Continuing Education in Amateur Radio
Frequency modulation, FM is widely used for a variety of radio communications applications. FM broadcasts on the VHF bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two way radio communications, and it is especially useful for mobile radio communications, being used in taxis, and many other forms of vehicle.
in view of its widespread use, frequency modulation, FM, is an important form of modulation, despite many forms of digital transmission being used these days.
FM, frequency modulation has been in use for many years. However its advantages were not immediately apparent. In the early days of wireless, it was thought that a narrower bandwidth was required to reduce noise and interference. As FM did not perform well under these conditions, AM predominated and FM was not used. However, Edwin Armstrong, an American engineer looked at the use of wideband FM for broadcasting and introduced the idea against the trend of the thinking of the time.
Since its first introduction the use of frequency modulation, FM has grown enormously. Now wideband FM is still regarded as a very high quality transmission medium for high quality broadcasting. FM, frequency modulation is also widely used for communications where it is resilient to variations in signal strength.
Frequency Modulation Basics
The Concept of Frequency Modulation
The amount by which the signal frequency varies is very important. This is known as the deviation and is normally quoted as the number of kiloHertz deviation. As an example the signal may have a deviation of ±3 kHz. In this case the carrier is made to move up and down by 3 kHz.
Narrowband FM, NBFM, and Wideband FM, WBFM
The level of deviation is important in many aspects. It obviously is important in determining the bandwidth of the overall signal. As a result the deviation used for FM is different between different applications. Broadcast stations in the VHF portion of the frequency spectrum between 88.5 and 108 MHz use large values of deviation, typically ±75 kHz. This is known as wideband FM (WBFM). These signals are capable of supporting high quality transmissions, but occupy a large amount of bandwidth. Usually 200 kHz is allowed for each wideband FM transmission. For radio communications purposes less bandwidth is used. Narrowband FM, NBFM often uses deviation figures of around ±3 kHz or possibly slightly more. As quality is not as important for radio communications applications, the much narrower bandwidth has advantages in terms of radio spectrum efficiency.
Improvement in Signal to Noise Ratio
It has already been mentioned that FM can give a better signal to noise ratio than AM when wide bandwidths are used. The amplitude noise can be removed by limiting the signal to remove it. In fact the greater the deviation the better the noise performance. When comparing an AM signal to an FM one an improvement equal to 3 D2 is obtained where D is the deviation ratio.
Pre-emphasis and de-emphasis when using frequency modulation
An additional improvement in signal to noise ratio can be achieved if the audio signal is pre-emphasised. To achieve this the lower level high frequency sounds are amplified to a greater degree than the lower frequency sounds before they are transmitted. Once at the receiver the signals are passed through a network with the opposite effect to restore a flat frequency response.
To achieve the pre-emphasis the signal is passed through a capacitor-resistor (CR) network. At frequencies above the cut-off frequency the signal increases in level by 6 dB per octave. Similarly at the receiver the response falls by the same amount.
Both the receiver and transmitter networks must match one another. In the UK the CR time constant is chosen to be 50µsS. For this the break frequency f1 is 3183 Hz. For broadcasting in North America values of 75µs with a break frequency of 2.1 kHz is used.
Pre-emphasising the audio for an FM signal is effective because the noise output from an FM system is proportional to the audio frequency. In order to reduce the level of this effect, the audio amplifier in the receiver must have a response that falls with frequency. In order to prevent the audio signal from losing the higher frequencies, the transmitter must increase the level of the higher frequencies to compensate. This can be achieved because the level of the high frequency sounds is usually less than those lower in frequency.
Phase modulation is a form of modulation that can be used for radio signals used for a variety of radio communications applications. As will be seen later, phase modulation, and frequency modulation are closely related and it is often used in many transmitters and receivers used for a variety of radio communications applications from two way radio communications links, mobile radio communications and even maritime mobile radio communications.
Phase modulation is also the basis for many forms of digital modulation based around phase shift keying, PSK which is a form of phase modulation. As various forms of phase shift keying are the favoured form of modulation for digital or data transmissions, this makes phase modulation particularly important.
Phase Modulation Basics
Before looking at phase modulation it is first necessary to look at phase itself. A radio frequency signal consists of an oscillating carrier in the form of a sine wave is the basis of the signal. The instantaneous amplitude follows this curve moving positive and then negative, returning to the start point after one complete cycle - it follows the curve of the sine wave. This can also be represented by the movement of a point around a circle, the phase at any given point being the angle between the start point and the point on the waveform as shown. Phase modulation
Phase modulation works by modulating the phase of the signal, i.e. changing the rate at which the point moves around the circle. This changes the phase of the signal from what it would have been if no modulation was applied. In other words the speed of rotation around the circle is modulated about the mean value. To achieve this it is necessary to change the frequency of the signal for a short time. In other words when phase modulation is applied to a signal there are frequency changes and vice versa. Phase and frequency are inseparably linked as phase is the integral of frequency. Frequency modulation can be changed to phase modulation by simply adding a CR network to the modulating signal that integrates the modulating signal. As such the information regarding sidebands, bandwidth and the like also hold true for phase modulation as they do for frequency modulation, bearing in mind their relationship.
Forms of Phase Modulation
Although phase modulation is used for some analogue transmissions, it is far more widely used as a digital form of modulation where it switches between different phases. This is known as phase shift keying, PSK, and there are many flavours of this. It is even possible to combine phase shift keying and amplitude keying in a form of modulation known as quadrature amplitude modulation, QAM.
The list below gives some of the forms of phase shift keying that are used:
- PM - Phase Modulation
- PSK - Phase Shift Keying
- BPSK - Binary Phase Shift Keying
- QPSK - Quadrature Phase Shift Keying
- 8 PSK - 8 Point Phase Shift Keying
- 16 PSK - 16 Point Phase Shift Keying
- QAM - Quadrature Amplitude Modulation
- 16 QAM - 16 Point Quadrature Amplitude Modulation
- 64 QAM - 64 Point Quadrature Amplitude Modulation
- MSK - Minimum Shift Keying
- GMSK - Gaussian filtered Minimum Shift Keying
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