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13_radio - Radio Waves Radio Electromagnetic Radiation...

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Unformatted text preview: Radio Waves Radio Electromagnetic Radiation Radio Transmission and Reception Modulation Techniques UCSD: Physics 8; 2 Electromagnetism • Electricity and magnetism are different facets of Electricity electromagnetism electromagnetism – recall that a static distribution of charges produces an electric field – charges in motion (an electrical current) produce a magnetic field – a changing magnetic field produces an electric field, moving charges • • Electric and Magnetic fields produce forces on charges An accelerating charge produces electromagnetic waves An accelerating (radiation) (radiation) • Both electric and magnetic fields can transport energy – Electric field energy used in electrical circuits & released in lightning – Magnetic field carries energy through transformer UCSD: Physics 8; 2 Electromagnetic Radiation • Interrelated electric and magnetic fields traveling through Interrelated space space • All electromagnetic radiation travels at c = 3× 108 m/s in All vacuum – the cosmic speed limit! the – real number is 299792458.0 m/s exactly UCSD: Physics 8; 2 Examples of Electromagnetic Radiation • • • • • • • • AM and FM radio waves (including TV signals) Cell phone communication links Microwaves Infrared radiation Light Light X-rays Gamma rays What distinguishes these from one another? UCSD: Physics 8; 2 Wavelength (Frequency) UCSD: Physics 8; 2 The Electromagnetic Spectrum • Relationship between frequency, speed and Relationship frequency speed wavelength wavelength f ·λ = c f is frequency, λ is wavelength, c is speed of light frequency wavelength speed • Different frequencies of electromagnetic radiation are Different better suited to different purposes better • The frequency of a radio wave determines its The propagation characteristics through various media propagation UCSD: Physics 8; 2 Generation of Radio Waves • Accelerating charges radiate EM energy • If charges oscillate back and forth, get time-varying fields + + + + − − − − + + + + E − − − − UCSD: Physics 8; 2 Generation of Radio Waves If charges oscillate back and forth, get time-varying magnetic fields too. Note that the magnetic fields are perpendicular to the electric field vectors + + + B − − − + − − − − − + + + + UCSD: Physics 8; 2 Polarization of Radio Waves Transmitting antenna E B UCSD: Physics 8; 2 Reception of Radio Waves E B Receiving antenna works best when ‘tuned’ to the wavelength of the signal, and has proper polarization Electrons in antenna are “jiggled” by passage of electromagnetic wave Optimum antenna length is λ/4: one-quarter wavelength UCSD: Physics 8; 2 Encoding Information on Radio Waves • What quantities characterize a radio wave? • Two common ways to carry analog information with Two radio waves radio – Amplitude Modulation (AM) – Frequency Modulation (FM): “static free” UCSD: Physics 8; 2 AM Radio • Amplitude Modulation (AM) uses changes in the Amplitude signal strength to convey information strength pressure modulation (sound) electromagnetic wave modulation UCSD: Physics 8; 2 AM Radio in Practice • Uses frequency range from 530 kHz to 1700 kHz – each station uses 9 kHz – spacing is 10 kHz (a little breathing room) → 117 channels – 9 kHz of bandwidth means 4.5 kHz is highest audio frequency that can be encoded • falls short of 20 kHz capability of human ear • Previous diagram is exaggerated: – audio signal changes slowly with respect to radio carrier • typical speech sound of 500 Hz varies 1000 times slower than carrier • thus will see 1000 cycles of carrier to every one cycle of audio UCSD: Physics 8; 2 FM Radio • Frequency Modulation (FM) uses changes in the Frequency wave’s frequency to convey information frequency pressure modulation (sound) electromagnetic wave modulation FM Radio in Practice UCSD: Physics 8; 2 • Spans 87.8 MHz to 108.0 MHz in 200 kHz intervals – 101 possible stations – example: 91X runs from 91.0–91.2 MHz (centered at 91.1) • Nominally uses 150 kHz around center – 75 kHz on each side – 30 kHz for L + R (mono) → 15 kHz audio capability – 30 kHz offset for stereo difference signal (L - R) • Again: figure exaggerated – 75 kHz from band center, modulation is > 1000 times slower than carrier, so many cycles go by before frequency noticeably changes UCSD: Physics 8; 2 AM vs. FM • FM is not inherently higher frequency than AM – these are just choices – aviation band is 108–136 MHz uses AM technique • Besides the greater bandwidth (leading to stereo and Besides higher audio frequencies), FM is superior in immunity to environmental influences to – there are lots of ways to mess with an EM-wave’s amplitude • pass under a bridge • re-orient the antenna – no natural processes mess with the frequency • FM still works in the face of amplitude foolery UCSD: Physics 8; 2 Frequency Allocation UCSD: Physics 8; 2 Converting back to sound: AM • AM is easy: just pass the AC signal from the antenna AM into a diode into – or better yet, a diode bridge – then use capacitor to smooth out bumps • but not so much as to smooth out audio bumps radio signal B D amplifier/ speaker UCSD: Physics 8; 2 Converting back to sound: FM • More sophisticated – need to compare instantaneous frequency to that of a reference source – then produce a voltage proportional to the difference – Compute L = [(L+R) + (L-R)]/2; R = [(L+R) - (L-R)]/2 – amplify the L and R voltages to send to speakers • Amplification is common to both schemes – intrinsic signal is far too weak to drive speaker UCSD: Physics 8; 2 Assignments • HW5: 12.E.24, 13.E.13, 13.E.15, 13.E.16, 13.P.7, HW5: 13.P.9, 13.P.11, plus additional required problems plus available on website available ...
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