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Lab8 - RLC Circuits Building An AM Radio(Left An AM radio...

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RLC Circuits – Building An AM Radio (Left) An AM radio station antenna tower; (Right) A circuit that tunes for AM frequencies. You will build this circuit in lab to receive AM transmissions from towers such as the one on the left. Objective: To understand how AM radio waves are produced and encoded. To learn how information is carried by a radio wave. To realize how radio waves are detected and decoded by building a working, tunable AM radio from an RLC (resistor-inductor-capacitor) circuit. Apparatus: SnapCircuits board, circuit elements (resistors, inductor “A1”, capacitors C2 & CV, inductor/antenna A1, amplifier U5), PC speakers, PC sound card interface, FFTScope software, battery banana-to-button cables (3) Introduction Earlier this semester you created electric fields by transferring charges (from friction and electrostatic induction). You also employed circuits to light up bulbs, create magnetic fields, and run electric motors. In this lab you will build a circuit to detect radio waves, which are used in every form of wireless communication – radio broadcasts, WiFi networks, cell phones, Bluetooth...even garage door openers and keyless car remotes. Radio waves belong to the portion of the electromagnetic spectrum which has the longest wavelengths. Note that this also corresponds to the lowest frequencies, since the product of the wavelength and frequency of these waves equals the speed of light: c = f = 3 x 10 8 m / s
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In the top part of the illustration below you will see that there are many other forms of electromagnetic radiation – examples are microwaves from your oven (12 cm wavelength), light (400-nm wavelength), x-rays at the doctor's office (10 Angstroms), and over-the-air television (1 m wavelength). Note that AM radio waves (frequency 530-1650 kHz, check your radio) have wavelengths between 180-570 meters; this requires a large antenna to transmit waves of this dimension – see the very tall tower at the beginning of this write-up. At the the bottom of the illustration above is a pictorial representation of how an AM broadcast travels from the station to your radio (left to right). Note the distinction between AM (amplitude modulation) and FM (frequency modulation) in the middle of the pictorial representation – a steady wave conveys no information unless it is modulated , which means that the waveform is varied slightly to transmit information. In AM, the amplitude is varied to carry the audio information (like transverse wave pulses); in FM, the frequency is varied instead (like longitudinal wave pulses). You will be building the RECEIVER, which receives the signal from the antenna, tunes to the desired frequency/wavelength, filters out any extraneous information, and amplifies it so that it can be reproduced by a speaker. At the very heart of the AM receiver is an RLC circuit which responds to a particular frequency that is a function of its capacitance C and inductance L. To understand C and L it is often useful to think of mechanical systems, such as a mass suspended from a spring bouncing up and down while submerged in a fluid. Here, the capacitance is analogous to a
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