1. Part IA Paper 3 Linear Circuits and Devices
A simple radio receiver
David Holburn
You can find a pointer to an HTML version of this presentation at: 1 1 1 1

2. 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 5

3. 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.

4. Modern Crystal Sets 5

5. Crystal Set Schematic 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

6. 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 inductor’s resistance
R1 is in JFET amplifier (gate resistor)
Output plotted vs. frequency of 100V incoming signal
Note the narrow steep-sided resonance curve 5

7. 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 5

8. 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 5