This document discusses angle modulation techniques, including frequency modulation (FM) and phase modulation (PM). It explains that FM directly varies the carrier frequency while PM directly varies the carrier phase. Both FM and PM occur simultaneously. It also describes the advantages of angle modulation, such as noise immunity, and the disadvantages, such as increased bandwidth compared to amplitude modulation. The key components of FM transmitters and receivers are outlined, including the FM modulator and demodulator. Two common types of FM demodulators are the quadrature detector and phase-locked loop detector.

1. ANGLE MODULATION

2. Angle Modulation

3. • 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

4. • 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

5. 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

6. 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

7. 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

8. FM Transmitter
Audio
Oscillator
FM
modulator
Carrier
generator
Output
amplifier
Antenna
These two blocks
are often combined
in one circuit

9. • 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)

10. 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

11. Superheterodyne Receiver for FM
Local oscillator
FM
Demodulator
RF
Section
Antenna
Audio-frequency
amplifier
Mixer
IF
Section
Speaker

12. FM Demodulator
• Two types of FM demodulator:
▫ Quadrature detector
▫ Phase-locked loop detector

13. Quadrature detector
Phase
comparator
circuit
Phase
shifting
circuit
Low
pass
filter
FM
input
signal
Audio
output
signal
• 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

14. Phase-locked loop (PLL) detector
Phase
comparator
circuit
Low
pass
filter
Amplifier
Voltage-controlled
oscillator
(VCO)
External
input
signal
Output
signal

15. 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