Cathode Ray Oscilloscope Introduction

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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.
• 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 lt----gt 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 (Dont 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 4div5ms/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 theres more !

• Dont 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