Us4303894a fm modulator using variable transistor capacitance. Frequency modulation fm is a process in which the carrier frequency is varied by the amplitude. A varactor is a diode for which its capacitance depends nonlinearly on the voltage applied to its terminals, and thus it is a nonlinear capacitor. Find powerpoint presentations and slides using the power of, find free. The first method can be accomplished by the use of a reverse biased diode, since the capacitance of such a diode varies with applied voltage.
In an fm modulation circuit, a variable capacitance circuit employs a transistor as a variable capacitance element. Scribd is the worlds largest social reading and publishing site. Varactor theory free download as powerpoint presentation. As an alternative, it is possible to realize dielectricfree actuation electrodes. In frequency modulators they can be placed across the resonant element within the generator and the audio applied to the diode. Pdf varactor diodebased circuit topologies, which can act as highq. Fig b shows the basic concept of a varactor frequency modulator. Semiconductor devices varactor diode this is a special pn junction diode with an inconsistent concentration of impurities in its pn materials. Even though the variable capacitance effect can be exhibited by the normal diodes pn junction diodes, but, varactor diodes are preferred for giving the desired capacitance changes as they are special types of diodes. The varactor diode, sometimes called tuning diode, is the diode whose. Before going to varactor diode, lets first take a look at the capacitor. Infineon has more than 30 years development expertise for the rf small signal discretes. The voice signal is applied from an input terminal 1 to the varactor diode 6 through a capacitor 2 and a. A varactor diode also known with the names varicap diode, varactor diode, tuning diode is a pn junction diode which acts as a variable capacitor under varying reverse bias voltage across its terminals.
Pdf distortionfree varactor diode topologies for rf adaptivity. How can you use a varactor diode in the generation of fm. A varactor diode is specifically designed for this. Varactor diodes are also termed as varicap diodes, in fact, these days they are usually termed as varactor diodes.
There are two types of fm modulators direct and indirect. Varactor diode definition, symbol, construction and. Varactor diode is a reverse biased pn junction diode, whose capacitance can be varied electrically. Linear gain voltage controlled oscillator with modulation compensation. For comprehensive details of different types of diodes, visit this page. As a result these diodes are also referred to as varicaps, tuning diodes, voltage variable capacitor diodes, parametric diodes and variable capacitor diodes. The different types of varactor diodes are available in the market such as hyper abrupt, abrupt and galliumarsenide varactor diodes.
Varactor diodes are used along with reactance modulator to provide automatic frequency correction for an fm transmitter. Distortionfree varactor diode topologies for rf adaptivity. Varactor diodes ideal for vco, vcxo, voltage controlled. In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage. It is well known that the operation of the pn junction depends on the bias applied which can be either forward or reverse in characteristic. Figure 1 shows a direct frequency modulator which uses a varactor diode.
The symbol of the varactor diode is shown in the above figure that includes a capacitor symbol at one end of the diode that signifies the characteristics of the variable capacitor of the varactor. The vco has an input voltage of 2v and is running at its free running. The l1 and c1 represent the tuned circuit of the carrier oscillator. Varactor diode is also sometimes referred to as varicap diode, tuning diode, variable reactance diode, or variable capacitance diode.
For free electrons, nregion is the higher concentration region and pregion is the lower. A varactor diode is a semiconductor diode whose junction capacitance varies linearly with applied voltage when the diode is reverse biased. The varactor diode modulator circuit is shown in fig5. View and download powerpoint presentations on varactor diode ppt. Varactor diode is a pn junction diode whose capacitance is varied by varying the reverse voltage. Varactor diode working, characteristics and its applications. The capacitance of d 1, of course, is controlled by two factors. In a normal pn junction diode, doping impurities are. This concept is modified to create a digital modulator as shown in fig.
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