Signal Generators Term 8

INTRODUCTION: A signal source is a vital component at a test setup, whether at the end of a production line, on the service bench, or in the research laboratory. Signal sources have a variety of applications including checking stage gain, frequency response, an alignment in receivers and in a wide range of other electronics equipment.

Signal sources provide a variety of waveforms for testing electronic cir­cuits, usually at low power. The various waveforms are generated by several different kinds of instruments, which range in complexity from simple fixed frequency sine-wave oscillators to highly sophisticated instruments such as might be used in testing complex communications equipment.

The difference between Oscillators and Generators: Although terminology is not universal. the 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. Although we speak of oscillators as "generating" a signal, it should be emphasized that no energy is created; it is simply converted from a do source into an ac energy at some specific frequency.  

REQUIREMENTS FOR OSCILLATION Basically, an oscillator is an amplifier with positive equation for an amplifier with positive feedback Af = A (1-1) 1+A β where Af = gain with feedback A = open-loop gain β = feedback factor Vi/Vo

Fig. 1-1 Closed – loop system consisting of amplifier with feed back Feedback is provided by the feedback, or phase-shift, network shown in Fig. 1-1. The output signal of the amplifier is fed back to the amplifier's input terminal through the phase-shift network, undergoing there a 180° shift in phase. Fig. 1-1 Closed – loop system consisting of amplifier with feed back

The amplifier also causes a 180° phase shift Therefore, the net phase shift in the circuit is zero, which is one condition that must be satisfied for sustained oscillation. A second requirement for oscillation is that the closed-loop gain, which is the product A , must be equal to. or greater than, unity. These two conditions are known as the Barkhausen criteria.

It is not necessary to supply an input signal to initiate oscillation It is not necessary to supply an input signal to initiate oscillation. A noise voltage or a transient is sufficient to start the process. It is usually desirable for the closed-loop gain to be greater than unity to ensure strong oscillations. However. if the gain is too high, the circuit operates at saturation and the output waveform will not be sinusoidal.

The phase-shift network consists of a combination of components with resistance and capacitance (RC) or inductance and capacitance (LC). A combination of inductance and capacitance is usually called a tuned circuit or a tank circuit. Oscillators that employ an RC combination have a distinct advantage over LC-type oscillators at low frequencies since the physical size of the required inductors is prohibitive.