Frequency Modulation Tutorial Includes:
Frequency modulation, FM Modulation index & deviation ratio FM sidebands, bandwidth FM demodulation FM slope detector FM ratio detector Foster Seeley detector PLL FM demodulator Quadrature demodulator MSK GMSK
Modulation formats: Modulation types & techniques Amplitude modulation Phase modulation Quadrature amplitude modulation
FM demodulation is also called FM detection and sometimes the phrase FM discrimination can be used.
FM demodulation basics
In any radio that is designed to receive frequency modulated signals there is some form of FM demodulator or detector.
This circuit takes in frequency modulated RF signals and takes the modulation from the signal to output only the modulation that had been applied at the transmitter.
In order to be able to demodulate FM it is necessary for the radio receiver to convert the frequency variations into voltage variations.
It is necessary to have a response that is as linear as possible over the required bandwidth.
The response that is normally seen for an FM demodulator / FM detector is known as an "S" curve for obvious reasons. There is a linear portion at the centre of the response curve and towards the edge the response becomes very distorted.
Types of FM demodulator
There are several types of FM detector / demodulator that can be used. Some types were more popular in the days when radios were made from discrete devices, but nowadays the PLL based detector and quadrature / coincidence detectors are the most widely used as they lend themselves to being incorporated into integrated circuits very easily and they do not require many, if any adjustments.
- Slope detection: This is a very simple form of FM demodulation and it relies on the selectivity of the receiver itself to provide the demodulation. It is not particularly effective and is not used except when the receiver does not have an FM capability. . . . . Read more about the FM slope detector.
- Ratio detector: This type of detector was one that was widely used when discrete components were used in transistor radios. . . . . Read more about the FM ratio detector.
- Foster Seeley FM : In the days when radio used discrete components, this was the other main contender for the FM demodulator in radios. . . . . Read more about the Foster Seeley FM discriminator.
- Phase locked loop demodulator: It is possible to use a phase locked loop to demodulate FM. The PLL FM detector provides excellent performance and does not require many, if any adjustments in manufacture. . . . . Read more about the PLL FM demodulator.
- Quadrature detector: The quadrature FM detector is now widely used in FM radio ICs. It is easy to implement and provides excellent levels of performance. . . . . Read more about the Quadrature FM demodulator.
These FM demodulators are used in different applications. Although the PLL FM detector and the quadrature detectors are most widely used, the Foster Seeley and ratio FM detectors are still used on some occasions.
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