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Wednesday, 5 May 2021

Superheterodyne Receiver Block Diagram

 

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.

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