A.M. Radio Receivers
A radio receiver is a device which reproduces the modulated or radio swells into sound swells.
In India, only breadth modulation is use for radio transmission and event.
Thus, similar radio receivers are call A.M. radio receivers. If you want to understand this full topic, you should to know about A.M. Diode Detector.
In order to reproduce the A.M. surge into sound swells, every radio receiver must perform the following functions
( i) The entering upstanding must block a portion of the fleeting radio swells.
(ii) The radio receiver must elect the asked radio surge from a number of radio swells interdicted by the entering upstanding.
For this purpose, tuned resemblant LC circuits must be use.
These circuits will elect only that radio frequence which is in reverberative with them.
(iii) The named radio surge must be amplify by the tuned frequence amplifiers.
( iv) The audio signal must be recover from the amplified radio surge.
(v) The audio signal must be amplify by suitable number of audio-amplifiers.
(vi) The amplified audio signal should be fed to the speaker for sound reduplication.
Types of A.M. Radio Receivers
A.M. radio receivers can be astronomically classified into two typesviz, straight radio receiver and superhetrodyne radio receiver.
The former was use in the early days of radio communication.
Still at present, all radio receivers are of superhetrodyne type. Straight radio receiver.
Fig-1 shows the block illustration of a straight radio receiver.
The upstanding is entering radio swells from different broadcasting stations.
The asked radio surge is name by the R.F. amplifier which employs a tuned resemblant circuit.
The named radio surge is amplify by the tuned r.f. amplifiers.
The amplified radio surge is fed to the sensor circuit. This
circuit excerpts the audio signal from the radio surge. The affair of the sensor is the audio signal
which is amplified by one or further stages of audio- modification. The amplified audio signal is fed to
the speaker for sound reduplication.
(i) In straight radio receivers, tuned circuits are use.
As it's necessary to change the value of variable capacitors ( gang capacitors) for tuning to the asked station.
Thus, there's a considerable variation of Q between the unrestricted and open positions of the variable capacitors.
This changes the perceptivity and selectivity of the radio receivers.
(ii) There's too important hindrance of conterminous stations.
The failings of straight radio receiver were overcome by the invention of superhetrodyne receiver by Major Edwin H.
Armstrong during the First World War.
At present, all ultramodern receivers use the superhetrodyne circuit.
In this type of radio receiver, the named radio frequence is convert to a fixed lower value, called intermediate frequence ( IF).
This is achieve by a special electronic circuit call mixer circuit. There's a original oscillator in the radio
receiver itself. This oscillator produces high frequence swells.
The named radio frequence is mix with the high frequence surge by the mixer circuit.
In this process, beats are produce and the mixer produces a frequence equal to the difference between original oscillator and radio surge frequence.
As explained latterly, the circuit is so design that oscillator always produces a frequence 455 kHz above the named radio frequence.
Thus, the mixer will always produce an intermediate frequence of 455 kHz anyhow of the station to which the receiver is tune.
For case, if 600 kHz station is tune, also original oscillator will produce a frequence of 1055 kHz.
Accordingly, the affair from the mixer will have a frequence of 455 kHz.
Fig-2 shows the superhetrodyne principle with a block illustration.
The named radio frequence f1 is mixed with a frequence f2 from a original oscillator.
The affair from the mixer is a difference ( i.e. f2 – f1) and is always 455 kHz anyhow of the station to which the receiver is tune.
The product of fixed intermediate frequence (455 kHz) is the salient point of superhetrodyne circuit.
At this fixed intermediate frequence, the amplifier circuits operate with maximum stability, selectivity and perceptivity.
As the conversion of incoming radio frequence to the intermediate frequence is achieve by heterodyning or beating the original oscillator against radio frequence.
Thus, this circuit is call * superhetrodyne circuit.