Transcript Paper E1 - Digital Circuits

Part IA Paper 3
Linear Circuits and Devices
A simple radio receiver
David Holburn
[email protected]
You can find a pointer to an HTML version of this presentation at:
http://www.eng.cam.ac.uk/~dmh
Simplest AM radio receiver
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The simplest possible radio receiver
Often called a crystal set (historic origin)
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Tuner - resonant LC circuit selects required frequency (station)
Detector - separates modulated audio signal from high frequency carrier
Earphone – converts electrical signal to sound
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.
Modern Crystal Sets
Crystal Set Schematic
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Here’s 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
Analysis of LC circuit with pSpice
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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 inductor’s resistance
R1 is in JFET amplifier (gate resistor)
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Output plotted vs. frequency of
100V incoming signal
Note the narrow steep-sided
resonance curve
Detector
Amplitude modulation
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Without the detector, the high-frequency alternating signal would not produce
any audible output from the headphones.
A pn-junction diode detector is used to extract the modulated audio signal
Basic property of diode – current flows essentially in one direction
 –ve half-cycles blocked by the diode
 +ve half-cycles pass unimpeded
A capacitor is needed to smooth the resultant rectified waveform
Headphones convert the electrical signal back into sound