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Design and construction of color TV PAN model - 2


PART I

RADIO – CASSETTE INTRODUCTION


CHAPTER 1: GENERAL CONCEPTS OF RADIO

A. PRINCIPLE OF RADIO TRANSMITTING AND RECEIVING:

I. ELECTROMAGNETIC RADIATION:


Radio waves used in information technology, infrared rays that we feel as heat on the skin or visible light from purple -> red, or ultraviolet rays, X-rays, gamma rays emitted from radioactive substances... are all waves with different frequencies of electromagnetic radiation. Electromagnetic radiation is also called electromagnetic waves, it can be converted to each other in the transmission space from electric field to magnetic field and vice versa.

Electromagnetic waves propagate in space at a speed of 300,000 km/s. If we call C the wave propagation speed, f the frequency and  the wavelength of the radiation, we have:

f  C


The frequency of electromagnetic waves is hertz (Hz). In radio communication technology

has a wavelength  in (m) or centimeters (cm) while other radiations such as light, X-rays, Gamma rays... have a wavelength in A 0with 1A 0 = 10 -10m.

II. ELECTRICAL SIGNALS:


In information technology, sound or image is converted into an electrical quantity in the form of current or voltage. The signal current or voltage is called an electrical signal. Common electrical signals are audio frequency (AF) signals and video signals (VF).

1. Audio frequency signal: (AF: audio frequency)

Audio signal is a signal with a frequency in the audible frequency range (20Hz- 20,000Hz). The device commonly used to convert sound into audio signal is a microphone.

2.Video signal: (VF: video frequency)

A video signal is an electrical signal whose intensity varies according to the brightness of the image elements. The maximum frequency of the video signal is proportional to the square of the number of lines of resolution of the image and its value is measured in Mhz.

III. ELECTRICAL SIGNAL MODULATION:


Voice and music after being converted into audio frequency signals, even though the power has been increased, still cannot be transmitted to the transmitting antenna to transmit information in the form of electromagnetic waves because:

- Audio signal broadcast through antenna cannot be transmitted far because the frequency is not high enough (below 20Khz).

- If the signal frequency is large enough to radiate, the efficiency of the work is low, the broadcasting station is very complicated, and the quality of the received signal is very poor.

Therefore, to transmit audio frequency signals in the form of electromagnetic radiation, people use modulation techniques. Using an audio frequency signal to modulate a high frequency signal to get another signal, the modulated signal contains both the transmitted audio frequency signal and has a high enough frequency to be easily radiated by the transmitting antenna into electromagnetic waves that propagate in space. The "modulation" process is to insert the audio frequency signal into the high frequency signal, using high frequency waves to "carry" the audio frequency waves. The high frequency waves are called carrier waves.

At the receiver, the audio frequency signal (containing the information to be transmitted) is separated from the high frequency signal and further amplified... and sent to the speaker to reproduce the audio signal.

In modulation techniques, we have amplitude modulation (AM) and frequency modulation (FM) used in sound systems.

1. AM signal modulation: (Amptitude Modulation)

Amplitude modulation is also known as amplitude modulation or amplitude modulation. Figure (1a) is an unmodulated high frequency (RF) signal. Figure (1b) is the audio frequency (AF) signal of the information to be transmitted and figure (1c) is the result of modulation, the modulated signal or also known as AM wave.

The amplitude modulated signal has the same frequency as the high frequency signal but the amplitude varies according to the audio frequency signal.

It has been shown that if the RF high frequency signal has frequency f 0amplitude modulated by an AF audio signal with frequency f, the AM signal has three components: carrier wave f 0and two frequency bands carrying f 0– f and f 0+ f (figure 2).

Difference (f 0+ f )- (f 0- f) = 2f = BW is called bandwidth, passband or spectrum. Radio stations usually have BW = 10Khz.



Figure a

Figure b



Figure c



a) RF high frequency signal


b) AF audio signal


c) AM amplitude modulated signal


Figure 1: Amplitude modulation.


f 0 -ff 0 f 0 +ff 0


Figure 2: Frequency spectrum of a high frequency signal modulated by an audio frequency signal.

a) By a monosyllable


b) By a frequency band


2. FM frequency modulation:

Frequency modulation is also known as frequency modulation.


Figure 3 shows the form of a high frequency signal that has been frequency modulated by an audio signal. The audio signal changes the frequency of the high frequency signal (carrier wave) while the amplitude remains the same.

Call f 0is the frequency of the unmodulated high frequency signal, after modulation, in the positive half cycle the frequency increases to f 0+f =f 1and in the second half of the sound cycle the frequency drops to f 0-f =f 2. The FM wave emitted has a frequency of f= f 0 f. Where f 0called the center frequency, f is called the frequency shift, frequency shift or frequency gain.

The bandwidth BW of an FM broadcast station is limited to 150Khz. BW = 2f =150Khz.


Figure 3: Audio frequency modulated wave.


3. Compare FM and AM modulated waves:

Compared with AM modulated waves, FM frequency modulated waves have the following advantages:


- Good sound quality, high anti-interference.


- FM transmitter provides fixed power with high working efficiency.


However, its disadvantage is that its bandwidth is too wide, so it is only suitable for short and ultra-short waves. Therefore, the transmission distance is only under 100km. From the above comparison, FM waves are often used to broadcast music for each local station because it has better quality than AM waves.

IV. RADIO WAVES:


Radio waves are called electromagnetic waves. The high-frequency signal (carrier wave) after being modulated (modulated) by the audio-frequency signal and amplified is emitted into space by the antenna in two parts: the electric field and the magnetic field of the same frequency lie on two planes perpendicular to each other and perpendicular to the direction of wave propagation.

The intensity of the wave propagating at a point depends on many factors:


- Transmitter antenna structure.


- Radio power.


- Transmission medium.


- Factors of weather, geography...


1. Bandwidth classification:

Type

frequency

wave

Step

wave

Name

call

Common name

use

Uses

1

10Khz- 30Khz

30km- 10km

Super low frequency

VLF

Long distance communication.

2

30Khz-

10km-

lower frequency

LF

Long distance communication

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Design and construction of color TV PAN model - 2

Radio waves are divided into 8 types for convenience and this division is not absolute.



300Khz

1km



sea.

3

300Khz

– 3Mhz

1000m-

100m

intermediate frequency

MF

Maritime communication.

4

3Mhz –

30Mhz

100m-

10m

High frequency

HF

Long distance communication and

medium.

5

30Mhz- 300Mh

10m-

1m

High frequency

VHF

Television, short distance communication.

6

300Mhz-

3Ghz

1m-

10cm

High breakage

frequency

UHF

Same as VHF

7

3Ghz- 30Ghz

10cm- 1cm

Ultra high frequency

SHF

Same as VHF

8

30Ghz-

300Ghz

1cm-

1mm

Pole

high frequency

EHF

Radio approach radar

Table 1: Bandwidth classification.

Note: Waves with frequencies of 2000Mhz or higher are called microwaves.


2. Propagation of electromagnetic waves:

There are two main paths for radio waves to travel from the transmitting antenna to the receiving antenna:


- Ground wave: wave propagates near the ground from the transmitter to the receiver directly.

Transmitting antenna

Receiving antenna

(b)

- Sky waves: waves propagate to the ionosphere of the atmosphere and reflect back to the receiver, the waves propagate indirectly.


Transmitting antenna

Ionospheric refraction


Receiving antenna

(a)



a) Ground waves


b) Sky waves

Figure 4: Radio wave propagation


The propagation of sky waves always changes with the weather, which means it always changes with the fluctuations of the two layers of atmosphere surrounding the earth. The propagation of ground waves is limited by terrain (mountains, seas, etc.) and the propagation range is difficult to exceed 100km due to the curvature of the earth (Figure 4).

From VHF waves and above (ultra short waves) do not reflect in the ionosphere and so go out into space, so they only propagate underground and are only suitable for nearby information.

Since HF waves (shortwave) are mainly transmitted as sky waves, they are reflected by the ionosphere, so the transmission distance is very long, suitable for long-distance information.

It should be noted that the farther away the transmitting antenna is, the weaker the wave energy, the weaker the received signal. The level of energy loss of the transmitted wave depends on the distance, terrain frequency, etc.

V. OVERVIEW OF THE BROADCAST SYSTEM:


To broadcast news programs, music, etc. everywhere, domestically and internationally, a radio broadcasting system must be used. The broadcasting system consists of three parts: a studio, a radio station, and a radio receiver.

The recording studio has the function of recording sound, converting speech and music into audio frequency signals, pre-amplifying, and providing audio signals to the broadcasting station.

The radio station receives audio frequency signals from the broadcasting studio, amplifies them, modulates them with high frequency waves into the station's signal, and then sends them to the transmitting antenna. At the transmitting antenna, the station's signal is radiated into radio waves and propagates in space.

antenna

Power amplifier

Create Oscillation

Frequency or Frequency Control


Sound Amplifier

Microphone


Figure (a)



Antenna


High frequency amplifier


Loudspeaker

Audio amplifier

Wave separation

Figure (b)

Figure 5: a: Simple block diagram of a broadcasting system b: Simple block diagram of a receiving system

At the radio receiver, the radio waves of the transmitting station are induced on the receiving antenna into the electromotive force of the signal. The signal is selected and sent to the high-frequency amplifier stage, which separates the waves to remove the high-frequency carrier wave, and retrieves the audio signal that the station needs to transmit. The amplifier increases the power level of the audio signal strong enough to be transmitted to the speaker (Figure 5).

B. QUALITY CRITERIA:

I. SENSITIVITY:


The sensitivity of a radio receiver is measured in millivolts per meter (mV/m) or V/m. The lower the signal voltage value, the more sensitive the receiver is and the farther away the receiver can receive stations with weak signals.

In fact, the radio receiver itself also has an internal noise level. This internal noise goes out to the speaker at the same time as the signal from the station to be received, so the general requirement is that the signal level going out to the speaker must be 3 to 4 times or more higher than the noise level, then the sensitivity will have practical meaning. Therefore, the sensitivity is determined with the condition: the signal/noise ratio (S/n) is 3/1, 4/1.

II. SELECTION:


The wave that the radio transmits includes a wide spectrum on both sides of the carrier frequency, on the other hand, at the same time there are many radio waves that are close together. Therefore, the frequency band of this wave may overlap a part of the frequency band of the neighboring wave, which can easily cause interference when receiving radio. A radio receiver with high selectivity is less likely to interfere when working. Selectivity is calculated according to the attenuation (in decibels-dB) of the signal when the resonance is shifted by 10Khz. The larger this value, the higher the selectivity.

III. WAVE BAND:


For the receiver, the general requirement is to be able to adjust the resonance at any frequency within the operating frequency range while still ensuring the machine's specifications. Because the operating frequency range of the shortwave band is very wide, it is divided into many consecutive wave bands, so that the adjustment of the reception is easy. The more wave bands the receiver has and the wider the shortwave band, the better.

IV. NOMINAL CAPACITY:


Rated output power is the power level that sends audio frequency to the speaker. The higher the speaker output power, the louder the sound will be. Radio receivers usually have a rated output power from 250mW to a few W.

V. TROPICALIZATION:


This is a very important criterion for receivers used in hot and humid climates like ours. To tropicalize, people can spray a layer of moisture-proof plastic on the printed circuit board and the components of the transformers are soaked and sealed to protect them from moisture.

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