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Chapter 2  Principles of radio transmission

2.1. AM Transmitter
2.2. FM Transmitter
2.3. Wavebands


Transfer of information (speech, music, image, computer data etc.) by radio can be presented in its simplest form with block - diagram as on Pic.2.1. That is a transmission realized by amplitude - modulated signal. Since, in our example, the information being transferred is the sound, the first step of such transmission is converting the sound into electrical signal, this being accomplished by a microphone. The low - frequency (LF) voltage at microphone output (Pic.2.1-a), that represents the electrical "image" of the sound being transferred, is being taken into the transmitter. There, under the effect of LF signal, the procedure called amplitude modulation is being carried out, and on its output high - frequency (HF) voltage is generated, its amplitude changing according to the current LF signal value. HF voltage creates HF current in the antenna, thus generating electromagnetic field around it. This field spreads through the ambient space, being symbolically shown on Pic.2.1 with dashed circles. Traveling at the speed of light (c=300 000 km/s), the electromagnetic field gets to the reception place, inducing the voltage in the reception antenna, as shown on Pic.2.1-c. This voltage has the same profile as the one on Pic.2.1-b, except it has much smaller amplitude. In the receiver, the amplification and detection are carried out first, resulting with the LF voltage on its output, that has the same profile as the one on Pic.2.1-a. This voltage is then transformed into sound by loudspeaker, that sound being exactly the same as the sound that acted upon the microphone. This, naturally, is the way it would be in ideal case. Back to reality, due to device imperfection as well as the influence of various disturbances, the sound being generated by the loudspeaker differs from the one that acts upon the microphone membrane. The block - diagram on Pic.2.1 (excluding the HF signal shape) is also applicable in case of radio transmission being carried out by frequency modulation. In that case frequency modulation is being carried out in the transmitter, under the effect of LF signal coming from the microphone, therefore HF signals on Pics.2.1-b and 2.1-c having constant amplitude, and their frequency being changed in accordance with the actual value of LF signal from the microphone. In fact, all types of radio transmission can be presented with Pic.2.1. First, the information being sent is always transformed into electrical signal through the appropriate converter. In telegraphy this converter is the pushbutton, in radiophony it's a microphone, in television engineering an image analysis cathode ray tube (CRT) etc. Then, with this "electrical image" of information, the modulation is being done. The modulated HF signal is being transferred into antenna and transmitted. On the reception place, the modulated signal from the reception antenna is being amplified and detected and then, again with the appropriate converter (pen recorder, loudspeaker, TV CRT etc.), the information is transformed back into its original form.

 

2.1 AM Transmitter

In order to better understand the way the radio transmitter works, block - diagram of a simple AM (amplitude modulated) signal transmitter is shown on Pic.2.2. The amplitude modulation is being performed in a stage called the modulator. Two signals are entering it: high frequency signal called the carrier (or the signal carrier), being created into the HF oscillator and amplified in the HF amplifier to the required signal level, and the low frequency (modulating) signal coming from the microphone or some other LF signal source (cassette player, record player, CD player etc.), being amplified in the LF amplifier. On modulator's output the amplitude modulated signal UAM is acquired. This signal is then amplified in the power amplifier, and then led to the emission antenna.

 

The shape and characteristics of the AM carrier, being taken from the HF amplifier into the modulator, are shown on Pic.2.3-a. As you can see, it is a HF voltage of constant amplitude US and frequency fS. On Pic.2.3-b the LF signal that appears at the input of the modulator at the moment t0 is shown. With this signal the modulation of the carrier's amplitude is being performed, therefore it is being called the modulating signal. The shape of the AM signal exiting the modulator is shown on Pic.2.3-c. From the point t0 this voltage has the same shape as that on Pic.2.3-a. From the moment t0 the amplitude of AM signal is being changed in accordance with the current value of the modulating signal, in such a way that the signal envelope (fictive line connecting the voltage peaks) has the same shape as the modulating signal.
Let's take a look at a practical example. Let the LF signal on Pic.2.3-b be, say, an electrical image of the tone being created by some musical instrument, and that the time gap between the points t0 and t2 is 1 ms. Suppose that carrier frequency is fS=1 MHz (approximately the frequency of radio Kladovo, exact value is 999 kHz). In that case, in period from t0 till t2 signals us on Pic.2.3-1 and uAM on 2.3-c should make a thousand oscillations and not just eighteen, as shown in the picture. Then It is clear that it isn't possible to draw a realistic picture, since all the lines would connect into a dark spot. The true picture of AM signal from this example is given on Pic.2.3-d. That is the picture that appears on screen of the oscilloscope, connected on the output of the modulator: light coloured lines representing the AM signal have interconnected, since they are thicker than the gap between them.
Block - diagram on Pic 2.2 is a simplified schematic of an AM transmitter. In reality there are some additional stages in professional transmitters that provide the necessary work stability, transmitter power supply, cooling for certain stages etc. For simple use, however, even simpler block diagrams exist, making the completion of an ordinary AM transmitter possible with just a few electronic components.

 

2.2. FM Transmitter

Block diagram of an FM (frequency modulated) transmitter is given on Pic.2.4. Information being transferred, i.e. the modulating signal, is a signal from some LF source. it is being amplified in LF amplifier and then led into the HF oscillator, where the carrier signal is being created. The carrier is a HF voltage of constant amplitude, whose frequency is, in the absence of modulating signal, equal to the transmitter's carrier frequency fS. In the oscillatory circuit of the HF oscillator a varicap (capacitive) diode is located. It is a diode whose capacitance depends upon the voltage between its ends, so when being exposed to LF voltage, its capacitance is changing in accordance with this voltage. Due to that frequency of the oscillator is also changing, i.e. the frequency modulation is being obtained. The FM signal from the HF oscillator is being proceeded to the power amplifier that provides the necessary output power of the transmission signal.
Voltage shapes in FM transmitter are given on Pic.2.5. Pic.2.5-a shows the LF modulating signal. The frequency modulation begins at moment t0 and the transmission frequency begins to change, as shown on Pic.2.5-b: Whilst current value of the LF signal is raising so is the trasmitter frequency, and when it is falling the frequency is also falling. As seen on Pic.2.5-c, the information (LF signal) is being implied in frequency change of the carrier.
The carrier frequencies of the radio difusion FM transmitters (that emmit the program for "broad audience") are placed in the waveband from 88 MHz til 108 MHz, the maximum frequency shift of the transmitter (during the modulation) being 75 kHz. Because of that the FM signal should be drawn much "thicker", but it would result in a black-square-shaped picture.

2.3. Wavebands

While considering problems related to the realization of the long - distance radio links, significant differences between the electromagnetic waves of various frequencies must be kept in mind. For example, low frequency waves (below 500 kHz) can bend themselves following Earth's curvature, while the HF waves are moving in streamlines, just as light. Some waves can be reverberated from the ionosphere, others are passing through it etc. According to characteristics of their outspread, radio waves can be classified into several groups or ranges: long, mid, short and ultra-short. Limits between the wavebands are not precise, with the raise of their frequency the waves are gradually losing some features while gaining some others. This division is shown in Table 1.





* LF low frequencies, MF mid frequencies, HF high frequencies, VHF very high frequencies, UHF ultra high frequencies, SHF super high frequencies, EHF extra high frequencies. Waves with wavelength smaller than 30 cm are also called the microwaves.
 

In the third table column the wavelengths are given. Wavelength () is distance that the wave passes moving at the speed of light (c=3*103 m/s), during the period that is equal to its oscillating period (T): =c*T. Having in mind that the wave frequency is f=1/T, one can easily get to the well known expression that gives the relation between the wavelength and the frequency:


 

Using this formula one can calculate the wavelength knowing the frequency and vice versa. For example, wavelength of an FM transmitter emitting at f=100 MHz frequency is L=3*108/100*106=3 m. Similar to that, wavelength of Radio Belgrade 1 is L=439 m, which makes its frequency equal to f=3*108/439=684 kHz.
Radio diffusion is being performed in certain parts of the wavebands given in Table 1, their boundary frequencies are (rounded values):


LW (long waves) 150 kHz (2km) 300 kHz (1 km)
MW (mid waves) 500 kHz (600 m) 1500 kHz (200 m)
SW (short waves) 6 MHz (50 m) 20 MHz (15 m)
FM (ultra short waves) 88 MHz (3.4 m) 108 MHz (2.78 m)


In LW, MW and SW the amplitude modulation is used, while in FM range it is the frequency modulation.

Here are the frequencies (in kHz) of some radio transmitters from the MW range, that can serve for tuning of the radio receivers being described in this issue: Timisoara 630, Belgrade1 684,  Bucharest 855 .

 

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