Transcript Cathode Ray Oscilloscope

Cathode Ray Oscilloscope
Introduction
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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
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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
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deflection plates
• 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
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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
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Ext from Ext Trig Input.
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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
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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
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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
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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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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
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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
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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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