Part IA Paper 3Linear Circuits and Devices

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A simple radio receiver

David Holburn dmh@eng.cam.ac.uk

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

Historic Radio Receiver (Crystal Set)

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.

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.

Modern Crystal Sets

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 100V incoming signal

Note the narrow steep-sided resonance curve

Detector

Amplitude modulation

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