Fixed and Variable AF Oscillator

      Signal generators are the sources of electrical signals used for the purpose of testing and operating different kinds of electrical equipment. A signal generator provides different types of waveforms such as sine, triangular, square, pulse etc., whereas an oscillator provides only sinusoidal signal at the output. 
The AF oscillators are divided into two types. They are as follows
1. Fixed frequency AF oscillator
2. Variable frequency AF oscillator. 

1. Fixed Frequency AF Oscillator 
Many instrument circuits contain oscillator as one of its integral parts to provide output signal within the specified fixed audio frequency range. This specified audio frequency range can be 1 kHz signal or 400 Hz signal. The 1 kHz frequency signal is used to execute a bridge circuit and 400 Hz frequency signal is used for audio testing. A fixed frequency AF oscillator employs an iron core transformer. Due to this a positive feedback is obtained through the inductive coupling placed between the primary winding and secondary winding of the transformer and hence fixed frequency oscillations are generated. 

2. Variable Frequency AF Oscillator 
It is a general purpose oscillator used in laboratory. It generates oscillations within the entire audio frequency range i.e. from 2O Hz to 20 kHz. This oscillator provides a pure, constant sine wave output throughout this AF range. The examples of variable AF oscillators used in laboratory are RC feedback oscillator, beat frequency oscillator.