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VIEWS: 125 PAGES: 52

  • pg 1
									ECE, ME – I SEM

Cathode Ray Oscillosope(CRO)

a) Block Diagram
b) Working
c) Application
Function Generator
a) Introduction

b) Block Diagram

c) Application

• What is an oscilloscope?

   A graph-displaying device of electrical signal
     X axis: Time
     Y axis: Voltage
     Z axis: Intensity or brightness

   Information given by oscilloscopes
       Time and voltage
       Frequency and phase
       DC and AC components
       Spectral analysis
       Rise and fall time
       Mathematical analysis

What can you do with oscilloscopes?
   Designing and repairing electronic equipment
   With the proper transducer (Ex: microphone)a)Electrical signal
    in response to physical stimuli, such as sound, mechanical stress,
    light,or heat.
   b)Engine vibrations.
   c)Brain waves

Theory of

Control panel of an oscilloscope
   Vertical Section
   Horizontal Section
   Trigger Section

Basic setting
   Vertical system
       attenuation or amplification of signal(volts/div)
   Horizontal system
       The Time base (sec/div)
   Trigger system(To stabilize a repeating signal and to
    trigger on a single event)

In digital circuits
   Measuring
       Logic level
       Timing
       Logic strength
       Rise and fall time
       Frequency
       Signal integrity
         Waveform distortion
         Noise level

In digital circuits
   Diagnosing
       Timing fault
       Proper fan-in and fan-out
       Proper pull-up and/or termination
       Collision
       Signal integrity
         Reflection
         Noise, crosstalk and ground bounce

       Open, short or stuck at 0 or 1

                                 Cathode-Ray Tube (CRT)

• Found in oscilloscopes, and similar devices are used in TV picture tubes
  and computer displays
• Use an electron beam

                                Cathode-Ray Tube (CRT)

               regulates the number of electrons that reach the anode and
Control Grid     hence control the brightness of the spot on the screen.
Focusing       ensures that electrons leaving the cathode in slightly different
anode            directions are focused down to a narrow beam and all arrive
                 at the same spot on the screen.

Cathode-Ray Tube (CRT)

Electron gun   consist of cathode, control grid, focusing anode, and
               accelerating anode
Deflecting     An electric field between the first pair of plates deflects the
plates         electrons horizontally, and an electric field between the
               second pair deflects them vertically. If no deflecting fields
               are present, the electrons travel in a straight line from the
               hole in the accelerating anode to the center of the screen,
               where they produce a bright spot.
                                 Principle Elements of a CRT

The interior of the tube is a very good vacuum, with a pressure of around
0.01 Pa (10−7 atm) or less.

The cathode, at the left end in the figure, is raised to a high temperature by the
heater, and electrons evaporate from the surface of the cathode.

The accelerating anode, with a small hole at its center, is maintained at a high
positive potential V1, of the order of 1 to 20kV, relative to the cathode.

This potential difference gives rise to an electric field directed from right to left in
the region between the accelerating anode and the cathode.

Electrons passing through the hole in the anode form a narrow beam and travel
with constant horizontal velocity from the anode to the fluorescent screen.

The area where the electrons strike the screen glows brightly.
Signal on the CRT

   The Function of this system is to provide an amplified
    signal (through vertical amplifier)of proper level to drive
    the vertical deflection Plates without introducing any
    appreciable distortion in the system.
   Voltage required for deflecting the electron beam-100 V(P-
    P) to 500 V. depending upon the accelerating voltage and
    construction of the tube.
 Function: Same as of vertical deflection system.
 There are two provisions for providing the
  horizontal input to horizontal deflection plates.ie
  externally and internally.
 Internally it is get through the output of vertical
                                         Measurement of OSC

1. Voltage Measurements (Volt/Div : 100mV/Div, Time/Div : 0.5ms/Div)

     a) Voltage Peak-to-Peak
     Vp-p= (V/Div) x No. of vert. div.
         = 100 mv/div x (3.8 x 2)
         = 0.76 V
     b) Voltage Peak
     Vp = (V/Div) x No. of vert. div.
         = 100 mv/div x (3.8)
         = 0.38 V

                                    Measurement of OSC

 2. Period and Frequency Measurements (Time/Div : 0.5ms/Div)
                                         a) Period, T
                                         T = (Time/Div) x (no. div/cycle)
                                           = 0.5ms/div x 10
3.8     Vp-p     Vp
                                           = 5ms

                 T                       b) Frequency, f
3.8                                      F = 1/T
                                           = 1/5ms
                                           = 200 Hz
       A         B
                                    Measurement of OSC

 3. Phase Measurements or Time Delay, TD (Time/Div : 0.5ms/Div)
                                          1 cycle = 10 div
                                          TD      = 2 div
3.8     Vp-p     Vp
                                          1 cycle : 10 div = 360o
                                          2 div = 72o

       A         B
   Lissajous Patterns methods(LP)

Electronic Engineer use LP to measure radio signal
They do this by analyzing the type of pattern an
unknown signal produces when it is combined with a
signal of a known frequency.
LP is determined by applying sinusoidal wave to
horizontal input(X) (unknown signal) and vertical
input(Y) (known signal). (use X-Y mode)
LP observed depends on the ratio of the two
frequency ( Horizontal/Vertical or Vertical/Horizontal)

(a) Frequency Measurement

(a) Frequency Measurement

 2:1            1:2


(b) Phase Angle

                   0o or Same phase



                  0<θ<90o or
                  270o< θ<360o

                  90o<θ<180o or
                  180o< θ<270o    26
                           (b) Phase Angle Measurement

3. Phase Measurements or Time Delay, TD               (Time/Div : 0.5ms/Div)

     θ- phase angle in degree   Yo-Y axis intercept        Ym-maximum vertical deflection

Front panel controls
Focus control :
This control adjusts CRT focus to obtain the sharpest, most-
 detailed trace. In practice, focus needs to be adjusted
 slightly when observing quite-different signals, which
 means that it needs to be an external control. Flat-panel
 displays do not need a focus control; their sharpness is
 always optimum.
Front panel controls(Cont’d)
Intensity control:

This adjusts trace brightness. On flat panels,
however, trace brightness is essentially
independent of sweep speed, because the internal
signal processing effectively synthesizes the
display from the digitized data.
Front panel controls(Cont’d)
Time base Controls :
These select the horizontal speed of the CRT's
 spot as it creates the trace; this process is
 commonly referred to as the sweep.
Quite a wide range of sweep speeds is generally
 provided, from seconds to as fast as picoseconds
 (in the fastest 'scopes) per division
Front panel controls(Cont’d)
   Holdoff control:
    Found on some better analog oscilloscopes, this
    varies the time (holdoff) during which the sweep
    circuit ignores triggers. It provides a stable display
    of some repetitive events in which some triggers
    would create confusing displays. It is usually set to
    minimum, because a longer time decreases the
    number of sweeps per second, resulting in a dimmer
Front panel controls(Cont’d)
 Vertical position control:The vertical
  position control moves the whole displayed
  trace up and down.
 Horizontal position control:The horizontal
  position control moves the display sidewise.
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
CRO Tube Controls
 POWER on / off
 Scale

 Illumination

 Focus. Create spot on screen

 Intensity. Brightness (Don’t burn a spot on your
Vertical Amp

 Position on display
 Sensitivity of vertical amp Calibrated. Cal fully
   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.
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
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.
Volts /Div switch
 Volts / Div
 Variable Fine adjustment

 these controls can have a Pull out switch position.
  May be 5 times mag.
 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
    A 10MHz CRO does not mean it will correctly measure signals at
    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
    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.
An oscilloscope displays voltage waveforms
Introduction to the Function

Example of Function Generator

What is a function generator?
 A function generator is a device that can produce
  various patterns of voltage at a variety of
  frequencies and amplitudes.
 It is used to test the response of circuits to
  common input signals. The electrical leads from
  the device are attached to the ground and signal
  input terminals of the device under test.

Features and controls
 Most function generators allow the user to choose
  the shape of the output from a small number of
 -Square wave - The signal goes directly from high
  to low voltage.
  -Sine wave - The signal curves like a sinusoid
  from high to low voltage.
  -Triangle wave - The signal goes from high to low
  voltage at a fixed rate

Features and controls
   The amplitude control on a function generator
    varies the voltage difference between the high
    and low voltage of the output signal.

   The direct current (DC) offset control on a
    function generator varies the average voltage of a
    signal relative to the ground.

Features and controls
 The frequency control of a function generator
  controls the rate at which output signal
  oscillates. On some function generators, the
  frequency control is a combination of different
 One set of controls chooses the broad frequency
  range (order of magnitude) and the other selects
  the precise frequency. This allows the function
  generator to handle the enormous variation in
  frequency scale needed for signals.

How to use a function generator?
   After powering on the function generator, the output signal
    needs to be configured to the desired shape. Typically, this
    means connecting the signal and ground leads to an
    oscilloscope to check the controls.
   Adjust the function generator until the output signal is
    correct, then attach the signal and ground leads from the
    function generator to the input and ground of the device
    under test.
   For some applications, the negative lead of the function
    generator should attach to a negative input of the device,
    but usually attaching to ground is sufficient.

Block Diagram

There are various forms of signal generators and which you choose depends
upon your intended use.

Various waveforms = generated by several different kind of instruments, which
range in complexity

        Simple fixed-frequency            Highly sophisticated instrument
          sine-wave oscillator            For testing complex com. Instr.

    Type of Signal Generators

    Working on high-end audio equipment, you'll want an audio oscillator, a low-
    distortion sine wave oscillator that can be used as a signal source for checking
    total harmonic distortion. These signal generators operate in the audio range,
    usually going from around 20 Hz or lower to over 20KHz

    Exp; Wien Bridge oscillator and phase-shift oscillator
2 Working on communications equipment, you'll need an RF signal generator.
  These have a frequency output of anywhere around 50KHz or lower to 50MHz,
  500MHz, 1GHz or more, depending upon the model

                 RF Generator                  RF Oscillators

 * RF = Radio Frequency
    Type of Signal Generators

    An all-around "signal generator" is the function generator which outputs a sine
    wave, triangle wave or square wave.

    Most have a frequency range of around 1 Hz or less to over 1MHz. Premium
    units can go from 0.001Hz to 40MHz and have all sorts of other bells and
    whistles built in. The function generator is probably the best choice for general
    electronic experimentation.

    Pulse generators produce a rectangular output waveform. The frequency
    range is from a fraction of a hertz to several megaherzt.

5 Sweep-frequency generators provide an RF sine-wave output that can be
  varied smoothly and continuously over an entire frequency band.


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