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# FM Generation and Detection - PowerPoint PPT Presentation

FM Generation and Detection. Analog and Digital Communications Autumn 2005-2006. Observation for. falls off quickly for .  Consider components only for. Significant frequency components lie within. Observation for. is insignificant for.  Consider components only for.

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### FM Generation and Detection

Analog and Digital Communications

Autumn 2005-2006

CS477: Analog and Digital Communications

falls off quickly for

 Consider components only for

Significant frequency components lie within

Observation for

is insignificant for

 Consider components only for

Significant frequency components lie within

FM Bandwidth

Consider Tone Modulation

CS477: Analog and Digital Communications

Usually approximated by

For worst case, use:

What is the corresponding value of

NO! A smaller may result in larger

FM Bandwidth: Single Tone

Is this the maximum value?

CS477: Analog and Digital Communications

For practical systems :

FM Bandwidth: Generic Signal

Estimate BW using worst case tone modulation!

Define Deviation ratio:

Approximations:

Carson’s rule:

Carson’s 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)

CS477: Analog and Digital Communications

CS477: Analog and Digital Communications

• 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

CS477: Analog and Digital Communications

900

NBPM Modulator

NBFM Output

Input

Generating NBFM Signals

Indirect method:

(NBPM/NBFM)

CS477: Analog and Digital Communications

Modulator

Frequency

Converter

Frequency Multiplier

Non-linear

Device

Bandpass

Filter

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!

CS477: Analog and Digital Communications

• 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)

CS477: Analog and Digital Communications

Limiter

Signal

Differentiator

Envelope

Detector

DC

Block

FM Detection: FM to AM

Differentiate the FM signal to get an AM signal!

Then use an Envelope Detector!

CS477: Analog and Digital Communications

Amplitude

Limiter+BPF

LPF

Phase-shift

Network

For small values of

FM Detection: Discriminator

CS477: Analog and Digital Communications

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!

CS477: Analog and Digital Communications

VCO

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.

CS477: Analog and Digital Communications