Modulation of Radio Waves
A blank page in a book conveys no information. In order to provide information there has to be some text or some pictures on that page. Similarly is a plain radio wave, it cannot be heard nor can it convey information. Special components can make it audible, but the only signal heard is a constant audio tone, still nothing is ‘read’. Some form of intelligence must be ‘impressed’ upon such a wave if it is to convey information. The process of impressing such information by changing the original signal is called ‘modulation’, and it is done in a variety of ways. In all cases the radio waves simply act as a vehicle for the information, so they are commonly called ‘carrier waves’. The waveform of information, which is being impressed upon the carrier wave, is called the ‘modulating wave’.
Technical limitations make audio frequency (AF) transmission non-effective. AF signals are transported by higher radio frequency (RF). The frequency which carries the AF is called carrier wave (CW). Superimposition of AF over the CW is called modulation. At the receiving end the AF is separated from the CW which is called demodulation. You can also define it as
THE PROCESS OF CHANGING SOME CHARACTERISTIC (AMPLITUDE, FREQUENCY , PHASE) OF A CARRIER WAVE IN ACCORDANCE WITH THE INTENSITY (AMPLITUDE) OF THE SIGNAL IS CALLED MODULATION.
Modulation is achieved by the following methods
- Keying : This is often called wireless telegraphy (w/t). It consists of starting and stopping the continuous carrier wave, breaking it up into dots and dashes. Therefore it is sometimes also described as ‘interrupted carrier wave’. The communication is by a code. Groups of these dots and dashes are assigned particular meanings, as in Morse code. The technique is primarily used for long-distance communication, however, some radio navigation facilities may break their carriers for identification by dots and dashes. The receiver requires a beat frequency oscillator (BFO) facility, to make the signals audible.
Amplitude Modulation (AM) : The amplitude of CW is varied by the amplitude of AF. This method is used for VHF RTF and LF/MF broadcast. This can be used either to radiate speech, music etc., or to transmit coded messages at audio frequencies. When a signal is amplitude modulated, its total amplitude varies between the sum of the amplitudes of the two signals and the difference between them. If sum of the amplitudes of A and B is 2 and the difference is 0 and the amplitude of the audio being carried varies between values 2 and 0. This is a measure of the modulation depth. Modulation depth is the extent to which the carrier is modulated and is expressed as a percentage.
Modulation Depth = (Amplitude of B/ Amplitude of A) X 100
THE RATIO OF THE AMPLITUDES OF THE SIGNAL TO THE UNMODULATED CARRIER WAVE EXPRESSED IN PERCENTAGE
Now comes importance of Modulation Depth
1) If depth is less than 50% audio signals are not very strong
2) If depth is more than 75% audio signals are strong and clear
3) If depth is more than 100% than there is distortion in reception and wastage of power
Remember the greater the modulation depth the lesser the range
WHENEVER A CONTINUOUS WAVE IS MODULATED BY A FREQUENCY LOWER THAN ITSELF, ADDITIONAL FREQUENCIES OCCUR ON EITHER SIDE OF THE CW FREQUENCY THESE ARE CALLED SIDE BANDS. THE INTELLIGENCE IS CARRIED IN THESE SIDE BANDS.
Amplitude modulation does not change the frequency of the carrier wave. However, adding or subtracting a signal to the carrier does in fact produce signals at slightly different frequencies, those at the sum and difference of the two waves. These signals at the new frequencies, when added together, are of the same strength as the modulating wave. If we measure frequencies, we can see that a simple amplitude modulated signal actually sends three waves, one at the original frequency, one at the sum of the two frequencies, and one at the difference between the two frequencies. Each of the two outside frequencies or sidebands is at half the original strength. Sidebands can be used to improve frequency band use.
2) Frequency Modulation(FM) : Another method of modulating a wave is by varying the frequency of the carrier wave. Frequency of the CW is varied in accordance with the change in amplitude of the AF, keeping the amplitude of the carrier constant. The demodulator at receiver end is called frequency discriminator.
FM is advantageous over AM because
(i) its transmitters are simpler (ii) less power required and (iii) reception is practically static free.
It is used in radio altimeter and VOR.
Sidebands. A frequency modulated signal, by its very structure, transmits signals at varying frequencies. These appear in a band extending in relation to the strength of the modulating signal, around the central carrier frequency, and are distributed equally on both sides of it. Unlike AM, where the resultant radiation consists of only three frequencies i.e. the carrier frequency, carrier frequency + audio frequency and the carrier frequency – audio frequency, a frequency-modulated signal carries with it a multiple of sidebands and consequently its bandwidth is greater. In the process of modulation, it is the sidebands and not the carrier which carries the intelligence in both AM as well as the FM. Therefore, the receiver must be capable of admitting an adequate range of frequencies on either side of the carrier when the carrier frequency is being tuned in. The receiver’s bandwidth may be broader than necessary for a particular reception. Since all information in an FM signal is contained in these sidebands, the receiver must be able to pick up all of them. The range of frequencies containing the sidebands is called the bandwidth. Because all the information from the modulating signal is contained in mirrored sidebands around the carrier wave, it is possible to transmit and receive only one group of the sidebands. The other sideband, and even the carrier wave, can be suppressed and electronically replaced in the receiver. If a transmitter uses only the upper sideband, and receivers are tuned to receive that, another transmitter can use the lower sidebands for its signal. This effectively reduces the bandwidth required, and allows more transmitters to use a busy frequency band. The following are examples of bandwidth requirements:
Speech transmission 3 kHz
Music between 10 and 15 kHz
Radar 3 to 10 MHz
Comparing Frequency Modulation and Amplitude Modulation
- Any signal which is modulated needs power. For a given power input, an unmodulated carrier wave will travel a larger distance than a modulated wave. An FM signal requires some extra power than unmodulated wave, and will travel a shorter distance, but an AM signal with maximum modulation requires about 50% more power than the basic carrier wave.
- Transmitters And Receivers. An AM transmitter is a complicated piece of equipment, whereas its receiver can be quite simple. In contrast, an FM transmitter is relatively simpler but the corresponding receiver is more complex.
- Static Interference. Static interference is caused by electrical disturbances in the The word ‘noise’ is usually reserved for interference from electrical components in transmitters and receivers. These disturbances occur over all frequencies, but are worse in the lower bands. They are much more of a problem to AM reception, because the interference is similar to an AM signal. Most static can be filtered out of an FM receiver.
3) Pulse Modulation : The modulating pulses amplitude-modulate carrier, giving it the shape of pulses. It is used in Radar.
If you speak this much on modulation then the examiner is most likely go to next question, however in case he asks further about need for Modulation then you can tell him
1) Requirement of practical antenna height (Lower the frequency Larger the antenna). Therefore modulating with a higher frequency the antenna size can be reduced and a lower frequency wave can be send from a smaller size antenna.
2) Operating Range : Lower the frequency lesser is the range (not surface waves).
3) It allows for wireless communication. Audio frequencies get attenuated and distorted when they travel a large distance. Modulation ensures that this distortion doesn’t happen.
Here at least I feel he will stop, however, if he insists on more than you can tell him about the limitations of Amplitude Modulation and they are:
1) Noise in reception, 2) Low Efficiency 3) Small operating Range and lack of audio quality
These are the reasons why our modern day radios have shifted from Amplitude modulated signals to Frequency Modulated signals..Basically modern day FM radios..
Advantages of FM
1) Noiseless reception 2) High Efficiency 3) Hi Fi reception
Disadvantages of FM
1) Complex receivers 2) operates in VHF so range is less
Radio waves are often designated by a 3-symbol code, which is used by ICAO as part of the definitions of all aviation communication and navigation systems. For example, a VOR signal is designated as ‘A9W’. The most important meanings of each part of the code are listed below. Several signals are described by more than one designator if the signal itself consists of more than one part. For example, a continuous wave signal with no modulation which for a short period is modulated in amplitude using a single modulating sub-carrier for identification purposes may be designated as NON A2A.
This tells the type of modulation on the main carrier wave. This includes:
N No modulation.
A Amplitude modulated, double sideband.
H Amplitude modulated, single sideband and carrier wave.
J Amplitude modulated, single sideband, suppressed carrier wave.
F Frequency modulated.
G Phase modulated.
P Pulse modulated, constant amplitude.
K Pulse modulated, amplitude modulated.
This designates the nature of the signal or signals modulating the main carrier:
0 No modulating symbol.
1 Single channel containing quantised or digital information without the use of a modulating sub-carrier.
2 Single channel containing quantised or digital information, using a modulating sub-carrier.
3 Single channel containing analogue information.
Two or more channels containing quantised or digital information.
7 Two or more channels containing analogue information.
8 Composite system comprising 1, 2 or 7 above, with 3 or 8 above. X Cases not otherwise covered.
Type of information transmitted. (This does not include information carried by the presence of the waves.)
N No information transmitted.
A Telegraphy – for aural reception.
B Telegraphy – for automatic reception.
D Data transmission, telemetry, telecommand.
E Telephony (including sound broadcasting).
F Television (video).
W Combination of the above.
X Cases not otherwise covered.
Well it seems long but frankly no examiner will ask the full thing. they will see how you started your answer and will shift over to next question but you however have to know the full thing so that there is no gap in your answer.