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Oct 11, 2005CS477: Analog and Digital Communications1 FM Generation and Detection Analog and Digital Communications Autumn 2005-2006

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Oct 11, 2005CS477: Analog and Digital Communications2 FM Bandwidth falls off quickly for Consider components only for Observation for is insignificant for Consider components only for Observation for Consider Tone Modulation Significant frequency components lie within

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Oct 11, 2005CS477: Analog and Digital Communications3 FM Bandwidth: Single Tone For worst case, use: depends upon the allowable distortion level Usually approximated by What is the corresponding value of Is this the maximum value? NO! A smaller may result in larger

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Oct 11, 2005CS477: Analog and Digital Communications4 FM Bandwidth: Generic Signal Estimate BW using worst case tone modulation! Define Deviation ratio: Carson’s rule: Carson’s rule is applied for extreme values of deviation ratio. It underestimates the BW for practical systems. Use (Do Example 5.2-1 and Exercise 5.2-1 in text) Approximations: For practical systems :

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Oct 11, 2005CS477: Analog and Digital Communications5 Transmission BW: Summary Condition(s)Single ToneGeneric Signal Carson’s Rule extreme Practical designs

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Oct 11, 2005CS477: Analog and Digital Communications6 Generating NBFM Signals Direct method Use a voltage-controlled oscillator Output frequency is proportional to input voltage Use information signal as the input voltage Restricted to low power applications Most circuits unstable at carrier frequency Not suitable for commercial FM broadcast Require sophisticated feedback control

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Oct 11, 2005CS477: Analog and Digital Communications7 Generating NBFM Signals Indirect method: (NBPM/NBFM) NBPM Modulator NBFM Output Input 90 0 (NBPM)

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Oct 11, 2005CS477: Analog and Digital Communications8 Generating WBFM Signals Direct Method Use a VCO (as in case of NBFM) Indirect Method Generate NBFM first NBFM Modulator Non-linear Device Bandpass Filter Frequency Multiplier Frequency Converter Frequency converter might be needed if the multiplier produces a carrier at a frequency different from the desired carrier frequency!

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Oct 11, 2005CS477: Analog and Digital Communications9 FM Detection Four primary methods Differentiator with envelope detector FM to AM conversion Phase-shift discriminator Approximates the differentiator Zero-crossing detector Frequency feedback Phase lock loops (PLL)

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Oct 11, 2005CS477: Analog and Digital Communications10 FM Detection: FM to AM Differentiate the FM signal to get an AM signal! Then use an Envelope Detector! Signal Differentiator Envelope Detector Amplitude Limiter DC Block

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Oct 11, 2005CS477: Analog and Digital Communications11 FM Detection: Discriminator Can we produce Phase-shift Network Amplitude Limiter+BPF LPF For small values of

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Oct 11, 2005CS477: Analog and Digital Communications12 FM Detection: Zero crossing detector Idea: Produce a pulse at each zero crossing; Integrate over a suitable period Higher frequencies produce more pulses; More pulses result in higher output values; Output values are proportional to message Want to track message but not the carrier!

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Oct 11, 2005CS477: Analog and Digital Communications13 FM Detection: Phase Lock Loops Basic Task of PLL: Lock the instantaneous angle of a VCO with the instantaneous angle of an external signal. i.e., lock (or track) the phase and frequency. VCO LPF

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