There are many types of modulation so there are many types of demodulators. Demodulation is defined as extracting the original informationcarrying signal from a modulated carrier wave. Chapter 5 radio frequency demodulation pdf version. Disadvantage of fft filters all the data manipulation is processor intensive and delays are an issue. Because the dfsk signal transmits only one frequency at a time, transmitter power efficiency can be much higher than an fdm transmitter, which has a linearity requirement to prevent intermodulation of tones. In this experiment, we will use a famous chip, the lm 566c to generate the fm signal.
The continuous analog signal st is a baseband signal with the information content voice or music to be transmitted. Modulation is the process of impressing a lowfrequency information signal baseband signal onto a higher frequency carrier signal modulation is done to bring information signals up to the radio frequency or higher signal. Implementation of fm demodulator algorithms on a high. Carrier is recovered by the carrier recovery circuit at the receiver. I and q components in communications signals and single. Then retune the transformer so that the fm carrier frequency falls at 455 khz. It requires no changes to the basic phase locked loop, itself, utilising the basic operation of the loop to provide the required output. The pll demodulates the fm signal using feedback force a voltagecontrolledoscillator vco to remain in phase with the carrier of the incoming signal.
To keep the compatibility the new device should be able to perform the analog modulation. Figure 9 shows a functional block diagram for an fm detector type remove out of band interference and then limited to remove am inter ference. Passing the derivative of the fm signal through an envelope detector will give the desired message signal at the output. Frequency modulation fm is a form of modulation in which changes in the carrier wave frequency correspond directly to changes in the baseband signal. The main reason behind using the fm modulations by the radio stations is the quality of the signal that can be recreated in the fm receiver. The most basic frequency demodulator consists of a frequency discriminator, which is a differentiator followed by an envelope detector, as shown in figure 3. Abstract analog modulations are more and more replaced by digital ones. Apr 16, 2017 demodulation of fm is done by frequency discrimination method and phase discrimination method. Slope detector can be constructed with the help of a tuned amplifier and an envelope detector. The analytical signal for an analog frequency modulated fm signal is. There are two types of am detectors or demodulators such as. This information recovery process is known as demodulation or detection.
Because both algorithms demand the same pre and after signal preparation, the demodulation. Once the signal has been received, filtered and amplified, it is necessary to recover the original modulation from the carrier. Referred to as the baseband signal carrier a higher frequency sinusoid example. An important concept in the understanding of fm is frequency deviation. In this experiment you will demodulate the fm signal using either slope detector or phase locked loop pll. It converts the instantaneous frequency deviation in an fm signal to. Describe fm demodulation circuit operation for the phaseshift and gatedbeam discriminators and the ratiodetector demodulator. Its amplitude a, which leads us to the class of systems designated as amplitude. The term and technology is 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.
However if st is sinusoidal, then the instantaneous frequency deviation of the anglemodulated signal is. Jan 18, 2012 when a modulated signal arrives at the receiving antenna, the embedded information must somehow be extracted by the receiver and separated from the hf carrier signal. Fm detector demodulators the fm detector demodulator treats the fsk signal as a simple fm signal with binary modulation. A pll can lock onto the frequency of an incoming waveform. Modulation and demodulation technology innovation and. However the fm modulator and demodulator circuits are complex compared to other modulation and demodulation techniques. Demodulation of fm slope and balanced slope detector. Learn about two techniques for recovering the baseband signal from a frequencymodulated carrier. Consequently, unlike amplitude modulation, the spectrum of an fm signal is. The message is recovered as the control input of the vco 4.
You will model the pll, and recover the message from the fm signal. Frequency modulation consists of encoding information onto a carrier signal by varying the frequency of the carrier. This senior project paper reports on the design and implementation of an fm receiver front end and of a digital phase lock loop design to demodulate fm broadcast signals. To recover the message signal from the fm signal, frequency demodulation must be performed. Introduction frequency modulation is a form of modulation, which represents information as. If you want a starting point, the equivalent function in matlab is fmdemod if you have the communications toolbox. Fm detectortype demodulators are somewhat complex for fdm work and cannot generally be used with interleaved fdm signals. Once the signal has been received, filtered and amplified, it is necessary to recover the. This process of detection is exactly opposite to that of modulation. Menu capture and decode fm radio fraida fund 10 february 2016 on education, software defined radio, wireless. An oscillator whose frequency is controlled by its input mt in this manner is called a voltage controlledoscillator.
The block diagram of square law demodulator is shown in fig 2. The process of recovering the message signal from the received modulated signal is known as demodulation. Demodulation for communications simulation matlab demod. Fm demodulation is also called fm detection and sometimes the phrase fm discrimination is used, although this term tends to be used with older circuits and technology. Phase locked loop fm demodulators are used in many radio receivers both domestic and professional for the demodulation of fm signals. Basic concept of modulation the information source typically a low frequency signal referred to as the baseband signal carrier a higher frequency sinusoid example. There are different types of modulation and so are demodulators.
This chapter describes the demodulation methods of am signal and fm signal. A complex baseband fm signal is generated from the conventional fm signal 1 as shown in figure 1. However, for a small class of signals specifically designed for pll demodulation, pll demodulators may perform better than fm detector demodulators. This experiment is meant to teach the basics of fm signal processing. Fm is considered an analog form of modulation because the baseband signal is typically an analog waveform without discrete, digital values.
In singlechannel efield sensing, the carrier we transmit is m, the sensed value is a, and the noise is n vectors. Demodulation is extracting the original informationbearing signal from a carrier wave. Restoring or recovering the message signal from the received modulated waveform that is generally corrupted by noise types of am detectors. Using matlab mfile and simulink to implement fm modulation and demodulation. Lets assume that by looking at the transfer curve obtained, you have decided that the fm carrier should fall at 452 khz. This circuit takes in frequency modulated rf signals and takes the modulation from the signal to output only the modulation that had been applied at the transmitter.
A noncoherent detector has only one input, namely, the modulated signal port. The second section will be the design of the fm radio front end. Two of them mixed and pll demodulator are are implemented on the dsp tms320c6711dsk. A demodulator is an electronic circuit that is mainly used to recover the information content from the modulated carrier wave. However the fm modulator and demodulator circuits are complex compared to other modulation and demodulation. The signal to noise ratio is comparatively very high at the output of fm demodulator circuits. Fm demodulation is a key process in the reception of a frequency modulated signal. Fm can adjust to rapid fading change of amplitude using automatic gain control agc. Jun 05, 2015 guide students to gain a working understanding of theories such as demodulation, bandwidth, and carsons rule. Fm modulation demodulation with pll general description package outline njw2307 is a fm modulation demodulation ic for audio signal in full duplex communication that operates from 3. Some phase modulation and quadrature amplitude modulation formats have a higher spectral efficiency for data transmission than frequency shift keying, a form of frequency modulation. Therefore, the following block diagram is an fm demodulator.
In an earlier semester project, four fm demodulation algorithms were developed and tested on their suitability. Remember to set up the vco module in 100 khz vco mode. Mixeramplifier allows fm ip signal to be am modulated by a noise ip prior to demodulation 3. The goal of this experiment is to become familiar with fm modulation and demodulation. This means that, in order to move the fm carrier up to 455 khz, the whole response curve. The analytical signal for double sideband, large carrier amplitude modulation dsblc am is. The way in which a phase locked loop, pll fm demodulator works is relatively straightforward.
Downmixing lowpass filtering decimation the multiplication with the square root of two is included to preserve the energy in the signal explained in section 3. The angle of the fm signal, assuming the value is 0 at t 0, is. Fm can be created by applying message signals directly to a voltagecontrolled oscillator vco, so that the vcos output will be a function of the input signal. In addition to audio signal, it can also be used in data pulse. Of course, you cannot apply ordinary amplitude demodulation to a frequency modulated signal. Signal manipulation based on frequency removing bin 210 212 performing an ifft inverse fast fourier transform and sending the data stream to a dac will recreate the analog signal without the carrier. Frequency modulation frequency modulation is realized by varying signal frequency in accordance with the modulating signal or message, as in figure 1. The fm demodulator is done with the help of a circuit called phase locked loop pll.
Describe fm demodulation circuit operation for the phaseshift and gatedbeam discriminators. Fsk demodulation the demodulation methods for fsk can be makes all positive voltages into binary 1s and all negative voltages into binary 0s. When a modulated signal arrives at the receiving antenna, the embedded information must somehow be extracted by the receiver and separated from the hf carrier signal. I recently compiled and posted a project on describing how a theremin an electronic musical instrument can be built using the analog discovery 2, the digilent analog parts kit, and a. If you do not specify the opt parameter, modulate uses a default of opt pimaxmaxx so the maximum phase excursion is. Fm is less vulnerable to signal interference from adjacent channels. Each channel sharing scheme a set of basis vectors. The am demodulator can be implemented by utilizing a balanced modulator. Frequency modulation the fm signal st defined by equation 2. Frequency modulation fm is a process in which the carrier frequency is varied by the amplitude of the modulating signal i. Teach the basics of frequency modulation and demodulation.
To understand the theoretical foundations for angle modulation as well as frequency modulation fm and demodulation. Teach the basics of frequency modulation and demodulation national instruments. The demodulator extracts the original baseband signal from the received modulated signal to summarize. Understanding quadrature demodulation radio frequency. Frequency modulation offers improved performance over amplitude modulation, but it is somewhat more difficult to extract the original information from an fm waveform. So, the message signal of the above derivative is contained in the frequency of the sinusoid and also in its magnitude. Frequency modulation fm is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. A phaselocked loop pll can be used to create a complex but highperformance circuit for fm demodulation. Dutton ee3 winter 2004 3 3 fm demodulation using the pll now that youve seen how the pll tracks a signal, we will explore one of its applications, fm demodulation. This complex representation of fm signal has a major advantage in the design of fm demodulator, which involves division and arctan. Fixedpoint dsp implementation of fm demodulationdecoding. The total mixing operation produces a real inphase and an imaginary quadraturephase baseband component. If youve read the page on amplitude demodulation, the preceding paragraph may have caused you to realize that a quadrature demodulator is actually composed of two amplitude demodulators.
In the simulation experiment section2, we used the vco to demodulate the information signal to make life easier. In the modulating process the carrier wave is generated by the lo circuit, and then combined with the baseband signal inside the mixer. Discuss the requirements for recovery of intelligence from an am signal and describe the theory of operation of the following am demodulators. You cant easily cook up firiir filters, for instance. It should take about 60120 minutes to run depending on your familiarity with the reference material, but you will need to have reserved that time in advance. In coherent detection or synchronous demodulation both the phase and frequency of carrier must be known at the detector. The left column is the shaped baseband signal spectrum, and the middle column is the spectrum of the modu lated signal that is transmitted and received. Fm, on the other hand, encodes all of the information in the temporal characteristics of the transmitted signal. By the time youre done, you should be sending and receiving your very own fm broadcasts from within the con.
Is a nonlinear function of the modulating signal mt, which makes frequency modulation a nonlinear modulation process. Upon completion of the experiment, the student will. In this case, using the 1,0,1 coefficient differentiators, the fm demodulator is simplified to that shown in figure 61b where the scaling operation is multiplication by. A pilot signal is transmitted outside the passband of modulated signal.
Phasemodulation and demodulation techniques are more complex than modulation and demodulation based on amplitude and frequency. The design approach is to design the fm demodulator as if it were an analog phase lock loop. Frequency modulation 1 objective to understand the principles of frequency modulation and demodulation. Fm demodulation in v93000 will also be briefly introduced. 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. January 22, 2019 january 25, 2019 by ian etheridge 2 comments. Introduction to iq demodulation of rfdata september 15, 1999 page 6 of 3. We call this type of modulator as synchronous detector or product detector. 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.
But quadrature amplitude demodulation can capture the frequencyencoded informationthis is simply the rather interesting nature of iq signals. Finds the pulse widths of a pulsewidth modulated signal y. Once a signal has been modulated, information is retrieved through a demodulation process. The fm is a very popular technique since they are widely used by the fm radio stations.
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