Chapter Two FM Transmitter and Receiver2.1. Overview In telecommunications and signal processing, frequencymodulation (FM) conveys information over a carrier wave by varying itsinstantaneous frequency. This contrasts with amplitude modulation, inwhich the amplitude of the carrier is varied while its frequency remainsconstant. In analog applications, the difference between the instantaneousand the base frequency of the carrier is directly proportional to theinstantaneous value of the input-signal amplitude. Digital data can be sentby shifting the carriers frequency among a range of settings, a techniqueknown as frequency-shift keying (FSK). FSK is widely used in data andfax modems, and can be used to send Morse code.  Radioteletype alsouses FSK. Frequency modulation is also used in telemetry, radar,seismic prospecting and newborn EEG seizure monitoring. Frequencymodulation is known as phase modulation when the carrier phasemodulation is the time integral of the FM signal. FM is widely used forbroadcasting music and speech, two-way radio systems, magnetic tape-recording systems and some video-transmission systems. In radiosystems, frequency modulation with sufficient bandwidth provides anadvantage in cancelling naturally-occurring noise.2.2. FM Broadcasting Throughout the world, the FM broadcast band falls within the VHFpart of the radio spectrum. Usually 87.5 to 108.0 MHz is used, or someportion thereof, with few exceptions such as in Soviet republics, andsome former Eastern Bloc countries, the older 65–74 MHz band is also
used. Assigned frequencies are at intervals of 30 kHz. In Japan, the band76–90 MHz is used. The frequency of an FM broadcast station (more strictly itsassigned nominal centre frequency) is usually an exact multiple of 100kHz.2.3. FM Modulation and Demodulation02.3.1. Modulation Modulation is the process of varying one or more properties of ahigh-frequency periodic waveform, called the carrier signal, with amodulating signal which typically contains information to be transmitted.This is done in a similar fashion to a musician modulating a tone (aperiodic waveform) from a musical instrument by varying its volume,timing and pitch. The three key parameters of a periodic waveform are itsamplitude ("volume"), its phase ("timing") and its frequency ("pitch").Any of these properties can be modified in accordance with a lowfrequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wavepulse train may also be used. In telecommunications, modulation is the process of conveying amessage signal, for example a digital bit stream or an analog audio signal,inside another signal that can be physically transmitted. Modulation of asine waveform is used to transform a baseband message signal into apassband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systemsor the public switched telephone network for instance, electrical signalscan only be transferred over a limited passband frequency spectrum, withspecific (non-zero) lower and upper cutoff frequencies. Modulating a
sine-wave carrier makes it possible to keep the frequency content of thetransferred signal as close as possible to the centre frequency (typicallythe carrier frequency) of the passband. A device that performs modulation is known as a modulator and adevice that performs the inverse operation of modulation is known as ademodulator (sometimes detector or demod). A device that can do bothoperations is a modem (from "modulator–demodulator").2.3.2. Demodulation Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave. A demodulator is anelectronic circuit (or computer program in a software-defined radio) thatis used to recover the information content from the modulated carrierwave. Frequency modulation or FM is more complex. It has numerousadvantages over AM, such as better fidelity and noise immunity.However, it is much more complex to both modulate and demodulate acarrier wave with FM, and AM predates it by several decades. There are several common types of FM demodulator: The quadrature detector, which phase shifts the signal by 90 degreesand multiplies it with the unshifted version. One of the terms that dropsout from this operation is the original information signal, which isselected and amplified. The signal is fed into a PLL and the error signal is used as thedemodulated signal. The most common is a Foster-Seeley discriminator. This is composedof an electronic filter which decreases the amplitude of some frequenciesrelative to others, followed by an AM demodulator. If the filter response
changes linearly with frequency, the final analog output will beproportional to the input frequency, as desired. A variant of the Foster-Seeley discriminator called the ratio detector Another method uses two AM demodulators, one tuned to the high endof the band and the other to the low end, and feed the outputs into adifference amp. Using a digital signal processor, as used in software-defined radio.
References 1. Stan Gibilisco (2002). "Teach yourself electricity and electronics". McGraw-Hill Professional. 2. David B. Rutledge (1999). "The Electronics of Radio". Cambridge University Press. 3. B. Boashash, 2003, “Time-Frequency Signal Analysis and Processing – A Comprehensive Reference”, Elsevier Science, Oxford. 4. "Transmission standards for FM sound broadcasting at VHF". ITU Rec. BS.450. International Telecommunications Union. 5. Kundu Sudakshina (2010). "Analog and Digital Communications". Pearson Education India. 6. "Demodulator - Definitions from Dictionary.com". dictionary.reference.com. Retrieved 2008-05-16. 7. "The ratio detector" 8.