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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

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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.

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• 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

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• 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 Variable Continuously

variable sweep rates. Cal is fully clockwise.• Position Controls horizontal position of trace.

• Horizontal variable controls 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.

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

• Coupling Selects 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