M3 Physics

• is an electronic device that receives and converts the desired radio waves back to audio or visual information. The receiver’s antenna detects and captures radio waves. The radio receiver can select the information required from the radio signals. The illustration below shows the components of a radio receiver.

  Radio Receiver
• It is a piece or length of wire. It captures radio waves. When exposed to radio waves, the wave induces a minimal alternating current in the antenna.

  Antenna
• It is a sensitive amplifier that amplifies the very weak radio frequency (RF) signal from the antenna to be processed by the tuner.

  RF amplifier
• It extracts signals of a particular frequency from different frequencies. The antenna captures radio waves of all the frequencies and sends them to the radio frequency (RF) amplifier, which amplifies them all.

  Tuner
• It is for the audio information to be separated from the carrier wave. A rectifying diode rectifies the alternating current signal in the AM signals. A direct current signal that feeds to an audio amplifier circuit is what remains on the alternating current once passed into a rectifying diode. The detector circuit is a bit more complicated for FM signals.

  Detector
• The weak signal that comes from the detector is amplified by this component using a simple transistor amplifier circuit.

  Audio Amplifier
• Despite the many designs of the radio receiver circuit, they share common but important features

  1. sensitivity
  2. selectivity
  3. fidelity
• Selectivity measures a radio receiver's performance to respond only to the radio signal tune in (such as a radio station) and to reject other signals nearby, such as another broadcast on an adjacent channel
• Receiver Sensitivity identifies and amplifies signals at the input of the receiver. It tells us the weakest signal that a receiver will be able to identify and process.
• The fidelity of a receiver is the ability to accurately reproduce, in its output, the signal that appears at its input. The wider the band passed by the frequency selection circuit, the greater is the fidelity.
• There are different types of a radio receiver, but the features and principles of operations are classified into two, namely:

  1. tuned radio frequency receiver or (TRF)
  2. Supersonic heterodyne radio receiver
• This was the first radio receiver used. It consists of a tuned circuit and a detector. Tune radio frequency was used in the early years of wireless technology.

  Tuned radio frequency (TRF)
• Tune Radio Frequency receiver consists of three main sections:

  1. Tune radio frequency stages,
  2. signal detector,
  3. audio amplifier
• These consist of one or more amplifying and tuning stages. Early sets often had several stages, each proving some gain and selectivity.

   

  Tuned radio frequency stages
• The detector enables the audio from the amplitude modulation signal to be extracted. It uses a diode to rectify a signal.

  Signal detector
• This is where the audio signal amplifies.

  Audio Amplifier
•  

  The TRF receiver gained popularity in 1920’s; however, the tuning took a little while for each stage in the early radios requires separate adjustment. Later superheterodyne receiver gained more popularity.

   
• changes the frequency through mechanical or electrical means. Adding one more tuned circuit allows the selection of one station at a time. Increasing one or more turned circuits will increase the selectivity of the radio receiver. Tuned amplification includes increased sensitivity and increased selectivity.

  Tuning
• reconstructs the information carried by the radio waves. The process of extracting the original information from the carrier wave is called demodulation. A diode, a capacitor, and a resistor make up the detector circuit. It removes high-frequency components. The AM demodulation has two stages.  First is the rectification of the amplifier by a diode. Second, the capacitor smooths out the amplitude of the rectified signal. The audio signal is the result of the two stages that will be sent to the audio amplifier to further amplify in the audio frequency amplifier (AF).

  Detector
• TRF receivers are easier to design. It has high sensitivity allowing broadcast frequency from 535 kHz to 1705 kHz. Problems of TRF receivers include difficulty in designing at very high frequency, poor audio quality, instability, and poor selectivity.
• It was developed to provide an additional level of selectivity. This uses a heterodyne or mixing process to convert signals done to a fixed intermediate frequency

  Superheterodyne Receiver 
• Tuning of this radio is through the effective changing of the local oscillator. Broadcast radio receivers, televisions, short wave receivers, and commercial radios have used the superheterodyne principle as the basis of their operation. It was one of the most successful forms of radio being used almost exclusively as the RF circuit design. It was Invented in 1918 and overcoming the selectivity issue making it popular for nearly 100 years.
• The superheterodyne was used in every form of radio from domestic broadcast radios to walkie talkies, television sets, through to hi-fi tuners, and professional communications radios, satellite base stations, and much more.

  superheterodyne
• Ganged tuning


  • Tuning amplifiers that will tune a single IF
• Receiving antenna


  Captures and transforms radio waves to an intermediate frequency (IF)
• Heterodyning


  Process of mixing or converting frequencies
• Intermediate frequency (IF)


  An electronic frequency that is intermediate between the input frequency and the output frequency
• Detector


  Reconstructs the information brought by the IF signal
• Local oscillator


  Provides a signal to mix with signals coming from the antenna
• RF amplifier


  Amplifies the signal and sends it to the mixer
• Selectivity in the IF amplifier


  • Some frequencies are rejected
• Mixer


  Circuit that generates an output signal which contains the sum and difference of the frequency of the two inputs
• AM broadcast band has the standard IF is 455 kHz. FM broadcast and television broadcast bands have standard IFs of 10.7 MHz and 44 MHz, respectively.
• The output from the detector may need amplification. A speaker is required to listen to the speech or music carried by the signals.