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" is used, although this term tends to be used with older circuits and technology.
FM demodulation is a key process in the reception of a frequency modulated signal. Once the signal has been received, filtered and amplified, it is necessary to recover the original modulation from the carrier. It is this process that is called demodulation or detection.
FM demodulator circuits are found in any receiver that uses FM: broadcast receivers, two way radios like walkie talkies and handheld radios that use FM, and any receiver where frequency modulation is used.
The terms: FM detector, FM discriminator and FM demodulator may appear rather differnet, but they are all used interchangeably, but refer to the same type of circuit. Possibly the term FM discriminator tends to be used a little more commonly for some of the circuits based on discrete components. The term FM demodulator is probably more widely used in the latest technical communications.
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 a frequency to voltage converter.
When the carrier frequency deviates to the lower end of the frequency range over which it deviates a lower voltage may be produced, then as it deviates higher in frequency, a higher voltage is produced.
Although it is easier to think of lower frequencies producing lower voltages, there is no need for this to be the case, it could be the other way around.
One of the chief requirements for the FM demodulator is that it should have a response that is as linear as possible over the required bandwidth. In other words a shift of a given frequency produces the same output change wherever it may be found on the curve. If the response is not linear, then distortion will be introduced.
A further requirement for the FM demodulator is that it should not be sensitive to amplitude variations. As the modulation is carried by only the frequency deviation, no amplitude sensitivity is wanted.
Any amplitude signal is likely to be noise, and by making the receiver insensitive to amplitude variations, the signal to noise ratio can be improved.
The resilience to noise is a major factor in providing low noise FM reception for applications like high fidelity audio broadcasts. It also means that for mobile radio, or handheld radio communications, the effects of signal level variations and fading due to movement is reduced.
If an FM demodulator is sensitive to amplitude variations as well as frequency variations, then the demodulator can be preceded a limiting amplifier stage. This stage runs into saturation when a signal of sufficient strength is present. By running in saturation, the amplitude variations are removed.
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.
As can be anticipated, the detector response curve cannot remain linear over a huge range of frequencies. Instead it should be sufficiently wide to accommodate the width of the deviation of the signal and a bit more to provide additional margin.
Types of FM demodulator
There are several types of FM detector / demodulator that can be used, and each type has its own characteristics, advantages and disadvantages.
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.
Additionally, the phase locked loop and coincidence detectors do not require inductors which are expensive to make when compared to capacitors and resistors, and even the integrated circuits. Also when it is possible to incorporate the detector into an IC, this reduces the component count which still further reduces costs while maintaining performance.
To improve the noise performance of the FM receiver, typically the IF stage may operate such that the IF amplifier is driven into limiting. This removes the amplitude variations, that will result in noise, and only allows through the frequency variations.
Driving the IF stage into limiting also removes any level variations, allowing for fading of various types - it is particularly useful for mobile radio communications because signal variations are a feature of any mobile radio communications system.
There main types of FM demodulator found in broadcast receivers, radio communication systems two way radios or walkie talkies / handheld radios, etc, are outlined below:
- 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.
This form of FM detection has very many limitations: the selectivity curve of the radio will not be at all linear and distortion will arise; the receiver will be sensitive to amplitude variations, etc.
Read more about . . . . FM slope detector.
- Ratio detector: This type of detector was one that was widely used when discrete components were used in transistor radios. The ratio detector required the use of a transformer that had a third winding to produce an additional signal which was phase shifted for the demodulation process. The ratio detector used two diodes along with a few resistors and capacitors.
Although it performed well, the ratio FM detector was an expensive form of detector in view of the transformer it used. As all wound components are more expensive than resistors and capacitors, these FM demodulators were expensive to make and after the introduction of integrated circuit technology where different circuits could be used, the ratio detector was rarely used. Nevertheless, in its day it performed well.
Read more about . . . . 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.
The Foster Seeley FM demodulator was very similar in many respects for the ratio detector. However instead of using a third winding on the transformer, it used a separate choke.
Like the Ratio detector the Foster Seeley detector fell out of widespread use when integrated circuits were introduced as other forms of FM demodulator were far easier to implement with ICs and their performance as superior.
Read more about . . . . 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. The other advantage of the PLL FM demodulator is that it is easily incorporated within an integrated circuit, and can therefore be added with very little incremental cost to an overall receiver chip, and hence the radio receiver.
The phase locked loop or PLL FM demodulator operated because the loop was set to track the instantaneous frequency of the incoming FM signal. To keep the loop in lock, the voltage controlled oscillator within the loop needed to track the frequency of the incoming signal. The tune voltage for the VCO varied in line with the instantaneous frequency of the signal, and hence provided the demodulated output of the audio or other modulation signal.
Read more about . . . . 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. The quadrature . coincidence form of FM demodulator is very easily incorporated into an integrated circuit and can be added with virtually no additional cost. This makes it a very attractive option for modern receiver designs. Many integrated circuits that are designed to provide the functionality of a compete receiver or an IF strip, incorporate a quadrature detector/ coincidence detector, and therefore FM demodulation can be added at virtually no cost to the final receiver.
Read more about . . . . Quadrature FM demodulator.
These FM demodulators are used in different applications. The different types of FM demodulator provide designers with a choice of approaches dependent upon the application: broadcast, two way radio communications including walkie talkies and handheld radios, high specification communications receivers and the like.
Although the PLL FM detector and the quadrature detectors are most widely used, along with phase locked loop based circuits. The Foster Seeley and ratio FM detectors are still used on some occasions, but they are normally only found in older radios using discrete components.
The FM discriminator or FM detector is a key element in any system using FM for broadcasting, radio communications, etc. Its performance will determine the overall performance of the whole system. Today, the detectors, discriminators of demodulators, whatever term is used to describe them, tend to major on the coincidence detector approach as this is very easy to implement, incoporate within an IC, and it does not require many additional components. In addition to this, it is able to provide a high level of performance.
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