Frequency Modulation (FM)

• Principles of Electronic Communication Systems by Louis E. Frenzel, Jr.

Basic Principles of Frequency Modulation (FM)

• Sine wave carriers are modified by varying frequency to transmit information
• Carrier amplitude remains constant in FM
• The carrier frequency shifts in proportion to the amplitude of the information signal
• The carrier frequency increases as the modulating signal amplitude increases
• Without modulation, the carrier is at its normal center or resting frequency
• Frequency deviation (f d) is the amount of change in carrier frequency produced by the modulating signal
• Frequency deviation rate indicates how many times per second the carrier frequency deviates above or below its center frequency
• The frequency of the modulating signal determines the frequency deviation rate
• Frequency-shift keying (FSK) modulation is used in binary data transfer in digital cell phones and low-speed computer modems
• In practice carrier waves are sine waves
• FM signals are represented graphically

Principles of Phase Modulation (PM)

• Phase-modulation (PM) signal occurs when phase shift of a constant-frequency carrier is varied, corresponding to a modulating signal
• Phase modulators create phase shift, a time separation between 2 sine waves with the same frequency
• Greater amplitude of the modulating signal leads to a greater phase shift
• Maximum frequency deviation from a phase modulator occurs when the modulating signal changes rapidly
• Frequency deviation is directly proportional to the amplitude of the modulating signal in both FM and PM
• Peak amplitudes of the modulating signal see the maximum lead or lag of phase shift in PM
• Carrier deviation in PM is proportional to the modulating frequency and the amplitude
• Deviation resulting from frequency variations of the modulating signal must be compensated to make PM compatible with FM
• Passing the intelligence signal through a low-pass RC network accomplishes the compensation
• RC low-pass filter is a frequency-correcting network, predistorter, or 1/f filter, attenuating higher modulating frequencies
• Indirect FM results from phase modulator usage
• Phase modulating a carrier with binary data is called phase-shift keying (PSK) or binary phase-shift keying (BPSK)
• PSK signal has constant frequency, but phase of signal shifts relative to a reference when a binary modulating signal occurs

Modulation Index and Sidebands

• Any modulation process produces sidebands
• Two side frequencies are produced by modulating a carrier with a constant-frequency sine wave
• Side frequencies represent the sum and difference of the carrier and modulating frequency
• An FM signal's bandwidth is wider than an AM signal’s with the same modulating signal
• Modulation index (m) is the ratio of frequency deviation to modulating frequency
• Communication systems using FM usually limit maximum frequency deviation and modulating frequency
• Max permitted frequency deviation in standard FM broadcasting is 75 kHz
• Max permitted modulating frequency in standard FM broadcasting is 15 kHz
• Resulting standard FM broadcasting modulation index is 5
• Bessel functions solve the equation expressing the phase angle in sine wave modulation
• Bessel coefficients are widely available, so memorizing or calculating them is unnecessary
• Symbol ! signifies factorial
• Narrowband FM (NBFM) occurs when the System's FM modulation index is less than π/2 = 1.57
• NBFM is used widely in communication to conserve spectrum space, but reduces signal-to-noise ratio
• Higher modulation index in FM means a greater number of significant sidebands, thus a wider bandwidth
• Bandwidth of an FM signal can be restricted by limiting the modulation index when spectrum conservation is necessary
• Where N is the number of significant sidebands the Bandwidth is BW = 2f_N

Noise-Suppression Effects of FM

• Noise from lightning, motors, automotive ignitions, and power-line switching produces transient signals
• Noise is typically narrow voltage spikes with high frequencies
• Noise (voltage spikes) add to a signal, causing interference
• Some noise completely obliterates signal information
• FM signals have constant modulated carrier amplitude
• FM receivers have limiter circuits to restrict amplitude of received signal
• Amplitude variations on the FM signal are clipped via limiter circuits
• Clipping does not affect content because information remains in frequency variations
• Noise interferes with FM signal, particularly with high-frequency modulating signal components
• Noise contains sharp energy spikes with many harmonics and high-frequency components
• Preemphasis is used to overcome the high-frequency noise
• Simple high-pass filter serves as the transmitter's pre-emphasis circuit
• Pre-emphasis amplifies only high-frequency components
• A simple low-pass filter can be used as a deemphasis circuit in a receiver.
• Deemphasis circuit returns the frequency response to its normal flat level
• Combining preemphasis and deemphasis increases the signal-to-noise ratio so that the high-frequency transmissions are stronger and won’t be masked by noise

Frequency Modulation Versus Amplitude Modulation

• FM offers significant advantages over AM
• FM provides superior immunity to noise via clipper limiter circuits in the receiver
• Interfering signals on the same frequency are rejected in FM, known as the capture effect.
• FM signals have a constant amplitude, thus no need for linear amplifiers to increase power, increasing transmitter efficiency
• FM uses considerably more frequency spectrum space
• FM has used more complex circuitry for modulation and demodulation
• Complex circuitry used in FM has disappeared with the proliferation of ICs, since ICs are inexpensive
• FM and PM are the most widely used modulation methods in electronic communication