Part IA Paper 3 Linear Circuits and Devices

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Part IA Paper 3Linear Circuits and Devices
A simple radio receiver David Holburn
dmh_at_eng.cam.ac.uk

• You can find a pointer to an HTML version of this
  presentation at http//www.eng.cam.ac.uk/dmh

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Simplest AM radio receiver

• The simplest possible radio receiver
• Often called a crystal set (historic origin)
• Tuner - resonant LC circuit selects required
  frequency (station)
• Detector - separates modulated audio signal from
  high frequency carrier
• Earphone converts electrical signal to sound

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Historic Radio Receiver (Crystal Set)
Above the detector - a fine crystal of galena
(lead sulphide). A a fine wire or cat's whisker
rests gently on its surface to made a rectifying
contact.

• The inductor is clearly visible at the rear of
  the baseboard. You can see the detector at left
  front. Terminals for headphones are visible on
  the right.

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Modern Crystal Sets
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Crystal Set Schematic

• Heres the schematic
• A parallel LC resonant circuit is used to select
  the required frequency
• A pn junction diode is used as detector
• Capacitor Cf bypasses radio frequencies to Earth
• Lower frequency audio signals pass through the
  headphones
• The headphones behave like a high value resistance

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Analysis of LC circuit with pSpice

• L1 and C1 chosen for resonance in Medium Wave
  (550 1600 kHz) band
• V1 represents antenna signal coupled to the
  resonant circuit via C3
• R4 is the inductors resistance
• R1 is in JFET amplifier (gate resistor)

• Output plotted vs. frequency of 100?V incoming
  signal
• Note the narrow steep-sided resonance curve

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Modulation

• Just transmitting a radio wave does not convey
  any useful information, but it can be used as a
  carrier, to carry a wanted signal.
• A simple way to send information using a carrier
  - switch it on and off
• To transmit speech or music, something more
  elaborate is needed
• Amplitude modulation is a simple modulation
  technique in which the amplitude of the carrier
  is varied in sympathy with the signal.
• Special forms of amplifier can be used for this
  purpose.
• Nowadays many other more complicated modulation
  schemes are in use.

Carrier wave
Signal
Modulated Carrier
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Detector

• Receiver LC circuit selects the desired carrier
  wave
• Detector is required to extract the signal from
  the modulated carrier
• Without this, the high-frequency alternating
  signal would not produce any audible output from
  the headphones.

• A pn-junction diode detector is used
• Basic property of diode current flows in only
  one direction (rectification)
• ve half-cycles blocked by the diode
• ve half-cycles pass unimpeded

pn-junction diode

• Capacitor smooths resultant rectified waveform
• Headphones convert electrical signal back into
  sound