In addition to commercial radio, it is used as a component of television signals, for satellite and microwave communications, and for digital data transmission. The carrier frequency 400hz, message signal frequency 10hz, and frequency deviation 100hz. Jun 05, 2015 guide students to gain a working understanding of theories such as demodulation, bandwidth, and carsons rule. In singlechannel efield sensing, the carrier we transmit is m, the sensed value is a, and the noise is n vectors. In any radio that is designed to receive frequency modulated signals there is some form of fm demodulator or detector.
Use the signal generator to generate an fm signal with the message signal being sine signal. During this lab you will create an fm signal by modulating an audio waveform onto a carrier, examine the spectrum of the modulated carrier, evaluate the modulated carrier when the modulation index is varied and. Vectors and modulation cse 466 interfacing 3 spose m and n are orthogonal unit vectors. In the first model, you are provided a fm structure that is very similar to the theoretical background of this experiment. The specification for amplitude and frequency are given in the mfile. The bandwidth of each is dependent on the message bit rate. When the centre frequency of the pll is selected or designed at the fm carrier frequency, the filtered or output voltage in the circuit shown in figure, is obviously the desired demodulated voltage, that varies in magnitude in proportion to the signal frequency. Fm demodulation is a key process in the reception of a frequency modulated signal. Theory of frequency modulation and phase modulation. In this paper, we will discuss fm theory, fm stereo multiplexing and the rds mechanism. What is fm, frequency modulation electronics notes. Frequency division multiplexing, fdma amplitude modulation and demodulation. The goal of this experiment is to become familiar with fm modulation and demodulation.
To understand the theoretical foundations for angle modulation as well as frequency modulation fm and demodulation. One approach to reduce this interference, known as. This demodulator is more complex than most asynchronous demodulators. Frequency discrimination slope detector major limitations. Frequency modulation keeps constant the amplitude of the highfrequency oscillation that is used as a signal and to vary the instantaneous frequency for presenting the information. Teach the basics of frequency modulation and demodulation. Quadrature demodulation can extract angle information that is relevant to both frequency modulation and phase modulation.
One approach to reduce this interference, known as frequency division multiplexing, allocates different carrier frequencies to different users or. There will be a minimum frequency separation required of the two tones. Amplitude modulation theory frequency spectrum power. Once a signal has been modulated, information is retrieved through a demodulation process. It reacts to all amplitude changes needs a limiter. For the sake of simplicity, ka 1 and ar ac have been chosen lowpass. A frequency demodulation with using frequency discriminator in this section we are going to build a simple fm demodulator. Frequency modulation keeps constant the amplitude of the high frequency oscillation that is used as a signal and to vary the instantaneous frequency for presenting the information. Guide students to gain a working understanding of theories such as demodulation, bandwidth, and carsons rule. The envelope detector extracts the amplitude, or envelope of the input signal of interest.
This type of demodulation is therefore called coherent demodulation or detection. Once the signal has been received, filtered and amplified, it is necessary to recover the original modulation from the carrier. Fm is considered an analog form of modulation because the baseband signal is typically an analog waveform without discrete, digital values. Frequency modulation fm is a form of modulation in which changes in the carrier wave frequency correspond directly to changes in the baseband signal. Frequency modulation is widely used on frequencies above 30 mhz, and it is particularly well known for its use for vhf fm broadcasting.
These sidebands are visible in the fft as a 1t1 frequency shift in either direction along the. It is also possible to change the frequency of a signal to give frequency modulation or fm. The fm demodulator is done with the help of a circuit called phase locked loop pll. In this experiment you will demodulate the fm signal using either slope detector or phase locked loop pll. Its amplitude a, which leads us to the class of systems designated as amplitude. A vibration with period t2 or a frequency of 1t2 can be seen, which is known as the carrier frequency. For example, the signal in a cell phone is a voice signal with a. Chapter 7 singlesidebandmodulationssb andfrequency. Fm modulation introduction in this lab, you will investigate frequency modulation fm and its properties.
Build simple envelope detector for fm demodulation. Its main advantage is that it is more resistant to additive noise than am. In the demodulator circuit the vco generates a frequency which matches the original carrier frequency and compares the phase of that with received fm wave using the. When transmitted over a linear, timeinvariant lti channel, and if noise is negligible, each sinusoid shows up at the receiver as a sinusoid of the same frequency, as we saw in chapter 12.
Frequencyshift keying fsk is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. Discuss the requirements for recovery of intelligence from an am signal and describe the theory of operation of the following am demodulators. The slope detection method revolves around a differentiation operation that exploits the instantaneous frequency of the fm signal. The simplest method for representing this kind of modulation is fresnels vectorial representation with.
Demodulation of dsbsc the modulator circuits can be used for demodulation, but replacing the bpf by a lpf of bandwidth b hz. The fm modulation and demodulation process is identical in stereo and monaural processes. It is relatively difficult to tune, as tuned circuit must be tuned to different frequency than carrier frequency. The signal output from a demodulator may represent sound an. The output of a fsk modulated wave is high in frequency for a binary high input and is low in frequency for a binary low input. Describe fm demodulation circuit operation for the phaseshift and. The very important conclusion to be made at this stage is that the bandwidth required for amplitude modulation theory is twice the frequency of the modulating signal. Frequency modulation fm it is a process in which the frequency of the carrier is varied in accordance with the instantaneous value of modulating voltage. Modulation allows us to transmit multiple signals in the same medium i. Connect to the two oscilloscope channels using the scope selector, as shown. Fm demodulator circuits are found in any receiver that uses fm.
The amount of change in frequency is determined by. Adjust the rf output frequency as 100 khz and amplitude as 0. Modulation is a process of translating information signal from low band frequency to high band frequency that is suits the transmission medium. Frequency shift keying fsk is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. From experiment 3, we know that the amplitude modulation signal utilizes the amplitude of audio signal to modulate high frequency carrier signal. Fsk frequency shift keying d1 63 fsk in f 1 f 2 decision out figure 4. We may define the instantaneous frequency of the angle modulated signal st as. A demodulator is an electronic circuit or computer program in a softwaredefined radio that is used to recover the information content from the modulated carrier wave. The mathematical representation of frequency modulation consists of a sinusoidal expression with the integral of the baseband signal added to the argument of the sine or cosine function.
Instantaneous frequency the instantaneousfrequencyof cos. The term and technology us used in both telecommunications and signal processing in analog frequency modulation, such as fm radio broadcasting of an audio signal representing voice or music, the instantaneous frequency deviation, the difference between the. Information signal is usually of low frequency, so it cannot travel far. In the second model, you will observe the frequency variations with respect to. The frequency modulator and demodulator structures are as explained below. A pll should have basic functional blocks like voltage controlled oscillator vco, phase comparator, low pass filter lpf and source follower. The modulation index can be used to make the frequency deviation more sensitive or less sensitive to variations in the baseband value. Therefore, when we receive the amplitude modulation signal, we need to restore the audio signal. The amplitude modulation shows as sidebands of the carrier frequency. Slide using a pilot tone slide 14 frequency translation slide 15 frequency translation cont. In this chapter the basic theory of fm modulation and demodulation will be presented and you will implement two types of demodulators, the frequency discriminator and the phaselocked loop. An fm signal can also be used to carry a stereo signal.
Radio systems can use a digital signal processor in conjunction with an analogtodigital converter to apply mathematical analysis to iq waveforms. It does not carry much information that we can relate to data or speech. The fm demodulation in v93000 will also be briefly introduced. The use of the regenerative detector for lowfrequency code reception, however, is usually avoided. Demodulation is extracting the original informationbearing signal from a carrier wave.
Frequency drift was a problem in early or inexpensive receivers. It is inefficient, as it is linear in very limited frequency range. The technology is used for communication systems such as telemetry, weather balloon radiosondes, caller id, garage door openers, and low frequency radio transmission in the vlf and elf bands. There are many types of modulation so there are many types of demodulators. The use of the regenerative detector for low frequency code reception, however, is usually avoided. Slide 9 coherent ssb demodulation slide 10 coherent demodulation cont. Frequency demodulation or detection can be obtained directly by using the pll circuit. Fm itself didnt have a problem apart from we couldnt recognize the fm transmitter potential. Quadrature frequency and phase demodulation radio frequency. Frequency modulation 1 objective to understand the principles of frequency modulation and demodulation. Iq transceiver frequency domain view demodulate using two sine waves in quadrature at receiver must align receiver lo signals in frequency and phase to transmitter lo signals proper alignment allows i and q signals to be recovered as shown transmitter output ff o 0 f o 11 f 0 receiver output ff 1 f 1 0 jj 2cos2. Oct 12, 2018 theory of frequency modulation and phase modulation. So lower the frequency of signal greater will be its wave length as. A smarter approach for reducing the amount of data without loosing essential information is to apply a complex baseband modulation technique with bandwidth reduction known as iqdemodulation.
The frequency of a radiofrequency channel can be explained best as the frequency of a carrier wave. Frequency demodulation electronic circuits and diagrams. Each channel sharing scheme a set of basis vectors. In the earlier time of wireless communication, it was measured that the required bandwidth of this was narrower, and necessary to decrease noise as well as interference. The receiver must generate a carrier frequency in phase and frequency synchronization with the incoming carrier. The fm input signal is first subjected to a limiter in order to eliminate any amplitude modulation noise present in the signal. It is this process that is called demodulation or detection. At low frequencies the detuning required to produce the proper audiobeat frequency is a considerable percentage of the signal frequency. Select the required frequency 2 khz, 4 khz, 6 khz form the switch positions for a.
While changing the amplitude of a radio signal is the most obvious method to modulate it, it is by no means the only way. A carrier wave is purely made up of constant frequency, slightly similar to a sine wave. Frequency modulation fm is a process in which the carrier frequency is varied by the amplitude of the modulating signal i. Then inner products dot products are 1 1 0 can interpret inner product as projection of vector 1 v1 onto vector 2 v2in other words, inner product of v1 and v2 tells us how much of vector 1 is there in the direction of vector 2. It needs a carrier signal of higher frequency for long distance destination. To implement the simulink models for fm including a basic sinusoid and a multimedia file music to analyze each signal in time and frequency domains using time scope and spectrum analyzer. The mfile will show these values in the matlab command window. Frequency shift keying fsk is the digital modulation technique in which the frequency of the carrier signal varies according to the digital signal changes. T1 first patch up according to figure 6, but omit the input x and y connections to the multiplier. Frequency modulation and phase modulation is a system in which the amplitude of the modulated carrier is kept constant, while its frequency and rate of change are varied by the modulating signal. In theory an fm signal contains an infinite number of side frequencies so that the. The fm or frequency modulation has been available approximately since am amplitude modulation although it has only some issues. T2 use the frequency counter to set the audio oscillator to about 1 khz.
The simplest method for representing this kind of modulation is fresnels vectorial representation with a consideration of the phase. Fm modulation and demodulation will be presented and you will implement two types of demodulators, the frequency. Chapter 8 frequency modulationfm fm was invented and commercialized after am. Oct 09, 2018 the two additional terms produced are the two sidebands outlined. Frequency modulation fm is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. Frequency modulation consists of encoding information onto a carrier signal by varying the frequency of the carrier. A smarter approach for reducing the amount of data without loosing essential information is to apply a complex baseband modulation technique with bandwidth reduction known as iq demodulation. The demodulation of a fm signal requires a circuit that yields an output. Build simple fm demodulator by using frequency discriminator. The last step is to subject the differentiated fm signal to an envelope detector. Frequency modulation frequency modulation is realized by varying signal frequency in accordance with the modulating signal or message, as in figure 1.
Describe fm demodulation circuit operation for the phaseshift and gatedbeam discriminators and the ratiodetector demodulator. Now the instantaneous frequency can be thought of as the time varying amplitude of the cosine carrier function originates. The concepts of amplitude modulation, modulation and demodulation, along with their. Slide11 demodulator using a hilbert transform slide12 demod. This chapter describes the essential principles behind. Frequency demodulation matlab fmdemod mathworks india. Fm concepts explained what is frequency modulation. Carrier modulation allows the transmission of modulating frequencies without the use of. For example, the signal in a cell phone is a voice. The two additional terms produced are the two sidebands outlined.