lab3 - Germanium Diode AM Radio 3.1 Introduction In this...

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1 LAB Germanium Diode AM Radio 3.1 Introduction In this laboratory exercise you will build a germanium diode based AM (Medium Wave) radio. Earliest radios used simple diode detector circuits. The diodes were made out of Galena crystals (also called cat’s whisker) and these radios were really simple. Despite all of the advances in modern electronics, there are thou- sands of crystal sets in daily use throughout the world. A crystal receiver is pow- ered solely by the radio waves that it pulls from the air. Figure 3.1 shows the picture of an early Ge diode radio. Amplitude Modulation The point to modulation is to take a message bearing signal and superimpose it upon a carrier signal for transmission. For ease of transmission carrier signals are generally high frequency for severable reasons: 1.For easy (low loss, low dispersion) propagation as electromagnetic waves 2.So that they may be simultaneously transmitted without interference from other signals 3.So as to enable the construction of small antennas (a fraction, usually a quarter of the wavelength) 4.So as to be able to multiplex; that is, to combine multiple signals for transmis- sion at the same time. Figure 3.1—An antique diode radio. This radio is a MW and SW radio that works without any power supply. 3
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Lab 3: Germanium Diode AM Radio 2 For example AM radio is 550-1600 KHz, FM radio is 88 MHz-108 MHz, TV is 52-88 MHz (channels 1-6), 174-216 MHz (channels 7-12) and 470-900 MHz (UHF). In Amplitude Modulation (AM), the amplitude of the high frequency signal is var- ied by a low frequency signal that needs to be transmitted. Figure 3.2 shows a message signal and a high frequency carrier signal multiplied to obtain the AM modulated signal. The carrier signal is the content transmitted by the radio station and the carrier is the frequency which the electromagnetic waves are transmitted. AM Detection Modern radios use complicated detection schemes to demodulate the transmitted electromagnetic waves to provide the kind of reception we have began to expect from our radios. However, the simplest way to demodulate the transmitted AM waveform is use an antenna, a tuned LC coil, a germanium diode and a earphone. In this lab we shall attempt to build the radio the earliest pioneers built using these simple components.
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lab3 - Germanium Diode AM Radio 3.1 Introduction In this...

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