Frequency Modulation (FM)

Manipulates frequency instead of amplitude. The amplitude of the information signal causes the resulting modulated signal frequency to increase (higher amplitude) or decrease (lower amplitude).

VCO (Voltage Controlled Oscillator)

Simple FM modulator

Modulation Index and Signal Bandwidth (β_fm)

• Frequency modulation can either be narrowband or sideband
• Narrowband if the modulation index β_fm << 1
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• Wideband if the modulation index β_fm >> 1
  • This is what FM radio stations use
  • Carson's Rule: The bandwidth of a wide band FM signal can be approximated as (2 * num_sidebands * info frequency). The number of sidebands can be replaced with β_fm+1 which equals 2*(Δf + f_i) Hz
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    • Note the matlab circuit is exactly the same as narrowband except the modulation index is larger

Differential Demodulation

• Take the derivative of the received signal

Receiving FM into Complex Baseband

• Quadrature Demodulation
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Complex Differentiation Discriminator (Non-coherent Demodulation)

• Demodulates the real and imaginary component separately
• Digital Version uses a differentiator, with FIR filters with components -1,0,1 or 1,-1
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  • When combined:
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Complex Delay Line Discriminator

• Simpler version of above that demodulates the real and imaginary components in parallel
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Phased Locked Loop Demodulator (Coherent demodulation)

• Just uses the real component of the signal
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Other concerns specific to commercial FM radio

• De-emphasis filtering
  • need to filter a 'pre-emphasis' which was added to the signal before transmission. The pre-emphasis filters signals above a certain frequency in order to maintain the modulation index and signal bandwidth.
• De-multiplexing the signal
  • The received commercial FM signal is multiplexed and contains mono, stereo, and a 'pilot' component, as well as possibly RDS digital information stream (song title, station, etc.)
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  • FM encoder and multiplexer:
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  • FM decoder and de-multiplexer:
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