Frequency Modulation CircuitThe varactor diode is a semiconductor diode whose junction capacitance changes appreciably with d.c. bias voltage. This diode is shunted with the tuned circuit (tank circuit) of the carrier oscillator as shown in the figure:
The capacitor C is kept much smaller than the diode capacitance (Cd), in order to keep the radio frequency (r.f) voltage from oscillator across the diode small as compared to V0. V0 is the polarizing voltage to maintain a reverse bias across the varactor diode. Also, reactance of C at the highest modulating frequency is kept large as compared to R, so that the shunting of the modulating signal through the tank circuit is avoided.
Phase Modulation vs Frequency Modulation
Below are given the difference between the Phase Modulation and Frequency Modulation:
|Sl. no||Frequency Modulation||Phase Modulation|
|1||Here the angular frequency of the signal is modified.||Here the Phase angle of the signal is modified.|
|2||The amplitude is kept constant||The Phase deviation is proportional to the amplitude of the modulating signal|
|3||The modulating index is proportional to amplitude of modulating voltage||The modulation Index is proportional to the modulating voltage only.|
The process of extracting a modulating signal from a modulated carrier is known as demodulation. Electronic circuits that perform the demodulation process are called FM detectors.
The detector performs the extraction in two steps:
- It converts the frequency modulated signal (FM) signal into a corresponding amplitude modulated signal (AM) signal by using frequency-dependent circuits, i.e. circuits whose output voltage depends on input frequency. Such circuits are called as frequency discriminators.
- The original modulating signal f (t) is recovered from this AM signal by using a linear diode envelope detector.