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In electronics and radio communication systems, when the transmitted electronic signal reaches the receiver, it will be not strong. Usually when the electronic signal has been transmitted over a long distance. Electronic signal that is transmitted together with a lot of other electronic signals collect noise of numerous types. Radio receivers give the receptiveness and selectivity that allows the full recovery of the initial information electronic signal. The radio receiver that is known to perform this is called the superheterodyne receiver. The superheterodyne receiver uses the heterodyning process to change a received electronic signal to an intermediate frequency (IF). The intermediate frequency can be easily operated than the initial radio carrier frequency. Practically, all radio and television receivers being manufactured in these days uses the superheterodyne principles of functionability.
The heterodyne receiver was invented in the 1900s. It is used today in virtually all electronic communication systems.
The Block Diagram of the Heterodyne Receiver
This diagram of the heterodyne receiver presents useful data regarding the transmission of the electronic signal, from the antenna to the loudspeaker.
The aerial is a transducer that receives and transforms electromagnetic waves into electric current. The aerial receives the radio signal that is not strong and forwards it to the RF amplifier, also named a low-noise amplifier.
The RF Amplifier
In electronic communication receivers, the RF amplifiers give the initial gain and initial selectivity in the radio receiver. The RF amplifiers are also known as pre-selectors.
Moving on, RF amplifiers reduces the oscillator radiation. Since that the local oscillator electronic signal is strong, it can pass through and be found at the input to the mixer or the first detector as indicated in the circuit diagram above. The RF amplifier that is placed between the mixer and the aerial separates the two, and thus minimizing any local oscillator radiation.
The magnitude of the amplification of the electronic signal that the RF amplifier carries out is thoroughly worked out in order to make sure that there is not too large load on the mixer when powerful electronic signals become present. The RF amplifier allows the electronic signals to be amplified so that they are capable to guarantee an admirable electronic signal to noise ratio is brought about.
Moreover, after an electronic signal have been tuned and amplified, it goes on to the one port of the mixer or the first detector.
The Mixer and the Local Oscillator
The output of the RF amplifier is forwarded to the input of the first detector. The first detector, on the other side, takes an input from the local oscillator. These two signals are then combined to attain the IF by the method of heterodyning.
Electronic components such as diodes are implemented as mixers or first detectors as they have a good signal-to-noise ratio. In these days complicated integrated circuits are being used.
In the superheterodyne receiver local oscillator electronic signal heterodynes with the input electronic signal to create a sum and difference frequencies (intermediate frequency electronic signals)
Furthermore, the local oscillator is designed so that its frequency could be altered over a comparatively wide range. As the local-oscillator frequency is altered, the mixer interprets a wide range of input frequencies to the intermediate frequency.
The intermediate frequency is created by mixing a local oscillator electronic signal with the incoming or input electronic signal. Because of that, the local oscillator is thus crucial part of the circuit to accurately carry out the process and the local oscillator ought to be tunable.
The IF Amplifier
The output of the mixer is an electronic intermediate frequency signal with the same modulation that is on the input electronic radio frequency signal. This electronic signal is then amplified by the intermediate frequency amplifiers stages and most of the receiver gain is collected during these stages. Because of the fact that the intermediate frequency is lower than the input electronic signal frequency, intermediate frequency amplifiers are easier to construct and good selectivity is easier to attain.
The Demodulators / Second Detector
The amplified intermediate frequency signal is forwarded to the demodulator or the second detector, which then recovers the original modulating data. The radios that are being produced in these days, the intermediate frequency signal is initially digitized by the analogue-to-digital converter and then transmitted to the digital signal processor. The demodulation processes are carried out by a programmed mathematical algorithm at the digital signal processor.
The Automatic Gain Control
Although not indicated in the block diagram above, the circuit known as the automatic gain control is found in the superheterodyne receiver. For the most part, the output electronic signal of a demodulator or the second detector is the initial modulating electronic signal. The amplitude of this electronic signal is directly proportional to the amplitude of the received electronic signal. This means that both the output electronic signal and the received electronic signal are properly related in magnitude.
The obtained electronic signal is rectified and filtered by the automatic gain control. To control the receiver gain the potential difference is usually supplied to the intermediate frequency amplifiers and quite often to the radio frequency amplifier. Automatic gain control circuits assists to manage a constant output potential difference over multiple input electronic signals. Automatic gain control circuits also assist in the prevention of the performance-degrading distortion.
The amplitude of the radio frequency electronic signal at the aerial of the receiver can range from a tiny value of a microvolt to thousands of microvolts. The range of this electronic signal is called the dynamic range. Electronic receivers are assembled and constructed with large gain so that electronic signals that are not strong can be received quite well. If a large electronic amplitude signal is supplied to a receiver, it causes the circuits to be overdriven and thus creating distortion and decreasing the quality.
Using the automatic gain control, the total gain of the receiver is automatically changed depending on the input electronic signal level. The electronic signal amplitude at the output of the detector or the de-modulator is properly related in size to the amplitude of the input electronic signal. If it is large, the automatic gain control circuit creates a big dc output voltage, and thus decreasing the gain of the intermediate frequency amplifiers. By carrying out this process of reduction in gain it gets rid of the distortion normally produced by a high-voltage input electronic signal. When the incoming electronic signal is not strong, the detector output is not high. The output of the automatic gain control will be a tiny dc potential difference. By doing this operation it consequently permit the gain of the intermediate frequency amplifiers to stay large, and thus supplying the highest amplification.
The Audio Amplifier
The regained electronic signal in the digital arrangement is changed back to analog by a digital-to-analog convertor.
Moving on the output from the de-modulator or the second detector is known as the recovered audio. The recovered audio is then put into the audio stages and there, the recovered audio is then amplified and forwarded on to the speakers.
The speaker is a transducer that converts an electrical signal into a sound signal. The output of the digital-to-analog convertor is quite often forwarded to an audio amplifier with enough potential difference and power gain to make the speaker to function.
The Necessity of AGC in AM
Automatic gain controls were put into action in the earliest radios for the reason of fading propagation which is also known as the slow differences in the amplitude of the received electronic signals which needed continuing changes in the receiver's gain so as to manage a relative output electronic signal. The job of the automatic gain control circuit is to guarantee a constant output signal level in spite of the signal's variations at the input.
The automatic gain control circuits are now seen in electronic systems in which wide amplitude changes in the output signal can cause a loss of data. Automatic gain control circuits are implemented in a lot of electronic systems where the magnitude of the input signal can change over a wide dynamic range. The task of the automatic gain control is to supply a constant output amplitude so that the electronic circuit following the automatic gain control circuit need a smaller dynamic range.
In the case that the electronic signal level alterations are much slower than the information rate that is obtained in the electronic signal, then an automatic gain control circuit can be employed to supply an electronic signal with a well stated average level to downstream circuits. The huge dynamic range of electronic signals that must be held by the receivers needs gain adjustment to thwart the overload of the stages and to change the demodulator or the detector input level for best performance.
Furthermore, the automatic gain control is the essential electronic component of superheterodyne receiver circuitry. The disadvantage of manual gain control in the receiver is that it is not able to supply a constant output. For instance, when a receiver is tuned from an electronic signal that is not strong to the one that is strong, its output will increase. Because of this, readjustments will be required to be taken. When a receiver is tuned to a certain electronic signal the output level can change widely if the input electronic signal strength alternates as a result of fading and adjustments. Because of the fact that the electronic signal alternations are very quick, continuous readjustments will be needed to be implemented and that is impractical.
This is where the automatic gain control is put into action.
The Necessity of AFC in FM
In radio electronic equipments, automatic frequency control is a means to automatically maintain a resonant circuit tuned to the frequency of an incoming radio signal. It is generally operated in radio receivers to keep the receiver tuned to the frequency of the wanted station.
The automatic frequency control circuit senses the difference between the actual frequency and the frequency that is wanted and then creates a control potential difference that is corresponding to the difference. A verticap which is an electronic component that has a variable capacitance that is a function of the voltage impressed on its terminals is implemented to keep the intermediate frequency constant.
An automatic frequency control circuit is used in an FM stereo radio receiver. Nearly all, modern radio receivers are mostly stable and drift-free, and no outside assistance is required to carry out with the tuning.
The frequency modulation of the superheterodyne receiver has an intermediate frequency of 10.7MHz and is tuned between the frequency range of 88MHz - 108MHz.
The range of the frequencies that the Local Oscillator ought to cover are:
Fs = 88MHz
Fo = 88MHz + 10.7MHz
Fs = 108MHz
Fo = 108MHz + 10.7MHz
98.7MHz - 118.7MHz
An acceptable bandwidth for the IF amplifiers will be:
The gain for the IF amplifiers will be:
IF amplified by the IF amplifier stages
The 'Capture effect' found in FM receiver
In general frequency modulation is regarded as admirable to amplitude modulation, despite the fact that they are both used to send electronic signals from one location to the other. Usually, frequency modulation presents some advantages over the amplitude modulation. The major advantage of frequency modulation over amplitude modulation is its immunity to noise.
Another benefit of the frequency modulation over the amplitude modulation is its capture effect feature. The frequency capture effect is a situation closely related with the frequency modulation reception whereby the stronger of two electronic signals at the similar frequency will be demodulated.
It completely gets rid of the electronic signal that not strong at the receiver limiter, where the electronic signal that is not strong is not amplified, but attenuated. In the situation where the two electronic signals are almost the same in strength, the receiver tends to switch from one to the other.
Moreover, some kinds of radio receiver circuits have powerful capture effect than others. The calculation of how well a receiver can remove a second electronic signal on the same frequency is known as the capture ratio. It is calculated as the smallest ratio of the power of two the two electronic signals that will result in the removal of the lower electronic signal.
However amplitude modulation, also called amplitude modulation radio, transmission is not accountable to this effect. Because of this, the aviation industry and other industries, have selected to use of amplitude modulation for communications rather than frequency modulation. By doing so, it permits diverse electronic signals to be communicated on the same channel.
Although the capture effect avoid the electronic signal that is not strong of two the frequency modulation electronic signals from being heard, when two stations are sending or broadcasting electronic signals of the frequency modulation of the same amplitude, first one may be captured and then the other.
The Method used to Demodulate an FM Signal.
Quadrature FM Detector/Demodulator
The quadrature demodulator is likely to happen to be the commonly operated FM detector. The quadrature demodulator operates a phase-shift circuit to make a phase shift of 90° at the unmodulated carrier frequency. The most commonly operated phase-shift arrangement is shown in Fig.1 . The frequency-modulated electronic signal is used through a very small capacitor (C1) to the parallel tuned circuit, which is then altered to resonate at the carrier frequency. At the resonate, the tuned circuit arises as a high value of pure resistance. The small capacitor has a very high reactance compared to the tuned circuit impedance. The output obtained across the tuned circuit at the carrier frequency is almost 90°. The output also leads the input. The balanced detector of differential amplifiers is usually operated as the phase detector. The phase detector produces a number of pulses with a width that changes with the quantity of phase shift between the two electronic signals. The electronic signals are passed through the RC low-pass filter to produce the original modulating signal.
Usually, the sinusoidal frequency modulation input electronic signals to the phase detector are at a top level and moves the differential amplifiers in the phase detector towards the cutoff and saturation. Moving on, the differential transistors behaves as switches, this signifies that the output is a series of pulses. No limiter is required if the input electronic signal is big enough. The duration of the output pulses is decided by the quantity of phase shift. The phase detector can be looked upon as an AND gate that has the output which is ON only when the two input pulses are ON and it is OFF if either one or both of the inputs are OFF.
Fig.2 displays the usual waveforms included in a quadrature demodulator. In the situation when there is no modulation, the two input electronic signals are completely 90° out of phase and will supply an output pulse with the signified width. If the frequency modulation electronic signal frequency increases, the amount of phase shift decreases, producing in a wider output pulse. The wider pulses averaged by the RC filter create a bigger average output voltage, that conform to the higher amplitude needed to make the higher carrier frequency. If the electronic signal frequency reduces, very large phase shift and narrower output voltage, which conforms to the original lower-amplitude modulating electronic signal.
How the ITU ensure compatibility between the Radio systems used in different countries?
ITU is an organisation that manages standards for information communication systems.
It is an agency of the UN that is based in Geneva in Switzerland. The organisation have 189 member states that usually meet to support cooperation and work out national interests. Numerous panels of the ITU put forward the standards for different areas within the communication sector. The ITU makes it possible for different countries to come together and discuss how the frequency spectrum is to be separated up and shared. Since a lot of the electronic signals generated in the spectrum do not carry for long distances, different states can then utilise these frequencies at the same time without interference. Some ranges of the frequency spectrum can transport the electronic signals all over the world. Because of this, different states ought to talk over with one another to formulate the usage of the high-frequency spectrum to avoid the interference.
Furthermore, there are standards, specifications and guidelines that countries pursue to guarantee compatibility between sending and obtaining the equipment in communication systems. Moving on, a lot of techniques are being implemented to be operated to modulate, multiplex, and process the intelligent electronic signal to be sent. In the situation that each system operated various techniques produced at the whim of the engineer, the systems will then be incompatible with one another and no communication could happen. Standards are implemented and pursued in order to ensure the compatibility when the electronic equipment is manufactured.
Adding to that, standards are clear and specified of rules of operations. They are the blueprints for manufacturing, and techniques of measurements that prove the description to the communication equipment. For instance: modulations techniques, multiplexing techniques, word length and bit formats, data transmission speeds, line coding methods, and cable and connector types are some of the specifications covered.
The ITU have many sectors. One of the sectors is known as radio-communication (ITU-R). Handling the international radio-frequency spectrum and satellite orbit resources , guaranteeing the rational, equitable, efficient and economical use of the radio-frequency spectrum by all the radio-communication services is the main task that is carried out by the ITU Radio-communication Sector (ITU-R).
In carrying out this task the ITU-R looks forward at making the conditions for the development and improvements of available and new radio-communication systems.
Also, another aim is to guarantee that the operations of radio-communication systems will take place without the interference. This is accomplished through the implementation of the Radio Regulations and Regional Agreements, and timely update of these instruments during the developments of the World and Regional Radio-commununication Conferences. In addition to that, radio standardization creates the suggestions purposed to guarantee the required accomplishments and excellence in using radio-communication systems. Moving on, it looks for methods to maintain spectrum and guarantee of the flexibility for future development and expansion.
Furthermore, the standards are implemented and managed by different organizations all over the world. ITU coordinate the worldwide and regional meetings to bring together representatives of governments and the telecommunications and ICT industry to swap opinions, knowledge and technology. During these meetings, they set up and comply upon the standards that are then made known to other to utilize and they also come to attain general agreement on the radio regulations and have systematic regulations for notifications, co-ordinations and registrations of radio frequencies in order to prevent dangerous interference between radio stations of various countries.
Why is regulation so important?
The regulation process is vital since it provides an environment for diverse countries to work together and exchange views and thoughts and the technology. Also, the regulation procedure makes sure that there is the compatibility between the Radio systems and other communication systems operated in diverse countries.
Also, the regulation is important because it guarantee that the operations of radio-communication systems will take place without the interference and makes possible the updating of the electronic instruments during the developments of the World and Regional Radio-commununication Conferences.
It presents the conditions for the improvement of available radio-communication systems and development of new radio-communication systems.
Finally, the regulation system supplies methods to maintain the spectrum and presents the guarantee of the flexibility for future development and expansion.