FM Generation and Detection

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FM Generation and Detection

• Analog and Digital Communications
• Autumn 2005-2006

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FM Bandwidth
Consider Tone Modulation
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FM Bandwidth Single Tone
Is this the maximum value?
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FM Bandwidth Generic Signal
Estimate BW using worst case tone modulation!
Define Deviation ratio
Approximations
Carsons rule
Carsons rule is applied for extreme values of
deviation ratio. It underestimates the BW for
practical systems.
(Do Example 5.2-1 and Exercise 5.2-1 in text)
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Transmission BW Summary
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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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Generating NBFM Signals
Indirect method
(NBPM/NBFM)
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Generating WBFM Signals
Direct Method ? Use a VCO (as in case of NBFM)
Indirect Method ? Generate NBFM first
Frequency converter might be needed if
the multiplier produces a carrier at a frequency
different from the desired carrier frequency!
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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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FM Detection FM to AM
Differentiate the FM signal to get an AM signal!
Then use an Envelope Detector!
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FM Detection Discriminator
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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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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.