Oscillators
An oscillator is an electronic device for generating an AC signal voltage. Oscillators generate sinusoidal or non-sinusoidal waveform from very low frequencies up to very high frequencies. The local oscillator in most present-day broadcast band AM saperhetrodynes will cover a range of frequencies from 1000 to 2100 KHz (approximately).
An oscillator is a circuit to generate alternating voltage of desired frequency and amplitude. It converts DC energy to an AC voltage. It has wide applications i.e. to test a stereo amplifier; an audio signal generator generates 20 KHz to 15 kHz at the transmitter and 47 MHz to 230 MHz frequency at receiver end. In radio, the carrier frequency varies from 550 KHz to 20 MHz for TV broadcasting in radio and TV receiver’s high frequency oscillators are required.
Basically an oscillator circuit is an amplifier that provides itself (through feedback) with an input signal. It is a nonrotating device for producing alternating current, the output frequency of which is determined by the characteristics of the device. The initial purpose of an Oscillator is to generate a given waveform of a constant peak amplitude and specific frequency and to maintain this waveform within certain limits of amplitude and frequency.
An oscillator must provide amplification and a portion of the output is feedback to sustain the input, as shown in Fig. 1. Enough power must be feedback to the input circuit for the oscillator to drive itself as in case of signal generator. The oscillator is self-driven, because the feedback signal is regenerative i.e. positive feedback.
Let us consider the basic requirement of oscillator circuit.
First, amplification is required to provide the necessary gain for the signal.
Second, sufficient regenerative feedback is required to sustain oscillations.
Third, a frequency determining device is needed to maintain the desired output frequency. In addition to the application, determine the types of oscillator to be used.
Feedback
Feedback is the process of transferring energy from a high-level point in a system to a low-level point. This means transferring energy from the output of an amplifier back to its input. If the output feedback signal opposes the input signal, the signal is degenerative or negative feedback However, if the feedback aids the input signal, the feedback is regenerative or positive feedbacks. Regenerative or positive feedback is one of the requirements to sustain oscillations in an Oscillator. This feedback can be applied in any of several ways to produce a practical Oscillator circuit.
A circuit, which produces electrical oscillations of any desired frequency, is called oscillatory circuit. This circuit consists of two reactive components namely inductor L and a capacitor C connected in parallel with each other. Such a circuit is also called LC or tank circuit.
The feedback signal is coupled from the tank circuit by two methods. The first method is to take some of energy from the inductor. This can be achieved by any one of the three ways shown in Fig. 2(a), (b) and (c). When an oscillator uses a tickler coil, as shown in Fig. 2(a), it is referred to as an Armstrong oscillator. When an oscillator used as tapped coil as shown 1(b) or a split coil as shown in Fig. 2(c), it is referred to as a Hartley oscillator. The second method of coupling the feedback signal is to use two capacitors in the tank circuit and tap the feedback signal between them. This is shown in Fig. 2(d) also oscillator using this method is called colpitts oscillator.
The use of positive feedback results in a feedback amplifier having closed loop gain Av greater than the open Loop gain Av. It results in instability and operation as an oscillatory circuit. An oscillator circuit provides a, constantly, varying amplified output signal at any desired frequency.
Classification of Oscillators
The electronic oscillators may be broadly classified into the following two categories.
The oscillators, which provide an output having a sine wave form, are called sinusoidal or harmonic oscillators. Such oscillators can provide output at frequencies ranging from 20 Hz to GHz.
Sinusoidal or Harmonic Oscillators
Tuned Circuit Oscillators
These oscillators use a tuned-circuit consisting of inductors (L) and capacitors (C) and are used to generate high frequency signals. Thus they are also known as radio frequency (CRT) oscillators. Such oscillators are Hartley and Colpitts oscillators etc.
RC Oscillators
These oscillators use resistors and capacitors and are used to generate low or audio-frequency signals. Thus they are also known as audio-frequency (A.F) oscillators. Such oscillators are phase-shift and wien-bridge oscillators.
Crystal Oscillators
These oscillators use quartz crystals and are used to generate highly stabilized output signal with frequencies up to 10 Mhz. The pierce oscillator is an example of a crystal oscillator.
Negative-resistance Oscillators
These oscillators use negative-resistance characteristic of the devices such as tunnel diodes. A tuned diode oscillator is an example of negative resistance oscillator.
Non-sinusoidal or Relaxation Oscillators
The oscillators, which provide an output having a square, rectangular or saw tooth waveform, are called non-sinusoidal or relaxation oscillators. Such oscillators can provide output at frequencies ranging from zero to 20 Mhz.
Factors Affecting the Stability of Oscillator
The frequency stability of an oscillator is a measure of its ability to maintain a constant frequency, over a long time interval. However, it has been found if an oscillator is set at a particular frequency, it does not maintain it for a long period. The cause of change in oscillation frequency or the factors affecting the stability of oscillator is given as under.
Operating point
The operating point of the active device i.e. bipolar transistor is selected in such a way that its operation is nonlinear region, changes the values of device parameters which, in turn affects the frequency stability of the oscillator.
Circuit Components
The values of circuit components (i.e. resistor, inductors and capacitors) change with the variation in temperature. Since such changes take place slowly, they also cause a drift in oscillator frequency.
Supply Voltage
The changes in DC supply voltage applied to the active device, shift the oscillator frequency. This problem can be avoided by using a highly regulated power supply
Output Load
A change in the output load may cause a change in the Q-factor of the tank circuit, hereby causing a change in oscillator output frequency.
Inter element Capacitances
Any change- in the inter element capacitances of a transistor (particularly collector-to-emitter capacitance), cause changes in the oscillator frequency and thus affects the frequency stability.
Stray Capacitances
The stray capacitances also affect the frequent stability of an oscillator. The effect of changes in inter element capacitances can be neutralized by putting an additional capacitor across the corresponding elements. However, it is difficult to avoid the effect of stray capacitances.