FREQUENCY MODULATION DEFINEDFM=varying/modifying the carrier frequency in accordance to the modulating signal.To produce FM, the modulating information signal varies the frequency of the carrier.The carrier frequency deviation is proportional to the amplitude of the information signal.The amplitude of the carrier remains constant during modulation.
What is frequency deviation?The amount of change in carrier frequency produced by the modulating signal.Or: the amount of frequency shift of the carrier above or below the center frequency.For FM, the maximum frequency deviation occurs at the maximum amplitude of the modulating signal.
PHASE MODULATIONTo produce phase modulation, the modulating signal causes the carrier to be phase shifted.The process of PM produces FM. PM is known as indirect FM.A phase shift occurs only if the modulating signal is varying.
PHASE MODULATIONNote: the carrier changes frequency only whenthe modulating signals amplitude is changing.
THE DIFFERENCE BETWEEN FM AND PMThe amount of carrier frequency deviation is directly proportional to the amplitude of the modulating signal in both FM and PM.In PM, the carrier frequency deviation is also proportional to the modulating signal frequency. This is not so in FM.To convert PM to FM, a low pass filter is used to roll off the modulating signal amplitude to offset the deviation increase with modulating signal frequency.
FM AND PMCOMPAREDWith FM, the maximum carrier frequency deviation occurs at the maximum modulating voltages.With PM, the maximum carrier frequency deviation occursat the maximum rate of change of the modulating signal.Modulatingsignal
PM PRODUCES MORE DEVIATIONFOR HIGHER FREQUENCY MODULATING SIGNALSModulatingsignals
FM PRODUCES THE SAMEDEVIATION FOR HIGHERFREQUENCY MODULATING SIGNALS(Both signals have the same peak amplitudeand will produce the same peak deviation.)
PM CAN BE MADE EQUIVALENT TO FM BY REDUCING THE AMPLITUDE OF THE HIGHER FREQUENCY MODULATING SIGNALS
FM SIDEBANDSFM and PM also produce sidebands.Unlike AM which produces one pair of sidebands for each modulating frequency, FM and PM produce an infinite number of sidebands.Only a small percentage of the sidebands have significant amplitude.The significant sidebands are determined by the modulation index (m).
MODULATION INDEXThe modulation index (m) is the ratio of the frequency deviation (fd) to the modulating signal frequency (fm).m = fd/ fmWhen the maximum values of frequency deviation and modulating signal frequency are used, the calculation produced is called the deviation ratio.The modulation index determines the number of significant sidebands.
SIDEBAND TABLEThe sideband table gives the relative amplitudes of the carrier and sidebands for a given modulation index.Only sidebands with an amplitude of 1% (.01) or more are considered significant.The sidebands are spaced from one another by an amount equal to the modulating signal frequency.
0.12Mod.IndexBESSEL FUNCTION TABLESidebands (Pairs)Carrier1st2nd0.001.003rd4th0.250.500.980.940.240.031.000.770.440.110.021.500.510.560.230.060.012.000.220.580.350.130.032.50-0.050.500.450.220.070.025th3.00-0.260.340.490.310.130.040.016th7th4.00-0.40-0.070.360.430.280.130.050.025.00-0.18-0.330.050.360.390.260.130.050.028th6.009th7.000.15-0.28-0.240.110.360.360.250.130.060.0210th8.000.00-0.30-0.170.160.350.3126.96.36.199-0.11-0.29-0.100.190.340.3188.8.131.52.060.030.300.060.0211th
BANDWIDTH OF AN FM SIGNALThe bandwidth (BW) of an FM signal is determined by the number of significant sidebands (N).BW = 2Nfm(max) where fm(max) is the maximum modulating frequency.Carsons rule can also be used to determine the bandwidth.BW = 2(fd(max) + fm(max)).
0.12Mod.IndexHOW TO DETERMINE BANDWIDTHSidebands (Pairs)Carrier1st2nd0.001.003rd4th0.250.500.980.940.240.031.000.770.440.110.021.500.510.560.230.060.012.000.220.580.350.130.032.50-0.050.500.450.220.070.025th3.00-0.260.340.490.310.130.040.016th7th4.00-0.40-0.070.360.430.280.130.050.025.00-0.18-0.330.050.360.390.260.130.050.028th6.009th7.000.15-0.28-0.240.110.360.360.250.130.060.0210th8.000.00-0.30-0.170.160.350.3184.108.40.206-0.11-0.29-0.100.190.340.3220.127.116.11.060.030.300.060.0211th
fC0.220.350.35Spectral display when the modulation index = 2
ADVANTAGES OF FM OVER AMLess sensitivity to noise. Noise is amplitude variations that are eliminated in the receiver.Capture effect. Only the strongest signal on the receive frequency gets through.Transmission efficiency. More efficient transmitter amplifiers can be used with FM.
DISADVANTAGES OF FMUses excessive spectrum space.As a result, FM can only be used at the high radio frequencies (VHF and above).
PREEMPHASIS AND DEEMPHASISHigh frequency noise that interferes with the high frequency modulating signals is overcome by amplifying high frequency signals more than low frequency signals before transmission (preemphasis).The original signal is demodulated at the receiver and passed through a low pass filter to correct for the high frequency emphasis.
FREQUENCY SHIFT KEYINGFSK is a variation of FM used to transmit binary data.In FSK, a binary 0 is transmitted as a lower carrier frequency while a binary 1 is transmitted as a higher carrier frequency.FSK is used in radio modems.