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IMPLEMENTATION OF FREQUENCY DEMODULATOR USING THE PLL DEMODULATION METHOD


There are many methods available for FM demodulation like slope detector, Foster-Seeley discriminator, Phase Lock Loop detector, Quadrature method, Ratio detector. In our project we are using the PLL demodulation method using IC LM565 which is FM demodulation IC. The LM565 and LM565C are general purpose phase locked loops containing a stable, highly linear voltage controlled oscillator for low distortion FM demodulation, and a double balanced phase detector with good carrier suppression. The VCO frequency is set with an external resistor and capacitor, and a tuning range of 10:1 can be obtained with the same capacitor. The characteristics of the closed loop system—bandwidth, response speed, capture and pull in range—may be adjusted over a wide range with an external resistor and capacitor. The loop may be broken between the VCO and the phase detector for insertion of a digital frequency divider to obtain frequency multiplication. Available TTL and DTL compatible phase detector input and square wave output. After the demodulation it follows, amplifier stage. In this amplifier stage IC TBA810 is used. This demodulator can work up to 10 mega Hz frequency. All this IC required 5,-5.15,-15 DC supply externally. The main aim of this project is to study MULTISIM and ULTIBOARD software and run & simulate our project in MULTISIM and how to make circuit layout of the required circuit. MULTISIM, Aimspice, Free PCB & Ultiboard softwares are utilized for implementation of circuit & PCB layout.

The process of removing the information signal from the carrier is termed demodulation. The challenge is to design a circuit (or algorithm) that will achieve this task optimally in the presence of noise, interference and varying signal strength, frequency and phase, whilst being compact, power efficient and cheap.

To detect an FM signal, it is necessary to have a circuit whose output voltage varies linearly with the frequency of the input signal. The slope detector is a very basic form of such a circuit, although its linearity of response is not a good. The tuning circuit tune to receive the signal on the slope of the response curve. The carrier amplitude is caused to vary with the frequency. There are four methods for the frequency demodulation technique. These are as follows:
1. Foster-Seeley Discriminator
2. Ratio Detector
3. Phase Lock Loop Detector
4. Quadrature Detector
Here IC LM565 is used as a Phase Lock Loop
Detector.
Mainly two sections are present as follows:
1. FM Demodulation by PLL.
2. Output audio amplifier section.

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