CHAPTER 3 OSCILOSCOPE AND SIGNAL CONDITIONING

Transcription

1 CHAPTER 3 OSCILOSCOPE AND SIGNAL CONDITIONING

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

3 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

4 Introduction to Signal Generator Cont d Types of signal generator: Standard Signal Generator Audio Signal Generator Radio-Frequency Signal Generator Function Generator Pulse Generator Sweep Generator Random Noise Generator

5 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.

6 OSCILLATOR CONT D 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.

7 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: A Vo Af 1 A V Where: A f = gain with feedback A = open-loop gain β = feedback factor, V i /V o s

8 REQUIREMENTS FOR OSCILLATION CONT D Gain for amplifier with positive feedback: Input Signal V s + - V i = V s - V f A V o Output Signal V f = βv o B Feedback Signal Closed-loop system consisting of amplifier with feedback

9 POSITIVE FEEDBACK AMPLIFIER OSCILLATOR CONT D 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, V f that is in phase with the original input signal.

10 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.

11 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.

12 POSITIVE FEEDBACK AMPLIFIER OSCILLATOR CONT D If a negative-feedback circuit has a loop gain that satisfies two conditions: Barkhausen Criteria Loop Gain, Aβ 1 Net Phase Shift = Phase Aβ = 0

13 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

14 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.

15 RADIO FREQUENCY OSCILLATOR CONT D Hartley oscillator Frequency of oscillation, f 2 L L LC Feedback factor, β 1 L A L L = L 1 + L 2 Barkhausen Criteria, Aβ 1 to sustain oscillation Solve the equation for inverting Amplifier A R f R i

16 EXAMPLE 1 Determine the frequency of oscillation and the minimum value of R f to sustain oscillation for the Hartley oscillator shown in figure: Rf +Vcc Ri=15k - + R +Vcc L1=10µH C=0.001µF L2=270µH

17 SOLUTION

18 EXAMPLE 2 Determine the minimum value of R f in figure below to sustain oscillation if L 2 =125µH Rf +Vcc Ri=10kΩ - + R +Vcc L1=15µH C=0.015µF L2

19 SOLUTION

20 EXAMPLE 3 Determine the value of L 2 in the circuit below if the frequency of oscillation is to be 100kHz R f +Vcc R i =10kΩ - + R +Vcc L 1 =15µH C=0.015µF L 2

21 SOLUTION

22 END OF CHAPTER 3