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CHAPTER 6.1
The Oscilloscope
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The measuring systems discussed so far
– Respond to the amplitude of the input
– produce a numerical output proportional to the input
• Analog on a scale readout; or
• Digital on a numeric readout
– do not give any time variation information about the input signal
– Are limited in the frequency of the input signal to which they can respond
To observe signal variation with time as well as amplitude another type of instrument
is needed:
– the cathode ray oscilloscope (CRO)
The CRO displays a voltage vs. time waveform on the screen of a cathode ray tube
(CRT) hence the name CRO.
The CRO has the following main subsystems:
– the display sub system - the CRT
– the vertical deflection sub system
– the horizontal subsystem
– the trigger subsystem
– the probes
We will examine all of these in detail
The CRT uses a beam of highly focussed electrons to produce an image on the CRT
face
Electrons have no weight and can therefore respond quickly to changes in input
– high frequency response
– > 1 GHz is available
The CRT has 3 elements
– the electron gun
– the deflection system
– the screen
•Refer to Figure 8-2 in handout
Observe the electron gun assembly
• The gun consists of
– a thermionic cathode; and
• The cathode produces electrons when heated
– accelerating and control electrodes
• There are two controls
– focus control
– intensity control
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The intensity control grid surrounds the cathode
A small -ve voltage controls the number of electrons passing through the small hole
– the INTENSITY control regulates this voltage
A set of accelerating and focussing grids (Anodes)
– accelerate the beam, ie increase the beam energy
– focus the beam
The anodes have voltages of 2 to 12kV
– the FOCUS control is connected to one of them
• the focus grid
The deflection system consists of 2 sets of deflection plates
– the vertical deflection plates
• controls the y-axis movement
– the horizontal deflection plates
• controls the x-axis
It is possible to deflect the beam in 2 ways
– electrostatically
• using electric charge
– electromagnetically
• using magnetic fields
The CRO uses electrostatic deflection
– smaller deflection angles are needed
• note shape of CRT
– higher frequency of operation
In contrast TV uses electromagnetic deflection
– The deflection yoke
Figure 8-3 shows the screen end of the CRT.
The voltages applied to the plates move the electron beam from the centre of the
screen.
The deflection D in cm from the centre of the CRT is given by:
D
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Ll Vd
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2d Va
Vd is the deflection voltage
Va is the accelerating voltage
The deflection sensitivity of the CRT is the deflection voltage needed to produce a
particular deflection Vd /D
– typical vertical sensitivity 10-20V/cm
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Recall from your labs that the vertical inputs can be as low as 20mV
Hence the need for vertical and horizontal amplifiers to create these values of
deflection voltage
SCREEN
• The inside of the CRT face (SCREEN) is coated with a PHOSPHOR
• The phosphor is a compound which emits light when struck by high energy
electrons
– called FLUORESCENCE
• When the electron beam is shut off, light is emitted for a short period after
– called PHOSPHORESCENCE
• The length of time the phosphor glows after being bombarded is called
PERSISTENCE
• Manufacturers vary the persistence of the phosphors to achieve certain levels of
performance
– shorter persistence is preferred for faster signal displays.
• The electron beam creates a TRACE as it moves across the CRT face
• The beam is shut off (BLANKING) and returned to the original position
• the beam is then RETRACED
• If the trace and retrace locations are identical, persistence will cause the display to
appear stationary
• HEAT is generated when the beam strikes the phosphor
– only 10% electron energy converted to light
• If intensity is high the screen could be burnt
– Keep INTENSITY control low
• The CRT screen has a grid etched on it
– the GRATICULE
– allows measurements to be taken
– there is sometimes a SCALE ILLUMINATION control
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CHAPTER 6.2
Scope Subsystems
Note: This handout is to be read in conjunction with the handout.
Vertical Subsystem
• Measurand is input here
– via a PROBE
• Can accept AC or DC signals
• Contains an attenuator
– set by the volts/division control
– scales the input signal
– analogous to the range selector on voltmeter
– 2mV/div to 20V/div typical
• Has the vertical amplifier
• Provides proper signal levels to drive the vertical deflection plates
Has the following controls:
• Input coupling control
– selects AC or DC coupling
• AC coupling
– capacitor inserted before attenuator
– blocks DC, passes AC
• DC coupling
– bypasses capacitor
– allows both AC and DC to be measured
• GND
– Disconnects source
– grounds input amplifier
– used to position trace on graticule
• 50
– Used for impedance matching
– puts accurate 50 load to ground
• Vertical position
– used to position the trace on the screen
– applies a DC voltage to the vertical deflection plates
Horizontal Subsystem
• Generates the horizontal/time base signal
– a sawtooth
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– This signal goes to the horizontal amplifier
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Causes electron beam to sweep horizontally across CRT face
Horizontal position of beam is proportional to elapsed time since start of sweep
– Horizontal axis calibrated in units of time
Horizontal amplifier provides proper signal levels to drive the horizontal deflection
plates
Has the following controls
• Time Base
– Sets the beam sweep rate
– units are sec/div
• Horizontal Position
– Shifts the display along the x-axis
• Horizontal Magnifier
– Increases the resolution of x-axis
Trigger Subsystem
• Recall the CRT is displaying a trace of a vertical input (amplitude) with time
• To produce a stable, usable display both vertical and horizontal sweeps must be
synchronised
• The Trigger section is responsible for this
• It uses either the vertical input or an external signal to develop the trigger pulse
• Pulse sent to horizontal section to initialise the sweep.
Trigger controls
• The LEVEL control
– Used to select a specific point on either the rising or falling edge of the input
signal that will be used for generating a trigger.
– Useful in applications where the input signal may be corrupted by noise.
– The LEVEL control selects a portion of the input signal that is not corrupted by
noise for use as the trigger input.
• The SLOPE control
– determines which edge: rising or falling, of the input signal will be used for
generating the trigger.
• The MODE is a multiple position selector
• AUTO
– Selects an internal oscillator that will trigger a sweep in the absence of an
external signal.
– Allows a baseline trace to be established before applying an input.
– Without the AUTO trigger a trace would not be produced on the screen.
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NORMAL ( or TRIGGERED)
In this position the trigger is generated from one of 3 sources set by the SOURCE
control
– INTERNAL - based on the input signal
– EXTERNAL - supplied by an external system for example a clock circuit
• Used extensively in digital systems
– AC LINE - derived from the AC power line frequency (60Hz locally)
CHAPTER 6.3
Scope Probes
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The probe performs the following functions:
– sensing the input signal;
– transferring the value to the inputs
The CRO probe may be modelled as follows:
PROBE HEAD
COAXIAL CABLE
Contains sensing circuitry
Passive Probes
resistors, capacitors
Active Probes
powered devices:
op-amps, FETs ,
transistors
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TERMINATION
An impedance,
typical value 50
Inexpensive probes
not terminated
Probe head
– Contains sensing circuitry
– Passive
• resistors, capacitors
– Active
• powered devices: op-amps, FETs , transistors
Coaxial cable
– A conductor with an external shield
• Prevents noise pickup
Termination
– An impedance that may or may not be present
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Transmission lines have a characteristic impedance
Maximum power transfer occurs when line is terminated by this impedance
Typical value 50
Inexpensive probes not terminated
CRO LOADING
• The CRO is basically a voltmeter
• It can therefore load a circuit like the typical voltmeter
• We can model the input of the CRO as a resistance in parallel with a capacitance
– Rin is typically 1M
– Cin is typically 30 - 50 pF
Typical scope input
Vin
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Rin
Cin
At DC to low frequencies, the input sees mostly Rin
As frequency increases the impedance of the Rin||Cin combination decreases
– The loading effect thus increases with increasing frequency
We can minimise the loading by using a compensated attenuating probe
Consider the following:
9Rin
PROBE
Rin
Cin
SCOPE
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Here a probe of input resistance 9Rin has been connected to the CRO
Therefore as far as a DC signal is concerned the Zin is 10Rin
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V is however Vin/10
– it has been attenuated by a factor of 10
The degradation due to Cin still exists
If the probe had an input capacitance as shown in the following modification:
Cin/9
9Rin
Rin
Cin
Figure 1
C1
Figure 2
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Vin
C2 V 2
The capacitors in Figure 1 are arranged as shown in Figure 2
Therefore:
V2
C1
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V1 C1  C2
If
C1 
C2
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9
V2 1
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V1 10
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Note that this result is independent of f
For an AC signal, V is again scaled by a factor of 10
– Total probe impedance 10Rin||Cin/10
This probe is called a 10:1 compensated attenuating probe
– Compensated because it has adjusted for Rin and Cin
– Attenuating because it scales the input signal down by a factor (of 10 in this case)
The final commercial model has one final modification:
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Cin/9
9Rin
Rin
CC
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Cin
The probe capacitance is adjustable.
The coaxial cable used for the probe lead has some capacitance Cc we calculate CT the
total capacitance as:
We adjust the probe C1 to be CT/9
Again the total input capacitance is CT/10
CHAPTER 6.4
DUAL CHANNEL SCOPES
NOTE: This handout is to be read in conjunction with the handout on oscilloscopes
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The majority of lab scopes are dual channel
– occasionally even more channels are provided
This is accomplished in 2 ways
– Using dual beams; or
– Using dual traces
We will examine both approaches
DUAL BEAM CRO
• Dual beam systems
– uses 2 separate beams in CRT
– separate vertical systems
• one for each beam
– may have separate or common time base systems
– High performance
– Very high cost
DUAL TRACE CRO
• Single beam system
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– Uses electronic switching to create the dual display
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Has 2 additional controls
– CHOP
– ALT(ernate)
A block diagram of the switching operation is as follows:
Vert 1
Mux
Vert Amp
Vert 2
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CHOP
– Rapidly switches between both channels
– both channels appear to occur together
– Preferred for slow signals
• CHOP rate must be faster than the highest signal frequency
ALT mode
– Short for ALTERNATE
Shows one complete sweep for 1st channel
Switches to show one complete sweep for other channel
Preferred for viewing high frequency signal
If CHOP were used for a high frequency signal a series of dots would be seen
– analogous to a sampled signal
Dual trace CROs can
– add signals
– subtract signals
– display an x-y plot
X-Y plots
– Time base is set to off
One signal channel is mapped to the x-axis
Other signal is mapped to the y-axis
A common application of this is the Lissajous figure
– Formed when sinusoidal signals drive both channels
– Used for measuring phase differences in signals
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CHAPTER 6.5
STORAGE OSCILLOSCOPES
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A storage scope captures a waveform for later retrieval and analysis
– One off events like clock or logic glitches
– very slow moving signals
Two types (like with meters)
– Analog
– Digital
Analog Types
– Variable persistence
– Bistable storage
Both types of scope ‘store’ the waveform in the CRT
– An image of the waveform is created on the screen ( as normal)
– The image is retained for a period of time depending on the mechanism used
Both storage types require a CRT with 2 sets of electron guns:
The WRITE gun
– the standard electron gun discussed before
The FLOOD guns
– A set of low energy electron guns
– produce collimated (parallel) low energy electrons
– used to read the stored image
Analog storage relies on secondary emission of electrons
Variable Persistence Storage
• Write gun and screen similar to conventional CRO
• The CRT is constructed with
– a collector mesh
– a storage mesh coated with a dielectric material.
– both being located behind the phosphor screen
• See figure 8-26 in handout
• Storage mesh is slightly -ve (-10V)
• Collector mesh at +100V
• High energy electrons from WRITE gun passes through collector and storage
meshes
• Secondary emission of electrons occurs on storage mesh
• Small +ve charges therefore remain on the mesh where beam passed
• The +ve region marks where the beam passed
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– an ‘image’ of the original beam is retained
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The collector mesh collects the secondary electrons.
Recalling Waveform
• Flood gun produces electrons
– low energy
– wide parallel beam
• Collector mesh attracts the electrons
– provides slight acceleration to storage mesh
• -ve voltage on storage mesh repels electrons except where +ve charges remain.
– recall that these were created during the write cycle
• Electrons pass through these points
• High +ve potential of screen (20kV) accelerates these electrons
– high energy electrons impact phosphor
– Fluorescence and Phosphorescence occur
– stored image is displayed
• Image slowly fades as +ve regions in storage mesh are neutralized
• Variable Persistence Scopes are useful for observing slow periodic signals
Bistable Storage
• See Figure 8-27 in handout
• Does not use a storage mesh
• Uses a special phosphor that has 2 stable states
• Recall that phosphor glows when impacted by high energy electrons
• During the write stage
– energetic electrons pass through phosphor
• image seen as before
• After Phosphorescence stage phosphor does not return to original energy state
– Returns to an intermediate second state
– Secondary emission leaves a small +ve charge in Phosphor
• Flood guns create a low energy collimated beam as before
– electron energy is too low to cause primary emission, but high enough to cause
phosphor to glow when they pass through the small +ve regions.
– These electrons are collected by metallic film
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