system

VERTICAL AND

HORIZONTAL

DEFLECTION PLATES

OSCILLOSCOPE: CONTROL PANEL

PanelPanelPanelPanel FunctionFunctionFunctionFunction

VOLTS/DIV Select the vertical deflection factor of the CRT display

TIME/DIV Select the horizontal deflection factor of the CRT display

INTEN (Intensity) Control

The INTEN (intensity) control (sometimes called BRIGHTNESS) adjusts the brightness of the beam on the CRT. The control is rotated in a clockwise direction to increase the intensity of the beam and should be adjusted to a minimum brightness level that is comfortable for viewing.

FOCUS and ASTIG (Astigmatism) Controls

The FOCUS control adjusts the beam size. The ASTIG (astigmatism) control adjusts the beam shape. The FOCUS and ASTIG controls are adjusted together to produce a small, clearly defined circular dotto produce a small, clearly defined circular dot

TRACE ROTATION Control

The TRACE ROTATION control allows for minor adjustments of the horizontal portion of the trace so that you can align it with the horizontal lines on the graticule.

BEAM FINDER Control When pushed, the BEAM FINDER pulls the beam onto the screen so that you can use the horizontal and vertical POSITION controls to center the spot.

Horizontal and Vertical POSITION Controls

The horizontal and vertical POSITION controls are used to position the trace. Because the graticule is often drawn to represent a graph, some oscilloscopes have the positioning controls labeled to correspond to the X and Y axes of the graph. The X axis represents horizontal movement; the Y axis represents the vertical movement.

AC-GDN-DC AC- Ac SignalGDN - grounding the signalDC- Dc Signal

FREQUENCY AND PERIOD.

� Frequency: measured in Hertz

� Definition: number of times the signal

repeats itself in one second

FREQUENCY AND PERIOD.

� Period

� Definition: the amount of time it takes the

signal to complete one cycle.

PHASE

� 1 cycle of a sine wave: 360⁰

�

Phase shift 90⁰Phase shift 90⁰

WAVEFORM MEASUREMENT (EXAMPLE)

� Given:

volt/div X: 50mV/div, Y: 200mV/div

time/div 1ms/div


� Peak to peak voltage :

Vpp = (volt/div) (no div peak to peak)

Vpp(X) = (50mV/div) ( 8 div ) = 0.4 V

Vpp(Y) = (200mV/div) ( 4 div ) = 0.8V


� Voltage peak: VVp X 2.02

4.0)( ==

VVp Y 4.02

8.0)( ==2

VppVp =


� Time

T = (Time/div) ( no div in 1 cycle)

T(X) = (1 ms/div) ( 5 div) = 5ms

T(Y) = (1 ms/div) ( 5 div ) = 5ms



CATHODE RAY OSCILLOSCOPE

� When electrons hit the screen the phosphor is excited

and emits light.

� Persistence. How long the display glows.� May need to reduce ambient light for older instruments.

� Connect a signal to Vertical deflection � Connect a signal to Vertical deflection

plate.

� At same time a voltage that increases

linearly with time (Ramp) is applied to the

Horizontal deflection plates.

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� This horizontal linear deflection is

produced by the Sweep generator.

� Sawtooth wave.

�

� When the sweep signal returns to zero ie

the end of the sweep, the beam flies back

to the start position. The beam is cut off

during the flyback time.

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CROS

� The display is made to appear stationary.

� This controlled by your adjustment

settings.

� The eye sees a waveform.

� X is <----> Horizontal

� Y is ^ Vertical Height of trace

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� The signal is amplified by the vertical amplifier, applied to the vertical plates.

� A portion of the vertical amp signal is applied to the Sweep Trigger.

� The sweep trigger generates a pulse coincident with a selected point in the cycle of the trigger signal.of the trigger signal.

� This pulse turns on the sweep generator initiating the sawtooth wave form.

� The sawtooth wave is amplified by the horizontal amp and applied to the horizontal deflection plates

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� The trigger can be based on 50 (60) Hz

� Provision is made for an external trigger.

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CRO TUBE CONTROLS

� POWER POWER POWER POWER on / off

� Scale

� Illumination

� FocusFocusFocusFocus. Create spot on screen

� IntensityIntensityIntensityIntensity. Brightness (Don’t burn a spot

on your screen)

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VERTICAL AMP

� PositionPositionPositionPosition on display

� SensitivitySensitivitySensitivitySensitivity of vertical amp Calibrated. Cal fully clockwise.

� VariableVariableVariableVariable sensitivity. Continuous range between calibrated steps.calibrated steps.

� AC AC AC AC ---- DC DC DC DC ---- GndGndGndGnd.� Selects desired coupling for incoming signal, or grounds

amp input. DC couples signal directly to amp. AC connects via a capacitor. (Blocks DC)

� Gnd = no signal. Gnd connects Y input to 0 volts. Checks

position of 0v on screen.

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VERTICAL AMP

� PositionPositionPositionPosition on display

� SensitivitySensitivitySensitivitySensitivity of vertical amp Calibrated. Cal fully clockwise.

� VariableVariableVariableVariable sensitivity. Continuous range between calibrated steps.calibrated steps.

� AC AC AC AC ---- DC DC DC DC ---- GndGndGndGnd.� Selects desired coupling for incoming signal, or grounds

amp input. DC couples signal directly to amp. AC connects via a capacitor. (Blocks DC)

� Gnd = no signal. Gnd connects Y input to 0 volts. Checks

position of 0v on screen.

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HORIZONTAL SWEEP

� Sweep time / Div (or CM)Sweep time / Div (or CM)Sweep time / Div (or CM)Sweep time / Div (or CM) Select desired

sweep rate, or admits external sig to horiz amp.

� Sweep time / Cm VariableSweep time / Cm VariableSweep time / Cm VariableSweep time / Cm Variable Continuously

variable sweep rates. Cal is fully clockwise.

� PositionPositionPositionPosition Controls horizontal position of trace.� PositionPositionPositionPosition Controls horizontal position of trace.

� Horizontal variableHorizontal variableHorizontal variableHorizontal variable controls attenuation of signal

applied to Horz amp through Ext Horiz connector.

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TRIGGER SET TO AUTO OR NORMAL

� TriggerTriggerTriggerTrigger selects timing of the beginning of

the Horizontal sweep.

� SlopeSlopeSlopeSlope selects trigger at +ve increasing or -ve

decreasing portion of signal.

CouplingCouplingCouplingCoupling Selects whether trigger is at a specific DC or � CouplingCouplingCouplingCoupling Selects whether trigger is at a specific DC or

AC level.

� Source: Source: Source: Source: IntIntIntInt from Vertical Amp

� ExtExtExtExt from Ext Trig Input.

� LineLineLineLine AC line 50 (60) HZ

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VOLTS /DIV SWITCH

� Volts / DivVolts / DivVolts / DivVolts / Div

� VariableVariableVariableVariable Fine adjustment

� these controls can have a Pull out switch

position. May be 5 times mag.position. May be 5 times mag.

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VERTICAL MODE

� The operation of vertical deflection plates

� Chan 1 and Chan 2 can each operate

separately.

� Dual. Ch1 and Ch2 are swept alternatively.� Dual. Ch1 and Ch2 are swept alternatively.

� Why Dual? Used to measure input and Output

signals of a device under test.

� Ch1 and Ch2 can be added

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TIME BASE

� Main, Max, Min, delay.� Selects the sweep for the main mix or delay mode and also X-

Y switch

� Time/Div provides selection of sweep rates.

Range of 0.1 Second, 50 to .1 mS, 50 to Range of 0.1 Second, 50 to .1 mS, 50 to

0.1uS per div. Note 5,2,1, sequence.

� To determine a frequency use reciprocal.

� Frequency = 1/time period (50Hz = 1/20mS)

� Time period = 1/Frequency (number of div * ?ms/div. Eg

4div*5ms/div = 20 ms)

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OTHER

� Comp TestComp TestComp TestComp Test. Allows individual components to be

tested. Connect via banana jacks to test

resistors, capacitors, diodes, transistors, etc

� Cal Cal Cal Cal delivers calibrated voltage e.g. 2v p-p 1KHz � Cal Cal Cal Cal delivers calibrated voltage e.g. 2v p-p 1KHz

square wave for setting scale.

� GNDGNDGNDGND. Earth terminal of scope

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CONNECTIONS

� Vertical InputVertical InputVertical InputVertical Input

� Horizontal Input

� External Trigger

Cal. Out� Cal. Out

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BANDWIDTH� A 10MHz CRO does not mean it will correctly measure

signals at 10MHz.

� Vertical Amps are not so wide-band as to amplify all signals. 10MHz is the 3dB point. A 10MHz signal of 1v will measure 0.707v on the screen.

� Clipping introduces odd order harmonics. A CRO � Clipping introduces odd order harmonics. A CRO operating near the max freq. will not show the

harmonics and you think you are reading a clean signal.

� Square waves begin to look like sine waves.

� A rule of thumb is 5 times. To measure 2MHZ use a 10MHz CRO. 3 times is suitable for most Amateur work.

� For 7MHz. Times 3 = 21. Use a 20 MHz CRO.

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EVERY CRO WILL BE DIFFERENT

� Many instruments made for specific work.

� Beam Finder push button

� Trace rotation

Chan 1 Vertical input. During X-Y operation this � Chan 1 Vertical input. During X-Y operation this

is X axis (abscissa)

� Chan 2 Vertical input Chan 2. During X-Y this

becomes ordinate input.

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MINI EXERCISES

� Obtain a trace

� Brightness

� Focus

Move trace up, down.� Move trace up, down.

� Move trace side ways

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CATHODE RAY OSCILLOSCOPE