Construction & Working of Audio Frequency Signal Generator

Construction & Working of Audio Frequency Signal Generator Points : construction & working of audio frequency signal generator Following fig is the conceptual block diagram of a signal generator having an internal audio oscillator for use as source of modulation.

The heart of the signal generator is a voltage — controlled oscillator (Vco). The frequency of the Vco is determined by the voltage present at the control input. If the control voltage is increased or decreased, the frequency of the Vco increases or decreases, respectively. So whatever signal is applied to the control voltage shows up as the frequency of the oscillator. This is exactly what is required for frequency modulation. The audio modulating signal drives the control input of the VCO to produce a frequency-modulated carrier. For amplitude modulation, a modulator circuit is added after the oscillator. The modulator circuit varies the amplitude of the Vco,s output without modifying its frequency. Thus, the output is an amplitude modulated signal. The actual block diagram used to implement the signal generator may be much more complicated (especially for high frequency generators), but the conceptual block diagram is still a valid tool for understanding signal generators.

The frequency accuracy and stability are very important in a signal generator used to test receivers. The pass band of the receiver is usually very narrow, much less than one percent of the frequency. An even better frequency accuracy is required for the test signal. Harmonic distortion is not usually very critical in receiver testing since the harmonics are well away from the frequency of receivers. For other high frequency applications, however, the harmonic distortion may be important). Amplitude accuracy is important for accurately measuring the receivers sensitivity.