Part IA Paper 3 Linear Circuits and Devices A simple radio receiver David Holburn dmh@eng.cam.ac.uk You can find a pointer to an HTML version of this presentation at: http://www.eng.cam.ac.uk/~dmh 1 1 1 1

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

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 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

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

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

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