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