Alexander Stepanovich Popov was invented the first radio receiver in 1896 and It was based on electromagnetic waves. A radio receiver is an electronic device that picks up the desired signal, rejects the unwanted signal amplifies the desired signal and demodulates the carrier signal to get back the original modulation frequency signal.
Characteristics of Radio receiver
Selectivity : It refers to the ability of a receiver to select a signal of desired frequency while rejecting all others.The bandwidth of a tuned circuit is a measure of the selectivity.
Sensitivity : The ability of the receiver to detect the weakest possible signal is known as sensitivity.The sensitivity of receiver mostly depends on the gain of the IF amplifiers.
Fidelity : The ability of a receiver to reproduce all the frequency components faithfully in the baseband signal is called fidelity.
How does a radio receiver works ?
Fig: Block diagram of working of radio receiver
- A radio receiver needs an antenna to help it pick the transmitter’s radio waves out of the air. An AM antenna is simply a wire or a metal stick that increases the amount of metal the transmitter’s waves can interact with.
- The use of a tuner is to separate one sine wave from the thousands of radio signals that the antenna receives. In this case, the tuner is tuned to receive the 680,000-hertz signal. Tuners work using a principle called resonance. That is, tuners resonate at, and amplify one particular frequency and ignore all the other frequencies in the air. Creating a resonator with a capacitor and an inductor is easy.
- The tuner causes the radio to receive just one sine wave frequency (in this case, 680,000 hertz). Now the receiver has to extract the DJ’s voice out of that sine wave. This is accomplished with a part of the radio called a detector or demodulator. In the case of an AM radio, the sensor is built with an electronic component called a diode. A diode allows current to flow through in one direction but not the other, so it clips off one side of the wave.
- The radio next amplifies the clipped signal and sends it to the speakers. The amplifier is made of one or more transistors
- Now from the speaker DJ’s voice will come out.
What are the types of radio receivers
- Tuned Radio Frequency (T.R.F) Receiver
- Superheterodyne 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. Popular in the 1920s, it could be tedious to operate because each stage must be separately tuned to the station’s frequency. By the mid-1930s, it was replaced by the superheterodyne receiver invented by Edwin Armstrong.
Typically a TRF receiver would consist of three main sections:
- Tuned radio frequency stages: This consisted of one of the more amplifying and tuning stages. Early sets often had several stages, each proving some gain and selectivity.
- 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.
- Audio amplifier: Audio stages to provide audio amplification were normally, but not always included.
Disadvantages of radio waves
- Since they used inductor and capacitor as tuning the element, the circuit is bulky and costly.
- They are not suitable to amplify audio frequencies
- If the band of the frequency is increased, design become complex.
Advantages of radio waves
- They amplify defined frequency.
- Signal to noise ratio at output is good.
- They are well suited for radio transmitters and receiver.
- The band of frequency over which amplification is required can be varied.
Superheterodyne Radio Theory
- RF tuning & amplification: This RF stage within the overall block diagram for the receiver provides initial tuning to remove the image signal. It also provides some amplification. If noise performance for the receiver is important, then this stage will be designed for optimum noise performance. This RF amplifier circuit block will also increase the signal level so that the noise introduced by later stages is at a lower level in comparison to the wanted signal.
- Local oscillator: The local oscillator circuit block can take a variety of forms. Early receivers used free running local oscillators. Today most receivers use frequency synthesisers, generally based on phase locked loops. These provide much higher levels of stability and enable frequencies to be programmed in a variety of ways.
- Mixer: Both the local oscillator and incoming signal enter this block within the superheterodyne receiver. The wanted signal does transform to the intermediate frequency.
- IF amplifier & filter: This superheterodyne receiver block provides the majority of gain and selectivity. High-performance filters like crystal filters may be used, although LC or ceramic filters may be used within domestic radios.
- Demodulator: The superheterodyne receiver block diagram only shows one demodulator, but in reality, radios may have one or more demodulators dependent upon the type of signals being the receiver.
- Audio amplifier: Once demodulated, the recovered audio is applied to an audio amplifier block to be amplified to the required level for loudspeakers or headphones. Alternatively, the improved modulation may be used for other applications.