Download Telecommunications: Radio Transmission and Reception - Amplitude and Frequency Modulation and more Study notes Basic Electronics in PDF only on Docsity! 1 Phys 351, Fall 2008 Lecture 15 1 Telecommunications: Radio (and TV) transmission and reception Frequency mixing, heterodyne-ing Modulation and demodulation Phys 351, Fall 2008 Lecture 15 2 Basic motivation and ideas Audio frequencies are 20Hz < Δω < 20kHz, but difficult to transmit and to receive directly because of interference from ambient EM. Trick: use circuits tuned to specific RF bands (500kHz - 100MHz) which are waves that are easy to transmit and mix in audio frequency waves as modulation of the radio frequency signal. Two main methods: Amplitude Modulation (AM): small changes in radio frequency electric field amplitude Frequency Modulation (FM): small changes in radio wave frequency Radio tuned to (“listens exclusively” to) a carrier frequency ω0 (e.g. 91.5MHz) with enough bandwidth Δω to span all encoded audio frequencies. ω0 Δω A m pl itu de ω Phys 351, Fall 2008 Lecture 15 3 Modulation methods Frequency mixing: trig identity: 2(cosω0t)(cosωAt) = cos[(ω0+ωA)t] + cos[(ω0−ωA)t] where audio frequency is ωA and carrier is ω0. AM: add amplitude of audio directly to carrier amplitude. FM: vary the carrier frequency by amount proportional to amplitude of audio signal at rate of change set by frequency of audio signal. Audio amplitude quiet loud 2π/ωA 2π/ωA Phys 351, Fall 2008 Lecture 15 4 For some value k < 1 (so that carrier always has positive amplitude) with k proportional to loudness, construct will give a pure tone at frequency ωA of amplitude k/2. So for an audio signal g(t) encode the AM radio signal as signal = [1+g(t)] (cosω0t) to get: where g(ω) is the Fourier transform of g(t) AM signal ω0 )(g 2 1 ! }t)cos{( 2 k }t)cos{( 2 k tcos )t](costcosk1[signal 0A0A0 0A !"!+!+!+!= !!+= ω signal(ω) ω0 ω signal(ω) ωA ωA Phys 351, Fall 2008 Lecture 15 5 Super-heterodyne “Super-het” receiver: Mixes carrier with local oscillator ω1 to make an intermediate frequency signal (455 kHz) to which internal circuits are tuned so that gains, etc can be controlled easily and noise excluded. Tuning dial sweeps both ω0 and ω1 to keep ωint = constant. ωint ω0 ω1 Phys 351, Fall 2008 Lecture 15 6 Super-het receiver × ω1 oscillator Tuning dial Wide-band RF amp an te nn a Tuned 455kHz ampmixer Quartz capacitor Rectifier/ audio filter audio amplifier Once signal is mixed down to ωint, it is filtered against a stable (quartz) oscillator and amplified The signal is rectified and filtered to remove ωint leaving audio frequency signal that is driven through speaker.
