311 angle modulation
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Transcript of 311 angle modulation
ANGLE MODULATION
Angle Modulation
• Difference between FM and PM lies in whether frequency or phase is directly varied by the info signal
• FM and PM occur concurrently in angle modulation
• Direct FM is indirect PM. Direct PM is indirect FM
• Direct FM vary frequency of constant-amplitude carrier proportional to info signal amplitude, at a rate equal to the info signal frequency
• Direct PM vary phase of constant-amplitude carrier proportional to info signal amplitude, at a rate equal to the info signal frequency
•FM and PM waveforms are identical except for the time relationship
• It is impossible to distinguish between FM and PM waveforms unless dynamic characteristic of information signal is known
•For FM – max frequency deviation (max change in carrier frequency) occurs during the maximum positive and negative peaks of information signal
•For PM – max frequency deviation (max change in carrier frequency) occurs during the zero crossings of information signal
Side frequencies
• In FM and PM, single-frequency information signal produces an infinite number of pairs of side frequencies, thus have infinite bandwidth
• Each side frequency is displaced from the carrier by an integral multiple of the modulating-signal frequency
Advantages of Angle Modulation• Noise immunity
▫Most noise cause amplitude variations in modulated wave
▫ In FM and PM, limiters are used to remove amplitude variations (hence noise also removed)
• Noise performance and signal-to-noise improvement▫Noise reduction by limiters improve SNR ratio
• Capture effect▫ If two signals are received with the same
frequency, the receiver will capture the stronger signal and the weaker signal is eliminated
▫However, if strengths of the signals are nearly the same, capture effect may cause signal to alternate in their dominance of the frequency
Disadvantages of Angle Modulation•Bandwidth: FM takes up more spectrum
space compared to AM•Modulation and demodulation circuits
required for FM and PM are usually far more complex than those of AM
FM Transmitter
Audio Oscillator
FM modulato
r
Carrier generato
r
Output amplifie
r
Antenna
These two blocks are often combined in one circuit
•Difference from AM transmitter: FM modulator is used in place of AM modulator
•FM modulator: ▫Changes carrier frequency according to
info signal▫Usually uses a varactor or voltage-variable
capacitor (VVC)
FM Receiver
•For FM receiver, the superheterodyne receiver can be used, like the AM receiver, except that FM demodulator is used in place of AM demodulator
Superheterodyne Receiver for FM
FMDemodulato
r
Local oscillator
RF Section
Antenna
Audio-frequency amplifier
MixerIF Section
Speaker
FM Demodulator
•Two types of FM demodulator:▫Quadrature detector▫Phase-locked loop detector
Quadrature detector
• Phase-shifting circuit determines the degree of phase shift by the exact frequency of the input signal at any particular instant
• Phase comparator circuit compares the phase of the original input signal with the output of the phase-shifting circuit▫ If shift = 90°, no change in output voltage▫ If shift > 90°, increase dc voltage level▫ If shift < 90°, decrease dc voltage level
• The changing dc voltage level (up and down) recreates the info signal
• Low pass filter – reduce amplitude of any high-frequency ripple and blocks dc offset
Phase comparato
r circuit
Phase shifting circuit
Low pass
filter
FM input signal
Audio output signal
Phase-locked loop (PLL) detector
Phase comparato
r circuit
Low pass
filter
Amplifier
Voltage-controlled oscillator
(VCO)
External input signal
Output signal
Phase-locked loop (PLL) detector• PLL has 3 operating states:
▫Free running VCO oscillates at its natural frequency
▫Capture External input signal is present and feedback
loop is complete PLL is in the process of acquiring frequency lock
▫Lock VCO frequency is locked onto (equal to) the
frequency of external input signal VCO frequency tracks (follows) changes in the
frequency of the external input signal, whether increase or decrease