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FM Transmitters

FM Transmitters. FCC Commercial Broadcast Specs f c : 88.1 – 107.9 Mhz , 200 kHz Spacing f c stability: +/- 2 kHz (+/-20 ppm) f m range: 50 Hz – 15 kHz (75uS pre-emphasis) D f c < 75 kHz (maximum carrier deviation) 100kW max (horizontal and/or vertical polarization).

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FM Transmitters

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  1. FM Transmitters • FCC Commercial Broadcast Specs • fc: 88.1 – 107.9 Mhz, 200 kHz Spacing • fcstability: +/- 2 kHz (+/-20 ppm) • fm range: 50 Hz – 15 kHz (75uS pre-emphasis) • Dfc < 75 kHz (maximum carrier deviation) • 100kW max (horizontal and/or vertical polarization) Direct FM using VCO fout fc ko fc , ko(Hz/v) VCO vm(t) fout vm

  2. Frequency Multipliers Desired Harmonic: N Unwanted Harmonics fout = Nfin fin Frequency Multipliers also multiply phase, hence modulation index!

  3. FM from PM…and vice versa fc N kp(rad/v) Phase Mod Equivalent Frequency Modulator Most analog phase modulators exhibit acceptable linearity only for |Df |< p/4. thus mp/f < p/4 . We can create higher modulation index using a frequency multiplier.

  4. PM from FM. . . while we’re at it. VCO Equivalent Phase Modulator

  5. Narrowband FMmp/f ~0.5 > only 1 sideband USB + LSB What if we were to delay the sidebands by p/2 ? USB LSB USB USB + LSB Carrier (stationary) LSB FM Phasor Df Hey Bret, do you remember plain ol’AM? …do you remember anything?

  6. Armstrong Method for WBFM

  7. Reactance Modulator (Voltage Variable Capacitance) Linear!

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