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Cathode Ray Oscilloscope Introduction PowerPoint PPT Presentation


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Cathode Ray Oscilloscope Introduction . Look and play Read instructions Break into smaller parts Decide on a simple measurement Increase your level of difficulty Record pathway. How you got there. Store directions and short-cuts. Cathode Ray Oscilloscope. - PowerPoint PPT Presentation

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Cathode Ray Oscilloscope Introduction

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Cathode ray oscilloscope introduction l.jpg

Cathode Ray Oscilloscope Introduction

  • Look and play

  • Read instructions

  • Break into smaller parts

  • Decide on a simple measurement

  • Increase your level of difficulty

  • Record pathway. How you got there.

  • Store directions and short-cuts


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

  • Horizontal deflection plates.

  • Vertical deflection plates


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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 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.

  • 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 on / off

  • Scale

  • Illumination

  • Focus. Create spot on screen

  • Intensity. Brightness (Don’t burn a spot on your screen)


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Vertical Amp

  • Position on display

  • Sensitivity of vertical amp Calibrated. Cal fully clockwise.

  • Variable sensitivity. Continuous range between calibrated steps.

  • AC - DC - Gnd.

  • 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)Select desired sweep rate, or admits external sig to horiz amp.

  • Sweep time / Cm VariableContinuously variable sweep rates. Cal is fully clockwise.

  • PositionControls horizontal position of trace.

  • Horizontal variablecontrols attenuation of signal applied to Horz amp through Ext Horiz connector.


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Trigger Set to Auto or normal

  • Trigger selects timing of the beginning of the Horizontal sweep.

  • Slopeselects trigger at +ve increasing or -ve decreasing portion of signal.

  • CouplingSelects whether trigger is at a specific DC or AC level.

  • Source: Int from Vertical Amp

  • Ext from Ext Trig Input.

  • Line AC line 50 (60) HZ


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Volts /Div switch

  • Volts / Div

  • Variable Fine adjustment

  • these controls can have a Pull out switch 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.

  • 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 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 Test. Allows individual components to be tested. Connect via banana jacks to test resistors, capacitors, diodes, transistors, etc

  • Cal delivers calibrated voltage e.g. 2v p-p 1KHz square wave for setting scale.

  • GND. Earth terminal of scope


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Connections

  • Vertical Input

  • Horizontal Input

  • External Trigger

  • 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 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 is X axis (abscissa)

  • Chan 2 Vertical input Chan 2. During X-Y this becomes ordinate input.


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And there’s more !

  • Don’t worry about it

  • Nothing is complex

  • Just Simplicity multiplied


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Operating

  • Power on

  • Intensity fully counter-clockwise

  • Vertical centering in center of range

  • Horizontal centering in center of range

  • Vertical at 0.2 or 5v / div. Try a range.

  • Timebase 10ms / div Change to suit.

  • Play ‘till operating for you.


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Mini exercises

  • Obtain a trace

  • Brightness

  • Focus

  • Move trace up, down.

  • Move trace side ways


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Measuring

  • Voltage RMS is 0.707 * Vp for Sine and Cosine waveforms.

  • Hint: Try using a multimeter in parallel until you are happy with the measured CRO readings.


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Mini exercise DC

  • Find a battery or a plugpack (Wall wart)

  • Determine approximate number of volts

  • Set vertical amp. Volts per Division

  • Cal. Control fully clockwise

  • DC (AC will show ripple component only)

  • Connect probe to battery

  • Read volts by number of volts per division on display.


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AC Sine-wave

  • Decide upon probable frequency

  • Set timebase

  • Obtain display

  • DC or AC ??

  • One or more cycles per division or whole display?

  • Volts per division (Vertical) Peak to Peak

  • Calculate RMS volts (Peak x 0.707)


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Square wave

  • Decide upon probable pulses per second

  • Set timebase, Obtain display.

  • One or more pulses per division or whole display?

  • DC or AC ??? Try it.

  • Volts per division. Vertical

  • Pulses per division. Horizontal

  • Measure volts, Length of pulse.


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Complex waves

  • AC ripple super imposed on a DC supply

  • Mixing two sine waves. Phase measurements. Lissajous patterns (X-Y)

  • Dual trace CROs

  • External Trigger ( Positive going and Negative going)

  • Noise

  • Frequency resolution of CROs.


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Read your operating manual

  • Read your operating manual

  • Read your operating manual

  • Enjoy reading your operating manual.

  • Test old projects, AM radios.

  • Audio or RF oscillators

  • Read your operating manual - ENJOY