Frequency Demodulation

Objective

To Study FM- Demodulation and to Observe the Demodulated Wave Characteristics.

Introduction

Demodulation is the act of extracting the original information- bearing signal from a modulated carrier wave or the process of separating the original information or signal from the modulated carrier wave. This process is used in the receivers to recover the original signal coming from the sender end n modulating form. We can say that its function is opposite to that of modulation process.

There are several ways to demodulation depending upon how parameters of the base-band signal are transmitted in the carrier signal, such as amplitude, frequency or phase. For a signal modulated with a linear modulation, like Amplitude modulation we can use a synchronous detector, for a signal modulated with an angular modulation use a Frequency demodulator.

There are several common types of FM demodulation, some of them are:

1. The Quadrature Detector, which phase shifts the signal by 90˚ and multiplies it with un-shifted version. One of the terms that drop out from this operation is the original information signal, which is selected and amplified.
2. The signal is fed into a PLL and the error signal is used as the demodulated signal.
3. The most common is a Foster-Seeley discriminator. This is composed of an electronic filter which decreases the amplitude of some frequencies relative to others, followed by an AM demodulator. If the filter response changes linearly with frequency the final analog output will be proportional to the input frequency, as desired.
4. A variant of the Foster Seeley discriminator called the Ratio detector.
5. Another method uses two AM demodulators; one tuned to high end of the band and other to the low end, and feed the output into a difference amplifier.

Slope Detector

The slope detector is a method of Frequency demodulation which converts the received FM signal to AM and demodulates with an envelope detector. Any circuit that outputs the time derivative of the input can perform the FM to AM conversion. In a FM signal the frequency is low, when the amplitude of the message signal is low and vice versa.Slope detection converts the FM signal to Amplitude modulated AM-FM signal using the slope of a filter circuit. Since the peak-to–peak amplitude of the FM signal is constant; on the up-slope of a band pass filter circuit is a lower frequency yield a lower voltage, a higher frequency will yield a higher voltage. When the output of the filter is envelope detected, the original message signal is recovered.