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Saturday, 29 May 2021

Radio Frequency Bandwidth of the Signal

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Radio Frequency Bandwidth of the Signal

The frequency range (i.e., bandwidth) required for a given transmission is depend on the bandwidth occupied by the modulation signals itself. For example, a high -fidelity audio- signal which occupies the range 50 Hz to 15 KHz requires a bandwidth of 300 to 3400 Hz for a telephone conservation. If the same carrier has been similarly modulated a more bandwidth signal the modulated wave bandwidth will also increases. However the transmitted signal bandwidth need not be exactly same as the bandwidth of original signal. Also it is necessary to know the bandwidth of modulating signal itself before finding the bandwidth of modulated signal. If the orrginal signals are sinusoidal in nature, the bandwidth is simply the frequency range between the lowest and the highest sine-wave signal. This creates no difficulty. However, if the modulating signals are non-sinusoidal in nature, then much more complexity arises.

                                                                

In the selection of a particular carrier frequency for a given application, there are number of considerations, but most important is the width of the frequency band covered by the signal components. In television broadcasting, where each radio frequencies channel width is 6 MHz, high carrier frequencies must be employed for interference free reception. A similar requirement applies to frequency modulated sound broadcasts. Radio frequency bandwidths covered by different types of signal and the carrier frequencies at which signals are normally transmitted are shown in table

 

Sl No:

Type of Signal

Radio Frequency Transmission Bandwidth

Typical Carrier Frequency Ranges

1

Telegraphy signals

80 Hz – 2 KHz

18 KHz – 30 MHz

2

Telephony signals and AM signals

10 KHz

500 KHz – 30 MHz

3

FM signals

150 KHz

88 MHz – 108 MHz

4

Facsimile signals

6 KHz

500 KHz – 30 MHz

5

Television signals

6 MHz

54 MHz – 216 MHz

6

Radar signals

2 MHz – 10 MHz

200 MHz – 30,000 MHz

.

Radio waves Classification

 

The frequencies, used for radio communication will range from 15 KHz to more than 30000 MHz. The selection of a particular carrier frequency for a particular application depends upon number of factors: From the Electromagnetic -Spectrum which extends through audio frequencies (20 Hz to 15 KHz), radio frequencies (15 KHz to 300,000 MHz), the infra-red region, the visible light region, the ultraviolet region and X-rays, γ-rays, cosmic rays etc., our interest at present for radio communication purposes is specially with radio frequencies and with audio frequencies to some extent. The radio and audio frequency range is subdivided as shown in table below. The wavelength can be find by the using the equation f = c/λ, where c is velocity of light, f is frequency and λ wavelength.

 

Sl No:

Classification

Frequency range

1

Audio frequency, AF

20 – 2500 Hz

2

High Audio Frequency, HAF

2500 – 5000 Hz

3

Very Low Frequency, VLF

10 – 30 KHz

4

Low Frequency, LF

30 – 300 KHz

5

Medium Frequency, MF

300 – 3,000 KHz

6

High Frequency, HF

3,000 – 30,000 KHz (3 – 30 MHz)

7

Very High Frequency, VHF

30 – 300 MHz

8

Ultra High Frequency, UHF

300 – 3,000 MHz

9

Super High Frequency, SHF

3,000 – 30,000 MHz (3 – 30 GHz)

10

Extremely High Frequency, EHF

30 – 300 GHz

 

Note: Frequencies more than about 2000 MHz are generally referred to as micro wave frequencies.


Sunday, 23 May 2021

Single Sideband Amplitude Modulation

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Single Sideband Amplitude Modulation

The general AM equation shows that when a carrier is amplitude-modulated by a single sine wave, the resulting signal consists of three frequencies — the original carrier frequency, the USB (fc + fm) and the LSB (fc — fm). Out of this, the carrier itself does not carry any information, and hence the power transmitted in the carrier signal will have no use. The two sidebands carry the same information. We need only one sideband at the receiver for the demodulation of the AM signal. We can improve the efficiency of the transmission by transmitting only one sideband (upper or lower one can be used). The resulting signal is referred to as single sideband SSB. As we know, the power transmitted,

 

PT = PCARR (1+ m2/2) = PCARR + PCARR m2/2

 

That is about 2/3rd of the power is transmitted by the carrier, which is a wastage. For the highest modulation index (m = 1), the efficiency of transmission is 33%. Under these conditions, 67% of power is transmitted by carrier, and this much power is waste. For values of m < 1, the efficiency is less than 33%. If the carrier is suppressed, only the side band power remains. As this is only PCARR m2/2 , a 2/3rd saving is effected at 100% modulation, and even more is saved as depth of modulation is reduced. If one of the sidebands is now also suppressed, the remaining power is PCARR m2/4 a further saving of 50% can be achieved over the carrier suppressed AM.

 

Single-sideband transmission (SSB) is a method of transmitting signals based on amplitude modulation in which only one sideband is transmitted. Essentially, the carrier and one sideband of an AM signal are removed, leaving only the other sideband. Assuming both sidebands are symmetric, no information is lost in the process. The required signal bandwidth is reduced and, since the final RF amplification is concentrated in a single sideband, effective power output is greater than normal AM. The carrier and removed sideband account for well over half of the power output of an AM transmitter.

 

To decode the signal at the receiving end the original AM mode is synthesized by adding a carrier signal to the lone sideband. The signal can then be demodulated as a standard AM signal. Because the synthesized carrier is locally generated, it of much higher quality than a transmitted one, which contributes to a higher quality received signal. An SSB signal cannot be demodulated by standard AM receivers because of the lack of a reference carrier signal. The major advantages of SSB over normal AM are

 

1. The power saving - Since the carrier is not transmitted, there is a reduction by 50% of the transmitted power. In AM, at 100% modulation, 1/2 of the power is comprised of the carrier; with the remaining (half) power in both sidebands.

 

2. Because in SSB, only one sideband is transmitted, there is a further power reduction by 50%.

 

3. Since SSB has only one sideband, the bandwidth required is only half than that required for normal AM.

Sunday, 16 May 2021

AM Modulator Block Diagram

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AM Transmitter Block Diagram

Note: Class C RF output amplifier is used in High Level Modulation.

Class B RF linear power amplifier is used in Low Level Modulation.

 

The general block diagram of an AM modulator is shown in the figure. A tuned circuit is used to generate AM. It is probable to create the output current of a class C amplifier relative to the modulating voltage by the application of this voltage in series by means of some of the 'dc' supply voltages for this amplifier.

 

In an AM transmitter, amplitude modulation can be done at any point after the radio frequency source. If the output stage in a transmitter is collector modulated, the system is known as high level modulation. In high level modulation, the power amplification of the RF signal takes place before modulation. If modulation is applied to any other point (base or emitter), the system is known as low level modulation. In low level modulation, the power amplification of the RF signal takes place after modulation.

 

The RF oscillator generates the radio frequency wave for modulation (i.e. the carrier wave). This is generally a crystal oscillator because it can generate highly stable frequency. The oscillator frequency shall not be affected by the loading of the next stages. Hence a buffer amplifier is required at the oscillator output. It will be a class A amplifier. The RF power amplifier raises the power of the RF signal to the required level for modulation. It will be a class-C amplifier.

 

The audio signal to be transmitted is applied to microphone. Microphone will converts sound wave into electrical audio frequency waves. Microphone is an audio transducer. The output signal from the microphone is very small in amplitude. The signal voltage must be raised to the sufficient level before power amplifications. AF pre-amplifier is used for this purpose. It will be a class A amplifier. The power of the AF signal must be raised to the required level before modulation AF power amplifier raises the power of the audio signal to the required level. It will be a class B amplifier.

 

The AM modulating amplifiers will modulate the RF signal by the AF signal from the output of the AF power amplifier. The transmitting antenna radiates the RF power from the output of the modulator into space.

Thursday, 13 May 2021

FM Transmitter Block Diagram with Explanation

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FM Transmitter Block Diagram with Explanation of Each Block

Block diagram of a low level FM broadcast transmitter is shown in figure. The master oscillator generates the RF signal (carrier) required for modulation. Master oscillator is generally a well defined LC oscillator. The buffer amplifier is used to make the oscillator frequency free from the loading of the next stages.


The frequency modulation is achieved by the reactance modulator. In reactance modulator, a voltage variable reactance is placed across the tank circuit of the master oscillator. It can be a varactor diode, FET, BJT or vacuum tube. The tank is tuned so that the oscillating frequency is equal to the desired carrier frequency. When we apply the modulating voltage, the reactance of the voltage variable reactance changes and hence the tank circuit reactance also varies. This changes the frequency of oscillation of the oscillator. When the modulating voltage is increasing positively, the oscillator frequency is also increasing. When the modulating voltage is zero, there is no reactance oscillation and the frequency of oscillation remains unchanged. When the modulating voltage is increasing negatively, the oscillator frequency will be decreasing. Thus the frequency modulation is obtained.


Since the reactance modulator operates on the tank circuit of an LC oscillator, the master oscillator cannot be crystal controlled. But it must have the stability of a crystal oscillator since it is the part of a commercial transmitter. If the frequency of the master oscillator shifts, the output frequency of the whole system must drift equally. Hence automatic frequency control (AFC) must be employed.


In AFC circuit, the master oscillator frequency is mixed with the frequency obtained from a crystal oscillator. The crystal oscillator will be tuned such that the resulting difference frequency of the output of the mixer will have usually about 1/20th of the master oscillator frequency This intermediate frequency (IF) signal from the output of the mixer is amplified by the IF amplifier and fed to the input of a phase discriminator. The discriminator output will be zero when the master oscillator frequency is equal to the carrier frequency (Centre frequency).


When the master oscillator frequency increases the discriminator produces a positive DC voltage. This voltage is fed in series with the reactance modulator. Then the master oscillator frequency decreases correspondingly. If the master oscillator frequency decreases, the discriminator output will be a negative DC voltage and the master oscillator frequency increases correspondingly. The phase discriminator will not react to normal frequency changes due to frequency modulation, since they are very small frequency changes. Discriminator will react only to slow changes in the incoming frequency. The RF power amplifier raises the power of the frequency modulated signal to a required level for transmission through the antenna. It will be a class C amplifier. The transmitting antenna radiates the RF power in to space.

Saturday, 8 May 2021

FM Receiver Block Diagram with Explanation

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Block Diagram of FM Receiver with Explanation

Standard broadcast for FM is 88-108 MHz. The maximum permissible deviation is 200 KHz. In FM the intermediate frequency is 10.7 MHz. In FM the operating frequencies are much higher than that in AM. It additionally contain a de-emphasis and limiter circuit. The method of demodulation is totally different for the methods used in AM detection. A superheterodyne FM receiver is shown in figure.

 

The first section is the RF section, which is a tunable circuit connected to the antenna terminals. It is used to select only the desired RF signal out off a number of frequencies to the receiver. The RF amplifier is a tuned voltage amplifier and it contains a parallel LC tunes circuit. This tuned circuit selects the desired RF signal from a number of frequencies to the receiver.

 

In the mixer the incoming signal frequency is mixed with the frequency generated by a local oscillator to convert it into a lower fixed frequency called intermediate frequency. It is 10.7 MHz in FM receivers. The local oscillator will be a high frequency oscillator. The RF amplifiers and the local oscillator are tunes together, so that difference frequency at the output of the mixer will be equal to the intermediate frequency. The local oscillator frequency always kept above the signal frequency by an amount equal to IF.

 

The output of the mixer is applied to the IF amplifier stages. The intermediate frequency and the bandwidth required in FM are higher than that in AM receivers. Typical bandwidth for a receiver operating in 88-108 MHz and IF of 10.7 MHz is 200 KHz. Two IF amplifier stages are often provided.

 

FM demodulation is totally different from AM demodulation. Balanced slope detector, Foster-Seeley discriminator and Ratio detectors are common types of demodulators used for FM detection. De-emphasis circuit is used to attenuate the high frequencies in order to compensate the boosting at the transmitter.

 

The amplitude of the FM signal remains constant. But by traveling from the transmitter to the receiver antenna, external sources produce unwanted variations in the signal amplitudes. These variations are easy to detect because the amplitude of the original FM signal remains constant. The limiter is a form of clipping device that does not produce an output, when the positive or negative amplitude of the FM signal exceeds a pre-determined level. So FM receivers can be integarated with amplitude limiters to take away the amplitude variation caused by noise. Hence FM reception is more immune to noise than AM reception.

 

There are different methods of obtain AGC in an FM receiver. The limiter user has leak type bias; this bias voltage changes proportional to the input voltage and is thus used for Automatic Gain Control. Occasionally a further Automatic Gain Control detector is used which takes positive output of the final IF amplifier and it rectifies and filters in the common manner.


Wednesday, 5 May 2021

Superheterodyne Receiver Block Diagram

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Superheterodyne receiver is the most popular type of radio receiver. In superheterodyne receiver, the selected RF signal is converted to a lower fixed frequency called intermediate frequency (IF) by the process of heterodyning (mixing) of two frequencies. This intermediate frequency is 455 KHz in AM receivers. The IF signal conveys the same information as the selected RF signal. The block diagram of a superheterodyne AM receiver is shown in the figure.

RF amplifier


The first section of a radio receiver is always a RF section, which is a tunable circuit connected to the antenna terminals. It is used to select only the desired RF signal out off a number of frequencies to the receiver. The RF amplifier is a tuned voltage amplifier and it contains a parallel LC tunes circuit. This tuned circuit selects the desired RF signal from a number of frequencies to the receiver.


The second purpose of the amplifier is the pre-amplification of the RF signal before mixing. When the RF signal reaches the antenna, a very weak voltage is induced in it. It is necessary first to amplify the RF signal to a suitable level for further processing. Usually only one RF amplifier stage is used in superheterodyne receivers.


Mixer


The amplified RF signal is fed to the mixer stage where the RF signal is mixed with a high frequency generated by a local oscillator. It contains a LC tuned circuit. Usually the local oscillator and RF amplifier are tuned together with the help of a gang capacitor. In standard broadcast receivers, the local oscillator frequency is always made higher than the RF signal frequency. The Mixing of the selected RF signal (fs) and the local oscillator frequency (fo) produce two additional frequencies — ( fs + fo) and ( fs — fo ). The difference in frequency (fs — fo) is always adjusted to be 455 KHz which is known as intermediate frequency, IF.  In the mixer stage, with the help of gang capacitor, the local oscillator frequency is always adjusted to be above the RF signal frequency by an amount equal to the intermediate frequency, 455 KHz.


That is, IF = Local oscillator frequency — Selected RF frequency


In standard broadcast receivers, the local oscillator frequency fo is made higher than the incoming signal frequency fs by an amount equal to the intermediate frequency fi.


i.e, fo = fs + fi  or  fs = fo – fi


Now a frequency fsi manages to reach the mixer, such that


fsi = fo + fi = fs + 2fi


This frequency will also produce an intermediate frequency fi when mixed with the local oscillator frequency fo. This has the effect of two stations have receiving simultaneously and is naturally undesirable. The frequency fsi is called the image frequency. The image frequency rejection of a radio receiver depends on the selectivity of the RF amplifiers and must be achieved before the IF stage. Once the spurious frequency enters the first amplifier, it becomes impossible to remove it from the wanted signal.


IF amplifier stage


The signal with frequency 455 KHz from the output of the mixer stage is applied to the IF amplifier stage IF amplifiers are fixed tuned amplifiers which are tuned to the intermediate frequency. These amplifiers give very high amplification to IF signal. To improve the gain and bandwidth, two or three IF amplifier stages are used. Each stage uses a pair of mutually coupled tuned circuit which is tuned to the required IF. They are called intermediate frequency transformer, IFT. The factors influencing the selection of intermediate frequency are


1. If IF is too high, selectivity and adjacent channel rejection become poorer and tracking become difficult.

2. If IF is low, image frequency rejection become poorer. It becomes worse if signal frequency is raised.

3. If IF is too low, selectivity becomes too sharp and resulting in cutoff of sidebands. Also high frequency stable local oscillator is also needed.


The value of IF for


• AM — KHz

• FM — 10.7 MHz

TV — 36 & 46 MHz

µ-wave & radar — 30, 60, 70 MHz


IF will influenced by high and low values. Hence a compromise is needed. So there will be two IF stages — the first one with a high IF value and the second one with a low IF value. The high IF pushes the image frequency farther away from the signal frequency and therefore permits much better attenuation of it. The second lower IF has all the properties of low fixed operating frequency, particularly sharp selectivity and hence good adjacent channel rejection.


Demodulator


The output of the IF amplifier is applied to the envelope detector where the audio signal is extracted from the AM signal. Diode is commonly used for AM demodulation. RF signal is suppressed by the filter circuit.


Automatic gain control (AGC) or Automatic volume control (AVC)


While we are tuning a radio receiver, the signal strength of different stations will be different. So the volume control has to be readjusted each time the receiver is tuned from one station to another. So the automatic gain control (AGC) or Automatic Volume Control (AVC) is employed in all modem receivers. AGC is a system in which the overall gain of radio receiver is changed automatically with change in strength of receiving signal in order to keep the output constant. The negative DC voltage obtained at the output of the AGC filter (in the envelope detector) is proportional to the receiving signal strength. This negative DC voltage is used for obtaining automatic gain control in simple AGC system. The negative DC voltage applied to a selected number of RF and IF stages. Negative bias voltage reduces the gain of the stages to which AGC is applied. Since the AGC voltage is proportional to the signal strength, when the signal strength is high, the AGC voltage produced will also be high and the reduction in gain will be high. When the signal strength is low the AGC voltage will also be low and there is less reduction in gain. Thus the overall gain of the radio receiver remains substantially constant.


Advantages of super heterodyne receiver


1. Better selectivity and better adjacent channel rejection.

2. Improved sensitivity

3. Gain is stable and no bandwidth variations over the tuning range.


Generation of the intermediate frequency in the mixer stage is an important characteristic of superheterodyne receiver. At this frequency, higher amplification and better gain stability can be obtained. In a superheterodyne receiver most of the selectivity and amplification is produced by the IF amplifiers. The most important factor regarding the sensitivity of a radio receiver is the gain of the intermediate frequency amplifiers. The IF amplifier provides most of the gain and therefore the sensitivity of the receiver. Since IF amplifiers use double tuned circuit there is no bandwidth variations over the tuning range. Since the characteristics of the IF amplifiers are independent of the frequency to which the receiver is tuned, the selectivity and the sensitivity of the superheterodyne receiver is made throughout its tuning range.

Tuesday, 4 May 2021

Tuned Radio Frequency (TRF) Receiver Block Diagram

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The TRF receiver is the simplest type of AM radio receiver. The block diagram of a Tuned Radio Frequency (TRF) receiver is shown in the figure. Infinite number of transmitters installed throughout the world radiates radio waves in space. In general, these transmitters radiate different frequencies. Electromagnetic waves surrounding an antenna will induce currents of their frequency in the antenna. A provision should be there in the receiver to select only the desired RF signal out of a number of frequencies to the receiver. This function of selecting the desired RF signal and rejecting the rest is achieved by the tuned voltage amplifiers in the RF amplifier stage. Tuned RF amplifiers contain a parallel LC tuned circuit. The desired RF signal is selected by the tuned circuit.

When the RF signal reaches the receiving antenna, a very weak voltage is induced in it. It is not possible to extract the audio signal from this voltage. It is necessary first to amplify the RF signal to a required level. This is achieved in a radio receiver with the help of tuned RF amplifier. Thus RF amplifier serves two purposes.


1. Selection of desired RF signal

2. Amplification of the selected RF signal to a suitable value. Usually two or three tuned RF amplifier stages are used.


The amplified RF signal is applied to the detector or demodulator stage where the audio signal is extracted from the audio signal. Diode detectors are the most common detector used for AM detection.


The demodulated signal amplitude will be very small in amplitude. In order to drive a loudspeaker, it must be first amplified. The audio amplifier stage includes the audio voltage and power amplifier. The audio voltage amplifier will be a class A amplifier and the power amplifier will be a class B push-pull amplifier. The voltage and power level of the audio signal from the output of the detector is raised in this stage. The signal gets sufficient energy to drive the loudspeaker. The output of the audio amplifier stage is applied to the loudspeaker. It will reproduce the original sound by converting the electrical audio frequency waves into sound waves.


Advantages of a TRF receiver


1. It is simple to design.

2. Its alignment is very easy.


Disadvantages of a TRF receiver


1. Poor selectivity and hence insufficient adjacent channel rejection.

2. Poor sensitivity

3. Instability of gain and bandwidth variation over the tuning range.


TRF receiver is suffered from the variation of the bandwidth over the tuning range. So the receiver will pickup adjacent stations as well as the one to which it is tuned. i.e., the selectivity of the TRF receiver varies with frequency. In TRF receiver the amplification of the signal also varies with the frequency. So the TRF receiver suffers from the instability of gain. These entire problems can be solved using superheterodyne receivers.


Significance of tuned amplifiers in radio receivers


Infinite number of transmitters installed throughout the world radiates radio waves in space. In general, these transmitters radiate different frequencies. Electromagnetic waves surrounding an antenna will induce currents of their frequency in the antenna. A provision should be there in the receiver to select only the desired RF signal out of a number of frequencies to the receiver. This function of selecting the desired RF signal and rejecting the rest is achieved by the tuned voltage amplifiers in the RF amplifier stage. Tuned RF amplifiers contain a parallel LC tuned circuit. The tuned RF amplifier contains a parallel tuned LC circuit. The RF amplifier also amplifies the selected RF signal to a suitable level. Also four frequencies are present at the output of the mixer.


1. Local oscillator frequency

2. The selected RF frequency

3. Local oscillator frequency + RF signal frequency

4. Local oscillator frequency – RF signal frequency


From these four signals, we have to select the difference frequency which is equal to 455 KHz. The fixed tuned amplifiers in the IF section are tuned to 455 KHz to select only this frequency and provides a high gain. Thus tuned amplifiers play a very significant role in radio reception.

Sunday, 2 May 2021

Main function of Radio Receiver

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Main functions of a radio receiver are,

1. Selection of RF signals from a number of different frequencies surrounding the antenna.


2. The amplification of selected RF signal.


3. Demodulation of audio signal from the modulated wave.


4. Amplification of the audio signal after detection to drive the loudspeaker.


Performance of a Radio receiver


The performance of a radio receiver depends on the following factors.


Sensitivity: Sensitivity of a radio receiver is its ability to amplify weak signals. It is defined as the minimum amount of radio frequency input voltage required to produce a desired amount of audio output. It is expressed in microvolt or in decibel. Sensitivity varies over the tuning band. In superheterodyne receivers, the factors determining the sensitivity are gain of IF and RF amplifiers and noise figure.


Selectivity: Selectivity of a radio receiver is called as its capability to select the signal of desired frequency and to reject the rest of the unwanted signal. It is selectivity, which determines the adjacent channel rejection of a receiver. Adjacent channel is a range of frequencies that just above or below the required channel. It is the characteristic that determines the extent to which the receiver is capable of distinguishing between the desired signal and signals of other frequencies. It varies with receiving frequency and become worst when the receiving frequency is raised. In superheterodyne receivers, selectivity mainly depend on the response of the IF section. RF section and mixers play a small part.


Fidelity: The ability of a radio receiver to reproduce the original sound exactly is called fidelity.