The non linear characteristics of varactor diode produce a distorted fm signal due to harmonics in the modulating signal. Vco has a center frequency f c and the input control voltage mt modulates the instantaneous frequency around this center frequency. Distorted fm signal is generated due to harmonics of modulating signal c. The oscillator does not change frequency, as is the case of direct fm. Punjab edusat society disadvantages of direct method in. What are the major advantages of the indirect method of. There are essentially two basic methods of generating frequency modulated signals namely. Direct fm modulation frequency of the carrier changes according to mt. It is used to perform direct frequency modulation on signals. Fm generation by direct method in analog communication by engineering funda duration.
Fm transmitters as previously stated, if a crystal. This method of fm generation is direct because the oscillator frequency is varied directly by the modulating signal, and the magnitude of frequency change is proportional to the amplitude of the. The direct methods cannot be used for the broadcast applications. The indirect method of reporting cash flows is useful for simplifying your operational activities to investors. Direct method frequency of oscillator of carrier 1. Pdf a method for direct generation of floor acceleration. The magnitude of the phase deviation is directly proportional to the amplitude of the modulating signal but independent of its frequency. Once the signal has been received, filtered and amplified, it is necessary to recover the. Armstrong method is used for generation of indirect fm signal.
Fm generation by direct method in analog communication by. Differentiator is used to convert the fm wave into a combination of am wave and fm wave. While changing the amplitude of a radio signal is the most obvious method to modulate it, it is by no means the only way. Multiple choice questions and answers on frequency modulation. The following figure shows the block diagram of fm demodulator using frequency discrimination method. This method is called the armstrong method of fm wave generation. Jul 08, 2015 methods of fm generation direct method reactance modulator indirect method armstrong methodvaractor modulator 11. The frequency of such an oscillator can be varied by an amount. Set the front panel gain control fully anticlockwise.
Vco produces an output signal, whose frequency is proportional to the input signal voltage. In this method of modulation the amplitude of the carrier wave is maintained constant. Set the onboard scaling resistor of the spectrum utilities module to obtain a full scale deflection fsd on the carrier. Frequency modulation fm is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave.
Advantage and disadvantage of fm generation using indirect method. Thus, crystal oscillators cannot be used in broadcast fm, but other oscillators can suffer from frequency drift. Dec 21, 2017 drawbacks of using direct method for generation of fm signal are. Direct modulation is usually accomplished by varying a capacitance in an lc oscillator. To overcome the disadvantage of direct method of fm wave. The direct method through focusing on everyday language, and using questions and answers lays an emphasis on teaching oral language.
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 deviation, the difference between the. Direct fm modulation frequency of the carrier changes according to mt advantages of direct fm. Fm generation methods fm signal generation they are two. This method is called as the direct method because we are generating a wide band fm wave directly. Frequency modulation fm the process of combining the message signal with the carrier. Frequency modulation is widely used for fm radio broadcasting. Direct modulation is usually accomplished by varying a capacitance in an lc oscillator or by changing the charging current applied to a capacitor. Does not give high stability to fm signal frequency b. Explain armstrong method of fm generation with the help of. The first method can be accomplished by the use of a reverse. Balanced frequency descriminator, zero crossing detector, phased lock loop, foster seeley discriminator, ratio detector, fm transmitter block diagram and explanation of each block.
In this method, voltage controlled oscillator vco is used to generate wbfm. Analog communication fm demodulators tutorialspoint. Indirect method of fm generation armstrong method learn. In radio transmission, an advantage of frequency modulation is that it.
The indirect method of generating wbfm was first prepared by e. Represent a frequency modulated passband signal, yt, as y t a cos 2. In the armstrong method of fm generation, the phase of the carrier is directly modulated in the combing network through summation, generating indirect frequency modulation. The direct method, which is also known as natural method or conversational method, has been popular since it enables students to communicate in the foreign language.
There are two types of fm modulators direct and indirect. Signals and modulation properties of amssb signals mathematical analysis of fm, noise amssb transmitter systems, fm applications, noise computation, introduction to receivers, fm and noise suppression, information and bandwidth, amssb detection, direct fm generation, the superheterodyne concept, indirect fm generation, amssb receiver subsystem. It is also used in telemetry, radar, seismic prospecting, and monitoring newborns for seizures via eeg, twoway radio systems, music synthesis, magnetic taperecording systems and some videotransmission systems. Fm fm generation by vco generation by vco generation by vco modules. It has better frequency stability than other method such as reactance. Fm nb t assume a bpf is included in this block to pass the signal with the highest carrier freuqnecy and reject all others a narrowband fm signal can be generated easily using the block. Fm modulators and transmitters sonoma state university. However if st is sinusoidal, then the instantaneous frequency deviation of the anglemodulated signal is. Method for improving frequency stability of fm system. Low distortion due to mproved signal to noise ratio about 25db w. Balanced zerocrossing fm detector this is a hybrid circuit analog and digital combination if fi fc.
In this procedure, a direct method is simulated, using a frequency differentiator and an envelope detector. Advantage of using direct method for generation of fm. It is also possible to change the frequency of a signal to give frequency modulation or fm. Introduction to fm using a vco a2 91 t1 before plugging in the vco, set the mode of operation to vco with the on board switch sw2. It can also be used to present investing and financial activities. Generation of fm waves, direct method, parametric variation method, varactor diode, reactance modulator, indirect method.
Fm generation by direct method in analog communication by engineering funda. A crystal oscillator can be used hence the frequency stability is very high. Smaller geographical interference between neighbouring stations. Army techniques publication atp 411, army motor transport operations, is the armys doctrine for the use of motor transportation in support of operations. Fm modulation system fm transmitter, reactance modulator. Armstrong fm transmitter indirect method of modulation, phase. In this video, i have explained fm generation by direct method by following outlines. Block diagram of direct method of generating wideband fm. Fm transmitters as previously stated, if a crystal oscillator. M armstrong so this method is properly known as armstrong method.
All that is needed is a linear voltagetofrequency converter. Advantage and disadvantage of fm generation using indirect. The second method, considered an indirect method, uses a phaselocked loop. Armstrong fm transmitter indirect method of modulation.
Pdf an efficient algorithm and its implementation for armstrongs. Armstrong method is used for the generation of options. Mdirect methodb techcommunication system4th sem lect27 duration. Fm transmitters as previously stated, if a crystal oscillator is used to provide the carrier signal, the frequency cannot be varied too much this is a characteristic of crystal oscillators. Another fm modulator which is widely used in transistorized circuitry uses a voltagevariable capacitor varactor.
In eecs 311 and eecs 4, you will study in detail amplifiers, feedback and oscillators. Indirect fm involves directly altering the phase of the carrier based on the input this is actually a form of direct phase modulation. The doctrine discussed in this manual is nested with fm 551, transportation operations, and supports the principles of transportation. Oct 19, 2015 fm signal generation they are two basic methods of generating frequency modulated signals 1. Jun 22, 2017 indirect method of fm genration gate achievers. The carrier signal is generated using a lc oscillator circuit. The frequency multiplier section, which features several frequency multipliers.
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. What is fm, frequency modulation electronics notes. In direct method, the modulating base band signal directly modulates the carrier. Explain armstrong method of fm generation with the help of a. Assume that we would like to demodulate the following fm signal which has a carrier frequency of. Armstrong method generates frequency modulated wave from a pulse modulated wave. Armstrong indirect method for wideband fm generation. Vco center frequency, f c frequency deviation constant, f d instantaneous frequency of fm signal. A very simple method for direct generation of floor response spectra was proposed by yasui et al. Audio oscillator preparation preparation a very simple and direct method of generating an fm signal is by the use of a voltage controlled oscillator vco. Pdf analog communication system pdf notes ac notes sw.
Introduction to fm using a vco a2 93 t10 revise your skills with the wave analyser. The exciter section contains the carrier oscillator, reactance modulator and the buffer amplifier. The varactor diode fm modulators are widely accepted because they are simple to use, reliable and have the stability of a crystal oscillator. Vco frequency multiplier mixer frequency multiplier fixed oscillator bandpass filter baseband signal wideband fm fig. To overcome the disadvantage of direct method of fm wave generations, in the indirect method a stable crystal oscillator is used to generate pm from which narrow band fm is obtained. The block diagram of the generation of wbfm wave is shown in the following figure. Fm demodulation is a key process in the reception of a frequency modulated signal. Use the front panel control fo to set the frequency to 10 khz. Frequency modulation is widely used on frequencies above 30 mhz, and it is particularly well known for its use for vhf fm broadcasting. Direct method for generation of fm signal drawbacks. Fm signal generation they are two basic methods of generating frequency modulated signals 1.
The varactor is simply a diode, or pn junction, that is designed to have a certain amount of capacitance between junctions. In this method the fm is obtained through phase modulation. In direct method of fm generation, as the carrier generation is directly affected by the modulating signal, it is not easy to get high order stability in carrier frequency. These points out the major advantage of phase modulation pm, or indirect fm, over direct fm. This video will help you to understand the generation of fm signals by using indirect method of frequency modulation or armstrong method. The fm modulator baseband block applies frequency modulation to a real input signal and returns a complex output signal. The exciter section contains the carrier oscillator, reactance modulator and the buffer amplifier the frequency multiplier section, which features several frequency multipliers the power output section, which includes a low. T3 use the frequency counter to monitor the vco frequency.
The magnitude of the phase deviation is directly proportional to the amplitude of the modulating signal but independent of. Jun 22, 2017 indirect method of fm genration duration. Then suitable frequency multiplying circuits are used to obtain the desired wide and fm. That is the phase modulator is crystal controlled for frequency. Direct method of fm generation here instantaneous frequency of the carrier wave is varied directly in accordance with the message signal by means of vco.