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Radio, AM and FM


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GCSE A2

 a

Simple Radio - System Diagram / Circuit Diagram

Radio-Simple.GIFSimple Receiver

 b

Terminology

 c

Advantages

  • It is simple to construct.
  • It's cheap.
 d

Disadvantages

  • Poor Sensitivity. This can be overcome with RF and AF amplification.
  • Very poor Selectivity because there is only one tuned circuit for filtering.
  • It is very difficult to add additional tuning stages.
    • This is because it is hard to manufacture exactly matched variable capacitors. They are very expensive.
    • This problem gets worse at higher frequencies.
 e

Carrier Wave, Information Signal and Modulation

 f

Amplitude Modulation

AM transmitters vary the amplitude of the carrier wave.

  • The amplitude of the carrier wave is proportional to the amplitude of the signal being modulated.
  • If the modulation signal frequency increases, the amplitude of the carrier changes at a greater rate.

Modulation-AM-FM.gif

  • A radio frequency carrier is needed.
  • As the audio waveform rises and falls, the carrier amplitude increases and decreases in direct proportion.
  • As the audio amplitude increases, the modulation of the carrier increases.
  • If the audio frequency rises, the carrier amplitude varies faster, in step with the audio signal.
  • The carrier frequency does not change.

AM-80.jpg

 g

AM Demodulation or Detection

  • A diode is used to pass the radio frequency signal in one direction only.
  • A low pass RC filter is used to pass the audio frequencies and decouple or smooth out the radio frequencies.
  • Silicon diodes are not ideal because 0.7 volts is needed before the diode starts to conduct.
  • Germanium diodes are better because only 0.2 volts is needed.
  • Active circuits are even better because these can mimic an ideal diode.

AM-Demodulation.jpg

 h

Frequency Modulation

FM transmitters vary the frequency of the carrier wave.

  • The frequency change of the carrier wave is proportional to the amplitude of the signal being modulated.
  • If the modulation signal frequency increases, the frequency change of the carrier changes at a greater rate.
  • The amplitude of the carrier does not change.

Modulation-AM-FM.gif

 i

FM on a Picoscope

  • The blue is the carrier.
  • The red is the sound wave being modulated.
  • Real life FM does not change the frequency so much and it would be hard to see on a trace like this.

Modulation-FM-1.gif

 j

Advantages of FM

The sound quality is better with a wider range of frequencies being faithfully reproduced.

FM is transmitted on the VHF band. There is less radio interference on this band so the signals are clearer.

FM is much less affected by amplitude noise from lightning strikes or industrial machinery so the sound is clearer.

At night AM radio signals from distant transmitters interfere with the local transmitters so AM radio is not so good.

 k

FM Disadvantages

The transmitters have a range of only 20 to 50 miles depending on the terrain so more transmitters are needed to cover the country.

AM transmitters on the Medium Frequency or Medium Wave band have a range up to several hundred miles. Fewer transmitters are needed to cover the country.

FM radios cost more to build because they are more complex than AM.

 

 

 

 

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