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Note for Analog Communication Systems - ACS by Adi Seshu

  • Analog Communication Systems - ACS
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Receivers Introduction :  Transmitted signals receive at receiving antenna.  Select the desired station signal and reject all other unwanted signals.  Amplify selected signal.  Demodulate the amplified signal.  Original modulating signal is power amplified.  Power amplified signal drives the loud speaker.  Loud speaker converts electrical signal into original sound information. AM Receivers Types:  Tuned Radio Frequency Receiver  AM Super heterodyne Receiver Tuned Radio Frequency (T.R.F) Receiver A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is usually composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and an audio frequency amplifier. Typically a TRF receiver would consist of three main sections: 1. Tuned radio frequency stages: This consisted of one of the more amplifying and tuning stages. Early sets often had several stages, each providing some gain and selectivity. This stage generally two or three RF simplifiers all tuning together along with detector. This is called Ganged tuning. This tuning section selects and amplifies the incoming frequency and simultaneously rejects all unwanted frequencies. Prepared by –M.Adiseshaiah,M.Tech,(Ph.D)., Assistant Professor , DVR&Dr HS MIC College of Technology

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2. Signal detector: The detector enabled the audio from the amplitude modulation signal to be extracted. It used a form of detection called envelope detection and used a diode to rectify the signal. 3. Audio amplifier: Audio stages to provide audio amplification were normally, but not always included. Disadvantages of TRF receiver : • Instability associated with high gain being achieved at one frequency by multistage RF amplifiers. If such an amplifier has a gain of 20,000 then if a small portion of the output leaked back to the input of the RF stage, then positive feedback and oscillation will result. Forcing the device to work as an oscillator instead of an amplifier.This type of leakage could result from stray capacitance coupling between to input and output terminals. • Poor selectivity. It is difficult to achieve sufficient selectivity at higher frequencies due to the use of single tuned circuits. Because it was not possible to use double tuned RF amplifiers in these receivers. • Bandwidth variation over the tuning range . Example: In AM Broadcast system , let us consider a tuned circuit is required to have a bandwidth of 10 Khz at a frequency of 550 Khz. Then the Q of this circuit must be Q=f/BW=550/10=55. At the other end of this AM broadcast is 1650 khz. Then Q=1650/10=165. However due to various losses which depend on frequency, such large increase in Q is not possible. Practical value of Q=120; Now BW = 1650/120 = 13.7 KHz. Due to increase in bandwidth Receiver may pick up adjacent stations. • The bandwidth of a tuned circuit doesn’t remain constant and increases with the frequency increase. • The need to have all RF stages track one another • Gain is non-uniform over a wide range of frequencies. Advantages 1. They amplify defined frequency. 2. Signal to noise ratio at output is good. 3. They are well suited for radio transmitters and receiver. 4. The band of frequency over which amplification is required can be varied. Prepared by –M.Adiseshaiah,M.Tech,(Ph.D)., Assistant Professor , DVR&Dr HS MIC College of Technology

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AM Superheterodyne Receiver RF Amplifier: It receives the signal from the antenna. The tuned circuit of RF amplifier is tuned to the desired signal frequency. It selects the wanted signal and amplifies it giving the required amplification to the signal. It improves the sensitivity and signal to noise ration of the receiver. Mixer stage: The mixer or frequency changer is a non-linear device that mixes the incoming signal of frequency fs with local oscillator frequency fo and generates the sum and difference frequencies fo + fs. The desired frequency is selected by tuned circuit the frequency is called intermediate frequency IF = fo- fs. And rejects all other frequencies. Local Oscillator: This is an LC oscillator which produces a sinusoidal signal of frequency fo. A constant frequency difference is maintained between the local oscillator signal and incoming RF signal by using capacitance tuning in which the capacitances are ganged together. IF Amplifer: The IF Amplifier is a fixed frequency amplifier, with very important function of rejecting adjacent unwanted frequencies. Most of the receiver gain is provided by this stage.More than one stage of IF Amplifier is used to get a good sensitivity and selectivity. Detector: The process in which modulated signal is converted back into original modulating signal is called demodulation. Demodulation of AM signal is done by diode detector circuit. AGC (Automatic Gain Control): AGC circuit is used to maintain a constant audio output irrespective of the variations at the input of the receiver. AGC is used to adjust the receiver gain automatically. Prepared by –M.Adiseshaiah,M.Tech,(Ph.D)., Assistant Professor , DVR&Dr HS MIC College of Technology

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Audio Voltage Amplifier: The output of detector which is Audio frequency signal is amplified with respect to voltage. It is applied to Power amplifier. Power Amplifier: The amplified audio signal is further amplified by a power amplifier to get a specified power level. Then it is given to the loudspeaker with sufficient power to drive the loud speaker. Loud Speaker: The loudspeaker converts the audio signal into sound signal. ____________________________________________________________________________________ Each Block Explanation in detail RF Section: The RF signal received through antenna and fed to the RF section. This circuit block performs two main functions: (i) Tuning or Pre-selection: Broadband tuning is applied to the RF stage. The purpose of this stage is to Select desired signal frequency and reject unwanted signal frequency i.e., image frequency. It must also be able to track the local oscillator so that as the receiver is tuned, so the RF tuning remains on the required frequency. (ii) Amplification: In terms of amplification, the level is carefully chosen so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved. The advantages of having an RF amplifier are as follows 1. Greater gain, i.e., better sensitivity 2. Improved image-frequency rejection 3. Improved signal-to-noise ratio 4. Improved rejection of adjacent unwanted signals, i.e., better selectivity 5. Better coupling of the receiver to the antenna (important at VHF and above) 6. Prevention of spurious frequencies from entering the mixer and heterodyning there to produce an interfering frequency equal to the IF from the desired signal 7. Prevention of re-radiation of the local oscillator through the antenna of the receiver Prepared by –M.Adiseshaiah,M.Tech,(Ph.D)., Assistant Professor , DVR&Dr HS MIC College of Technology

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