The Ins and Outs of Radio Receivers
- Date: 2007-07-01 - Word Count: 631
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Signal that has to be transmitted is impressed onto a carrier wave in any of the modulation methods, and it is then amplified and applied to the transmitting antenna. The modulated signal is radiated, propagated and collected by the receiving antenna .The signal at this point is generally quite weak, therefore the receiver must first amplify the received signals. Since the signal is quite likely to be complied by lots of other unwanted signals, probably at neighboring frequencies, it must be selected and the other rejected .Finally, since modulation took place in transmitter, the reverse process of this, demodulation, must be performed in the receiver to recover the original modulating voltages.
It is understood that a receiver has the function of selecting the desired signal from all other unwanted signals, amplifying and demodulating it, and displaying it in the desired manner. This outline of functions that must be performed shows that the major difference between receivers of various types is likely to be in the way in which they demodulate the received signal. This will depend on the type of modulation employed.
Of the various forms of receiver proposed at one time or another, only two have any real practical or commercial significance. They are the Tuned Radio Frequency receiver and the Superheterodyne receiver. Only the second of these is used to a large extent today, but it is easy to explain the operation of the tuned radio frequency receiver since it is simpler of two. The best way of justifying the existence and overwhelming popularity of the Superheterodyne receiver is by showing the shortcomings of the tuned radio frequency type.
Until shortly before World War II, most radio receivers were of Tuned Radio Frequency type. It is simple logical receiver; a person with little knowledge of communication would probably expect all radio receivers to have this form. The virtues of this type, which is now not used expect as a fixed frequency receiver in the special applications, are its simplicity and high sensitivity. It must also be mentioned that the tuned radio frequency receiver was first introduced; it was a great improvement on the types used previously mainly crystal, regenerative and super regenerative receivers.
In the superheterodyne receiver the incoming signal voltage is combined with a signal generated in the receiver. The local oscillator voltage is normally converted into a signal of lower fixed frequency. The signal at this intermediate frequency contains the same modulations of the original carrier, and it is now amplified and detected to reproduce the original information. The superhet has the same essential components as the receiver in addition to the mixer, local oscillator and intermediate frequency amplifier.
A constant frequency difference is maintained between the local oscillator and the radio frequency tuned circuits, normally through capacitance tuning in which all the capacitors are ganged together and operated in unison by one control knob. The intermediate frequency amplifier generally uses two or three transformers, each consisting of a pair of mutually coupled tuned circuits. With this large number of double tuned circuits operating at a constant specially chosen frequency, the intermediate frequency amplifier provides most of the gain and bandwidth requirements of the receiver. Since the characteristics of the intermediate frequency amplifier are independent of the frequency to which the receiver is tuned, the selectivity and sensitivity of the superhet are usually fairly uniform throughout its tuning range and not subject to the variations that affect the Tuned Radio Frequency receiver. The radio frequency circuits are now used mainly to select the wanted frequency to reject interference such as the image frequency and to reduce the noise figure of the receiver.
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Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect xm radio, xm satellite radio
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It is understood that a receiver has the function of selecting the desired signal from all other unwanted signals, amplifying and demodulating it, and displaying it in the desired manner. This outline of functions that must be performed shows that the major difference between receivers of various types is likely to be in the way in which they demodulate the received signal. This will depend on the type of modulation employed.
Of the various forms of receiver proposed at one time or another, only two have any real practical or commercial significance. They are the Tuned Radio Frequency receiver and the Superheterodyne receiver. Only the second of these is used to a large extent today, but it is easy to explain the operation of the tuned radio frequency receiver since it is simpler of two. The best way of justifying the existence and overwhelming popularity of the Superheterodyne receiver is by showing the shortcomings of the tuned radio frequency type.
Until shortly before World War II, most radio receivers were of Tuned Radio Frequency type. It is simple logical receiver; a person with little knowledge of communication would probably expect all radio receivers to have this form. The virtues of this type, which is now not used expect as a fixed frequency receiver in the special applications, are its simplicity and high sensitivity. It must also be mentioned that the tuned radio frequency receiver was first introduced; it was a great improvement on the types used previously mainly crystal, regenerative and super regenerative receivers.
In the superheterodyne receiver the incoming signal voltage is combined with a signal generated in the receiver. The local oscillator voltage is normally converted into a signal of lower fixed frequency. The signal at this intermediate frequency contains the same modulations of the original carrier, and it is now amplified and detected to reproduce the original information. The superhet has the same essential components as the receiver in addition to the mixer, local oscillator and intermediate frequency amplifier.
A constant frequency difference is maintained between the local oscillator and the radio frequency tuned circuits, normally through capacitance tuning in which all the capacitors are ganged together and operated in unison by one control knob. The intermediate frequency amplifier generally uses two or three transformers, each consisting of a pair of mutually coupled tuned circuits. With this large number of double tuned circuits operating at a constant specially chosen frequency, the intermediate frequency amplifier provides most of the gain and bandwidth requirements of the receiver. Since the characteristics of the intermediate frequency amplifier are independent of the frequency to which the receiver is tuned, the selectivity and sensitivity of the superhet are usually fairly uniform throughout its tuning range and not subject to the variations that affect the Tuned Radio Frequency receiver. The radio frequency circuits are now used mainly to select the wanted frequency to reject interference such as the image frequency and to reduce the noise figure of the receiver.
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Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect xm radio, xm satellite radio
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