1. SIGNAL GENERATORS

2. 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

5. 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

8. 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.

9. 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.

10. 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
β = feedback factor, Vi/Vo

11. REQUIREMENTS FOR OSCILLATION CONT’D
Gain for amplifier with positive feedback:
Closed-loop system consisting of amplifier with feedback

12. 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, Vf that is in phase with the original input signal.

13. 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.

14. 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.

15. 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 = 0 or 360 degree

16. 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

17. 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.

18. RADIO FREQUENCY OSCILLATOR CONT’D
Frequency of oscillation,
Hartley oscillator
L = L1 + L2
Feedback factor, β
Barkhausen Criteria, Aβ ≥ 1 to sustain oscillation
Solve the equation for inverting Amplifier