OSCILOSCOPE

1
CHAPTER 3

• OSCILOSCOPE
• AND
• SIGNAL CONDITIONING

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OUTLINE

• Introduction to Signal Generator
• Oscillator
• Requirement for Oscillation
• Positive Feedback Amplifier Oscillator
• Radio Frequency Oscillator

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Introduction to Signal Generator

• Device that generate signal of various frequency
  and amplitude.
• Common and vital equipment in any electronic
  laboratory.
• Signal generator characteristics
• Stable and distortion-free signal
• Controllable amplitude values
• Controllable frequency values
• DC offset

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Introduction to Signal Generator Contd

• Types of signal generator
• Standard Signal Generator
• Audio Signal Generator
• Radio-Frequency Signal Generator
• Function Generator
• Pulse Generator
• Sweep Generator
• Random Noise Generator

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OSCILLATOR

• Oscillator is a circuit that generates an ac
  output signal without requiring any externally
  applied output signal.
• The only input power to an oscillator is the d.c
  power supply.
• Term Oscillator is generally used for an
  instrument that provides only a sinusoidal output
  signal, and the term generator is applied to an
  instrument that provides several output
  waveforms, including sine wave, square wave,
  triangular wave and pulse trains as well as
  amplitude modulation of the output signal.

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OSCILLATOR CONTD

• ADVANTAGES
• An oscillators is a non-rotating device.
  Consequently, there is little wear and tear and
  hence longer life.
• Due to the absence of moving parts, the operation
  of an oscillator is quite silent.
• An oscillator can produce waves from 20 Hz to
  extremely high frequencies 100 MHz
• The frequency of oscillations can be easily
  changed when desired.
• It has good frequency stability.
• It has very high efficiency.

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REQUIREMENTS FOR OSCILLATION

• Basically, an oscillator is an amplifier with
  positive feedback.
• The signal regenerate and sustain itself.
• The gain equation for an amplifier with positive
  feedback is
• Where

Af gain with feedback A open-loop gain ß
feedback factor, Vi/Vo
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REQUIREMENTS FOR OSCILLATION CONTD

• Gain for amplifier with positive feedback

• Closed-loop system consisting of amplifier with
  feedback

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POSITIVE FEEDBACK AMPLIFIER OSCILLATOR CONTD

• A transistor amplifier with proper positive
  feedback can act as an oscillator. You must
  remember that a positive feedback amplifier is
  the one that produces a feedback voltage, Vf that
  is in phase with the original input signal.

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POSITIVE FEEDBACK AMPLIFIER OSCILLATOR

• A phase shift of 180 is produced by the
  amplifier and a further phase shift of 180 is
  introduced by feedback network.
• The signal get shifted by 360.
• This signal is then feedback to the input. The
  feedback voltage is in phase with the input
  signal.
• The circuit is producing oscillations in the
  output.
• However, this circuit has an input signal. But,
  then this is inconsistent with our definition of
  an oscillator which states that an oscillator is
  a circuit that produces oscillations without any
  external signal source.

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POSITIVE FEEDBACK AMPLIFIER OSCILLATOR

• If we open the switch S in the figure, we will
  get the circuit as shown
• It means that the input signal is removed.
  However, the feedback voltage is still applied to
  the input signal. The amplifier will respond to
  this signal in the same way that it did to the
  input signal and that is the feedback voltage
  will be amplified and sent to the output.
• The function of the feedback network is to send a
  portion of the output back to the input.
  Therefore, the amplifier receives another input
  cycle and another output cycle is produced. This
  process will continue so long as the amplifier is
  turned on.
• Therefore, the amplifier will produce sinusoidal
  output with no external signal source.

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POSITIVE FEEDBACK AMPLIFIER OSCILLATOR CONTD

• If a negative-feedback circuit has a loop gain
  that satisfies two conditions

Barkhausen Criteria
Loop Gain, Aß 1
Net Phase Shift 0 _at_ Phase Aß 0
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TYPES OF OSCILLATOR

• Oscillator categorized in two types
• i) Audio Oscillators
• ii) Radio Frequency Oscillators
• There are two common types of Audio Oscillators
• i)Wien Bridge Oscillator
• ii) Phase-Shift Oscillator
• Both of which employ RC feedback network
• The Wien Bridge offers some very attractive
  features, including a straightforward design, a
  relatively pure sine-wave output and very stable
  frequency

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RADIO FREQUENCY OSCILLATOR

• There are of many types of radio frequency, but
  the famous is HARTLEY OSCILLATOR.
• Radio frequency must satisfy the same basic
  criteria for oscillation, that is Barkhausen
  criteria.
• The phase-shift network for RF oscillators is an
  inductance-capacitance (LC) network.
• This LC combination which generally referred to
  as a tank circuit, acts as a filter to pass the
  desired oscillating frequency and block all other
  frequencies.

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RADIO FREQUENCY OSCILLATOR CONTD
Frequency of oscillation,
Hartley oscillator
L L1 L2
Feedback factor, ß
Barkhausen Criteria, Aß 1 to sustain oscillation
Solve the equation for inverting Amplifier
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EXAMPLE 1
Determine the frequency of oscillation and
the minimum value of Rf to sustain oscillation
for the Hartley oscillator shown in figure
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SOLUTION
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EXAMPLE 2
Determine the minimum value of Rf in figure below
to sustain oscillation if L2125µH
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SOLUTION
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EXAMPLE 3
Determine the value of L2 in the circuit below if
the frequency of oscillation is to be 100kHz
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SOLUTION
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END OF CHAPTER 3