Presentation on theme: "Part IA Paper 3 Linear Circuits and Devices"— Presentation transcript:
1 Part IA Paper 3 Linear Circuits and Devices A simple radio receiverDavid HolburnYou can find a pointer to an HTML version of this presentation at:1111
2 Simplest AM radio receiver The simplest possible radio receiverOften called a crystal set (historic origin)Tuner - resonant LC circuit selects required frequency (station)Detector - separates modulated audio signal from high frequency carrierEarphone – converts electrical signal to sound5
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.
5 Crystal Set Schematic Here’s the schematic … A parallel LC resonant circuit is used to select the required frequencyA pn junction diode is used as detectorCapacitor Cf bypasses radio frequencies to EarthLower frequency audio signals pass through the headphonesThe 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) bandV1 represents antenna signal coupled to the resonant circuit via C3R4 is the inductor’s resistanceR1 is in JFET amplifier (gate resistor)Output plotted vs. frequency of 100V incoming signalNote the narrow steep-sided resonance curve5
7 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 signalBasic property of diode – current flows essentially in one direction–ve half-cycles blocked by the diode+ve half-cycles pass unimpededA capacitor is needed to smooth the resultant rectified waveformHeadphones convert the electrical signal back into sound5