Chapter 5 radio frequency demodulation pdf version. Frequency modulation offers improved performance over amplitude modulation, but it is somewhat more difficult to extract the original information from an fm waveform. Fm is considered an analog form of modulation because the baseband signal is typically an analog waveform without discrete, digital values. Its amplitude a, which leads us to the class of systems designated as amplitude. Demodulation of fm slope and balanced slope detector. Teach the basics of frequency modulation and demodulation national instruments. Passing the derivative of the fm signal through an envelope detector will give the desired message signal at the output. Remember to set up the vco module in 100 khz vco mode. The signal to noise ratio is comparatively very high at the output of fm demodulator circuits. This means that, in order to move the fm carrier up to 455 khz, the whole response curve.
Apr 16, 2017 demodulation of fm is done by frequency discrimination method and phase discrimination method. 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. Phase locked loop fm demodulators are used in many radio receivers both domestic and professional for the demodulation of fm signals. In the modulating process the carrier wave is generated by the lo circuit, and then combined with the baseband signal inside the mixer. There are different types of modulation and so are demodulators. If you do not specify the opt parameter, modulate uses a default of opt pimaxmaxx so the maximum phase excursion is. Learn about two techniques for recovering the baseband signal from a frequencymodulated carrier. Basic concept of modulation the information source typically a low frequency signal referred to as the baseband signal carrier a higher frequency sinusoid example. You cant easily cook up firiir filters, for instance. The continuous analog signal st is a baseband signal with the information content voice or music to be transmitted. Finds the pulse widths of a pulsewidth modulated signal y. 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. Demodulation is defined as extracting the original informationcarrying signal from a modulated carrier wave. Fm detector demodulators the fm detector demodulator treats the fsk signal as a simple fm signal with binary modulation.
Each channel sharing scheme a set of basis vectors. Demodulation for communications simulation matlab demod. However the fm modulator and demodulator circuits are complex compared to other modulation and demodulation. Upon completion of the experiment, the student will. Describe fm demodulation circuit operation for the phaseshift and gatedbeam discriminators and the ratiodetector demodulator. By the time youre done, you should be sending and receiving your very own fm broadcasts from within the con.
Frequency modulation fm is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The design approach is to design the fm demodulator as if it were an analog phase lock loop. Lets assume that by looking at the transfer curve obtained, you have decided that the fm carrier should fall at 452 khz. 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 demodulation is a key process in the reception of a frequency modulated signal. Fsk demodulation the demodulation methods for fsk can be makes all positive voltages into binary 1s and all negative voltages into binary 0s.
Fm is less vulnerable to signal interference from adjacent channels. Referred to as the baseband signal carrier a higher frequency sinusoid example. Disadvantage of fft filters all the data manipulation is processor intensive and delays are an issue. Introduction to iq demodulation of rfdata september 15, 1999 page 6 of 3. The way in which a phase locked loop, pll fm demodulator works is relatively straightforward. 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. So, the message signal of the above derivative is contained in the frequency of the sinusoid and also in its magnitude. The process of recovering the message signal from the received modulated signal is known as demodulation. 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. Of course, you cannot apply ordinary amplitude demodulation to a frequency modulated signal. The fm is a very popular technique since they are widely used by the fm radio stations. Once a signal has been modulated, information is retrieved through a demodulation process. The message is recovered as the control input of the vco 4.
In coherent detection or synchronous demodulation both the phase and frequency of carrier must be known at the detector. Fm can adjust to rapid fading change of amplitude using automatic gain control agc. Introduction frequency modulation is a form of modulation, which represents information as. January 22, 2019 january 25, 2019 by ian etheridge 2 comments. Abstract analog modulations are more and more replaced by digital ones. This complex representation of fm signal has a major advantage in the design of fm demodulator, which involves division and arctan. 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. Then retune the transformer so that the fm carrier frequency falls at 455 khz. The most basic frequency demodulator consists of a frequency discriminator, which is a differentiator followed by an envelope detector, as shown in figure 3. To keep the compatibility the new device should be able to perform the analog modulation. The second section will be the design of the fm radio front end.
The analytical signal for double sideband, large carrier amplitude modulation dsblc am is. 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. Fm demodulation in v93000 will also be briefly introduced. Is a nonlinear function of the modulating signal mt, which makes frequency modulation a nonlinear modulation process. Mixeramplifier allows fm ip signal to be am modulated by a noise ip prior to demodulation 3.
The angle of the fm signal, assuming the value is 0 at t 0, is. Implementation of fm demodulator algorithms on a high. Carrier is recovered by the carrier recovery circuit at the receiver. Teach the basics of frequency modulation and demodulation. Describe fm demodulation circuit operation for the phaseshift and gatedbeam discriminators. 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 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 pilot signal is transmitted outside the passband of modulated signal. Once the signal has been received, filtered and amplified, it is necessary to recover the original modulation from the carrier. A pll can lock onto the frequency of an incoming waveform. It converts the instantaneous frequency deviation in an fm signal to. A noncoherent detector has only one input, namely, the modulated signal port. Fm, on the other hand, encodes all of the information in the temporal characteristics of the transmitted signal. 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.
If you want a starting point, the equivalent function in matlab is fmdemod if you have the communications toolbox. The pll demodulates the fm signal using feedback force a voltagecontrolledoscillator vco to remain in phase with the carrier of the incoming signal. Consequently, unlike amplitude modulation, the spectrum of an fm signal is. However the fm modulator and demodulator circuits are complex compared to other modulation and demodulation techniques. It requires no changes to the basic phase locked loop, itself, utilising the basic operation of the loop to provide the required output. Frequency modulation fm is a process in which the carrier frequency is varied by the amplitude of the modulating signal i. Therefore, the following block diagram is an fm demodulator. 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. 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. An important concept in the understanding of fm is frequency deviation. In this experiment, we will use a famous chip, the lm 566c to generate the fm signal.
A complex baseband fm signal is generated from the conventional fm signal 1 as shown in figure 1. 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. Modulation and demodulation technology innovation and. Frequency modulation consists of encoding information onto a carrier signal by varying the frequency of the carrier.
Understanding quadrature demodulation radio frequency. However, for a small class of signals specifically designed for pll demodulation, pll demodulators may perform better than fm detector demodulators. 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. To understand the theoretical foundations for angle modulation as well as frequency modulation fm and demodulation. Frequency modulation the fm signal st defined by equation 2. Fixedpoint dsp implementation of fm demodulationdecoding. The analytical signal for an analog frequency modulated fm signal is. A phaselocked loop pll can be used to create a complex but highperformance circuit for fm demodulation.
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. The pll fm demodulator provides a very attractive option in many instances, offering exceedingly low levels of distortion, and the ability to be incorporated into integrated circuit technology. 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. The am demodulator can be implemented by utilizing a balanced modulator. Fm detectortype demodulators are somewhat complex for fdm work and cannot generally be used with interleaved fdm signals. The fm demodulator is done with the help of a circuit called phase locked loop pll. 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. We call this type of modulator as synchronous detector or product detector. There are two types of am detectors or demodulators such as. Because both algorithms demand the same pre and after signal preparation, the demodulation.
Menu capture and decode fm radio fraida fund 10 february 2016 on education, software defined radio, wireless. Demodulation is extracting the original informationbearing signal from a carrier wave. In this experiment you will demodulate the fm signal using either slope detector or phase locked loop pll. Frequency modulation 1 objective to understand the principles of frequency modulation and demodulation. I and q components in communications signals and single. 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 demodulator extracts the original baseband signal from the received modulated signal to summarize. The goal of this experiment is to become familiar with fm modulation and demodulation.
An oscillator whose frequency is controlled by its input mt in this manner is called a voltage controlledoscillator. 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. Frequency modulation fm is a form of modulation in which changes in the carrier wave frequency correspond directly to changes in the baseband signal. Discuss the requirements for recovery of intelligence from an am signal and describe the theory of operation of the following am demodulators. Frequency modulation frequency modulation is realized by varying signal frequency in accordance with the modulating signal or message, as in figure 1. Two of them mixed and pll demodulator are are implemented on the dsp tms320c6711dsk. 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.
In an earlier semester project, four fm demodulation algorithms were developed and tested on their suitability. This information recovery process is known as demodulation or detection. There are many types of modulation so there are many types of demodulators. In singlechannel efield sensing, the carrier we transmit is m, the sensed value is a, and the noise is n vectors. 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.
The total mixing operation produces a real inphase and an imaginary quadraturephase baseband component. Restoring or recovering the message signal from the received modulated waveform that is generally corrupted by noise types of am detectors. In the simulation experiment section2, we used the vco to demodulate the information signal to make life easier. This chapter describes the demodulation methods of am signal and fm 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.
A demodulator is an electronic circuit that is mainly used to recover the information content from the modulated carrier wave. In addition to audio signal, it can also be used in data pulse. To recover the message signal from the fm signal, frequency demodulation must be performed. Using matlab mfile and simulink to implement fm modulation and demodulation. Slope detector can be constructed with the help of a tuned amplifier and an envelope detector. 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. This experiment is meant to teach the basics of fm signal processing. But quadrature amplitude demodulation can capture the frequencyencoded informationthis is simply the rather interesting nature of iq signals. You will model the pll, and recover the message from the fm signal. However if st is sinusoidal, then the instantaneous frequency deviation of the anglemodulated signal is. The block diagram of square law demodulator is shown in fig 2.1588 116 1557 667 827 413 1556 959 1 691 1583 637 261 41 100 1551 782 411 64 1357 883 1499 962 731 788 541 423 277 609 1060 1368 1229 1251 102 614 252 1467 1488 147 783 405 124 1135 509 600 603