CommunicationCommunication
ElectronicsElectronics
Principles & ApplicationsPrinciples & Applications
Chapter 4
Frequency Modulation
FREQUENCY MODULATIONDEFINED
•FM=varying/modifying the carrierfrequency in accordance to the modulatingsignal.
•To produce FM, the modulatinginformation signal varies the frequency ofthe carrier.
•The carrier frequency deviation isproportional to the amplitude of theinformation signal.
•The amplitude of the carrier remainsconstant during modulation.
What is frequency deviation?
•The amount of change in carrier frequencyproduced by the modulating signal.
•Or: the amount of frequency shift of the carrierabove or below the center frequency.
•For FM, the maximum frequency deviationoccurs at the maximum amplitude of themodulating signal.
Resting fc
Increasing fc
Increasing fc
Decreasing fc
Resting fc
Modulating signal
Carrier
FM
PHASE MODULATION
•To produce phase modulation, themodulating signal causes the carrierto be phase shifted.
•The process of PM produces FM.PM is known as indirect FM.
•A phase shift occurs only if themodulating signal is varying.
PHASE MODULATION
No change
No change
No change
No change
Freq. increase
Freq. increase
Freq. decrease
Note: the carrier changes frequency only when
the modulating signal’s amplitude is changing.
Modulation
Carrier
THE DIFFERENCE BETWEENFM AND PM
•The amount of carrier frequency deviation isdirectly proportional to the amplitude of themodulating signal in both FM and PM.
•In PM, the carrier frequency deviation is alsoproportional to the modulating signalfrequency. This is not so in FM.
•To convert PM to FM, a low pass filter is usedto roll off the modulating signal amplitude tooffset the deviation increase with modulatingsignal frequency.
FM AND PM
COMPARED
With FM, the maximum carrier frequency deviation occursat the maximum modulating voltages.
With PM, the maximum carrier frequency deviation occurs
at the maximum rate of change of the modulating signal.
Modulating
signal
PM PRODUCES MORE DEVIATION
FOR HIGHER FREQUENCYMODULATING SIGNALS
Modulating
signals
Shallow slope (smaller rate of change)
Steep slope (greater rate of change)
FM PRODUCES THE SAME
DEVIATION FOR HIGHER
FREQUENCY MODULATING SIGNALS
Modulating
signals
(Both signals have the same peak amplitude
and will produce the same peak deviation.)
PM CAN BE MADE EQUIVALENT TO FMBY REDUCING THE AMPLITUDE OFTHE HIGHER FREQUENCYMODULATING SIGNALS
Modulating
signals
The slope is now the same for both modulating signals.
FM SIDEBANDS
•FM and PM also produce sidebands.
•Unlike AM which produces one pair ofsidebands for each modulating frequency,FM and PM produce an infinite number ofsidebands.
•Only a small percentage of the sidebandshave significant amplitude.
•The significant sidebands are determinedby the modulation index (m).
MODULATION INDEX
•The modulation index (m) is the ratio ofthe frequency deviation (fd) to themodulating signal frequency (fm).
•m = fd/ fm
•When the maximum values of frequencydeviation and modulating signal frequencyare used, the calculation produced is calledthe deviation ratio.
•The modulation index determines thenumber of significant sidebands.
SIDEBAND TABLE
•The sideband table gives the relativeamplitudes of the carrier and sidebandsfor a given modulation index.
•Only sidebands with an amplitude of 1%(.01) or more are considered significant.
•The sidebands are spaced from oneanother by an amount equal to themodulating signal frequency.
0.12
Mod.
Index
BESSEL FUNCTION TABLE
Sidebands (Pairs)
Carrier
1st
2nd
0.00
1.00
3rd
4th
0.25
0.50
0.98
0.94
0.24
0.03
1.00
0.77
0.44
0.11
0.02
1.50
0.51
0.56
0.23
0.06
0.01
2.00
0.22
0.58
0.35
0.13
0.03
2.50
-0.05
0.50
0.45
0.22
0.07
0.02
5th
3.00
-0.26
0.34
0.49
0.31
0.13
0.04
0.01
6th
7th
4.00
-0.40
-0.07
0.36
0.43
0.28
0.13
0.05
0.02
5.00
-0.18
-0.33
0.05
0.36
0.39
0.26
0.13
0.05
0.02
8th
6.00
9th
7.00
0.15
-0.28
-0.24
0.11
0.36
0.36
0.25
0.13
0.06
0.02
10th
8.00
0.00
-0.30
-0.17
0.16
0.35
0.34
0.23
0.13
0.23
-0.11
-0.29
-0.10
0.19
0.34
0.32
0.22
0.13
0.17
0.06
0.03
0.30
0.06
0.02
11th
BANDWIDTH OF AN FM SIGNAL
•The bandwidth (BW) of an FM signalis determined by the number ofsignificant sidebands (N).
•BW = 2Nfm(max) where fm(max) is themaximum modulating frequency.
•Carson’s rule can also be used todetermine the bandwidth.
•BW = 2(fd(max) + fm(max)).
0.12
Mod.
Index
HOW TO DETERMINE BANDWIDTH
Sidebands (Pairs)
Carrier
1st
2nd
0.00
1.00
3rd
4th
0.25
0.50
0.98
0.94
0.24
0.03
1.00
0.77
0.44
0.11
0.02
1.50
0.51
0.56
0.23
0.06
0.01
2.00
0.22
0.58
0.35
0.13
0.03
2.50
-0.05
0.50
0.45
0.22
0.07
0.02
5th
3.00
-0.26
0.34
0.49
0.31
0.13
0.04
0.01
6th
7th
4.00
-0.40
-0.07
0.36
0.43
0.28
0.13
0.05
0.02
5.00
-0.18
-0.33
0.05
0.36
0.39
0.26
0.13
0.05
0.02
8th
6.00
9th
7.00
0.15
-0.28
-0.24
0.11
0.36
0.36
0.25
0.13
0.06
0.02
10th
8.00
0.00
-0.30
-0.17
0.16
0.35
0.34
0.23
0.13
0.23
-0.11
-0.29
-0.10
0.19
0.34
0.32
0.22
0.13
0.17
0.06
0.03
0.30
0.06
0.02
11th
fC
0.22
0.58
0.58
0.35
0.35
0.13
0.13
0.03
0.03
Spectral display when the modulation index = 2
ADVANTAGES OF FM OVER AM
•Less sensitivity to noise. Noise isamplitude variations that are eliminatedin the receiver.
•Capture effect. Only the strongest signalon the receive frequency gets through.
•Transmission efficiency. More efficienttransmitter amplifiers can be used withFM.
DISADVANTAGES OF FM
•Uses excessive spectrum space.
•As a result, FM can only beused at the high radiofrequencies (VHF and above).
PREEMPHASIS AND DEEMPHASIS
•High frequency noise that interferes withthe high frequency modulating signals isovercome by amplifying high frequencysignals more than low frequency signalsbefore transmission (preemphasis).
•The original signal is demodulated at thereceiver and passed through a low passfilter to correct for the high frequencyemphasis.
FREQUENCY SHIFT KEYING
•FSK is a variation of FM used totransmit binary data.
•In FSK, a binary 0 is transmitted asa lower carrier frequency while abinary 1 is transmitted as a highercarrier frequency.
•FSK is used in radio modems.