With the switch open, only a small leakage current flows between source and drain. This type of circuit can also be used as an amplitude modulator. Hence keeping c and r constant, the effective capacitance c e offered by the reactance tube varies. Find out information about reactancetube modulator.
Eg vcolinear integratedcircuit direct fm modulator highfrequency deviations and high modulation indices. An electrontube circuit, used to produce phase or frequency modulation, in which the reactance is varied in accordance with the instantaneous amplitude of. The fieldeffect transistor fet is a type of transistor which uses an electric field to control the flow of current. Fet reactance modulator 3 terminal reactance that can be connected across tank circuit. Larger reactance leads to smaller currents for the same voltage applied. Oct 19, 2018 the circuit shown is the basic circuit of a fet reactance modulator, which behaves as a threeterminal reactance that may be connected across the tank circuit of the oscillator to be frequencymodulated. In the stabilized reactance modulator afc system, a. The practice has shown that the input diode byv1040, easily gets defective when connecting the supply voltage the diode gets an internal short circuit.
It takes the original signal that has both sidebands and a carrier signal, and then modulates it so that only the. The modulating voltage v m is applied at the control grid of the reactance tube as shown in figure 1. Fm reactance modulator a direct fm modulator using a jfet as. The lecture describes the working of fet reactance modulator for fm generation. Fm modulation system fm transmitter, reactance modulator. Direct pm angle modulation in which the frequency of the carrier is deviated indirectly by the modulating signal. Notes two signal generators are used in this circuit, one. Fet balanced modulator for ssb under repositorycircuits. How does one select the values of components used in amplitude modulation circuit using a transistor common emitter amplifier circuit. Find out information about reactance tube modulator.
Am modulator this circuit uses two signal generators to simulate an amplitude modulated rf carrier wave. Theory to determine z, a voltage v is applied to the terminals aa between which the impedance is to be measured, and the resulting current i is calculated. Its a colpitts and instead of a separate transistor as the reactance modulated device, the capacitance of the oscillator transistor is modulated directly by the small differences in basecollector voltage caused by the signal from the microphone. The j fet or junction fet is a form of fet where the gate is formed by using a diode junction onto the channel. A balanced modulator is a device that modifies a signal, usually in the form of an amplitude modulated am radio signal. The practice has shown that the input diode byv1040, easily gets defective when connecting the supply voltage the diode gets an internal short. Therefore, the value of reactance introduced into the tuned circuit varies directly with the transconductance of the reactance tube. Modulation is defined as the process by which some characteristics of a carrier signal isvaried in accordance with a modulating signal. Ppt frequency modulation fm powerpoint presentation free. Fet circuit configurations are the common source, common gate, and common drain formats. Before the invention of transistor, reactance tube modulation was popularly used to produce frequency modulation.
Each have their own characteristics of voltage and current gain as well as input and output impedance. A simple c apacitive r eactance c ircuit, utilizing a firstorder feedback ne t work and operating at a f requency of 250 khz, yielded a low q effective capacitance but ser ved to verify the mode l em. This circuit uses two signal generators to simulate an amplitude modulated rf carrier wave. Some contain automatic circuitry to shut the transmitter. Two signal generators are used in this circuit, one representing a high frequency 200khz rf carrier, vg2, the other signal generator is used to inject a 1khz audio signal. And again a class a bias circuit is used for this device. The transistor reactance modulator shown in the figure is to be used in an fm transmitter. Clean up take your circuit s apart and put components away. When using the mosfet as a switch we can drive the. It is a key requirement of the fet circuit design to ensure the junction remains reverse biased for satisfactory. The reactance modulator takes its name from the fact that the impedance of the circuit acts as a reactance capacitive or inductive that is connected in parallel with the resonant circuit of the oscillator. There for there is no current provided to the drain of the gaas fet when there is no negative bias on the gate. Varactor diodes are used along with reactance modulator to provide automatic frequency correction for an fm transmitter. Either the mobile or the fixed base station can have power outputs as high as 150 w.
Explain how and why we use capacitive coupling of the signal at the fet gates. Fm reactance modulator a direct fm modulator using a jfet. The varactor diode modulator circuit is shown in fig5. Sep 19, 2016 advantages of direct method simplicity of the modulator circuit low cost disadvantages of direct method this type of modulator is not stable enough.
When the fet is off the output signal is equal to the input multiplied by 1, when the fet is on the output is 0 multiplied by 0. Hoag a reactance modulator changes the frequency of the tank circuit of the oscillator by changing its reactance. Fets control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source. Clean up take your circuits apart and put components away. Explain why the modulator modulates, and how the demodulator recovers the original signal. The isolation is maintained by ensuring that the diode junction remains reverse biased when operated within the circuit. There are various designs of phase modulators that are referred to as reactance modulators and the lc phase modulator is one form of a reactance modulator. The circuit shown is the basic circuit of fet reactance modulator which behaves as a three terminal reactance that may be connected across the.
May 26, 2017 viewing fm frequency modulation signal shape. Inductive reactance is a property exhibited by an inductor, and inductive reactance exists based on the fact that an electric current produces a magnetic field around it. Sma connectors were used for the rf input and output. It is a function of various parameters such as temperature, device ageing etc. We have presented six simple, lowcost circuits that illustrate many of the principles of fet operation. We here briefly consider i the reactance tube modulation ii reactance modulation using bjt and iii modulator using varactor diode. Resistors in the reactance modulator circuit will be precision types with close tolerance. In the context of an ac circuit although this concept applies any time current is changing, this magnetic field is constantly changing as a result of current that oscillates.
Aug 15, 20 the circuit above is essentially the same thing. Eg vcolinear integrated circuit direct fm modulator highfrequency deviations and high modulation indices. Reactance modulator circuit operation and troubleshooting. Fieldeffect transistor reactance circuits by gary dale clark captain, united states marine corps b. The alternative to an ic interface is a discrete design. Jan 06, 2018 fet reactance modulator 3 terminal reactance that can be connected across tank circuit. The base band signal is referred to as the modulating signal and the output of the modulation process is called as the modulation signal. The output can be used to simulate the response of lc and tank circuits. Frequency modulation methods electronics tutorials. Reactance is similar to electric resistance, but it differs in several respects reactance is used to compute amplitude and phase changes of sinusoidal. For low fm signal outputs check capacitors c1, c2, c5, c6 and c7 with a capacitor checker. When a modulating signal is applied to the grid of v1, both e 1 and i p change, causing transconductance to vary with the modulating signal. Fieldeffect transistor circuits, may 1967 electronics. The lc modulator is a circuit that is resonant at the frequency that is to be phase modulated.
The rc circuit now appears to be purely capacitive. Ppt frequency modulation fm powerpoint presentation. This is accomplished by a combination of a resistor, a condenser, and a vacuum tube. The value of this reactance is proportional to the trasconductance of the device. The trimcapacitors were used to tune out the internal reactance of the device. Fieldeffect transistor circuits, may 1967 electronics world. The circuit shown is the basic circuit of fet reactance modulator which behaves as a three terminal reactance that may be connected across the tank circuit of the oscillator to be frequency modulated. Q1, along with its associated circuitry, is the oscillator. The circuit shown is the basic circuit of a fet reactance modulator, which behaves as a threeterminal reactance that may be connected across the tank circuit of the oscillator to be frequencymodulated. Let the modulating and carrier signals be denoted as m t and c t a c cos. Leakages of this level put the burden of circuit performance on clean, solderresin free, low leakage circuit layout. The jfet or junction fet is a form of fet where the gate is formed by using a diode junction onto the channel.
Switching modulator is similar to the square law modulator. Generation of frequency modulation varactor diode modulator. The choice of the fet circuit configuration or topology is one of the key design parameters on which the overall circuit design is based. Figure 212 shows a typical frequencymodulated oscillator stage operated as a reactance modulator. This is accomplished by a combination of a resistor, a condenser, and a vacuum tube the modulator connected across the tank circuit of the oscillator as in fig. Q2 is the modulator and is connected to the circuit so that its collectortoemitter capacitance c ce is in parallel with a portion of the rf oscillator coil, l1. Indirect method this method of fm generation is also known as armstrong method of fm generation. Some of these transmitters can be damaged if not operated into the proper load impedance, usually 50ohm. How can you use a varactor diode in the generation of fm. This may be caused by static charge exceeding the maximum reverse voltage of the diode. Any one of these capacitors could cause low output. The most likely types of fm radio a technician will be called to service are an automotive mobile, a fixed base station, or a handheld portable. A ptype control electrode or gate surrounds and is joined to the surface of the middle section of the ntype bar, thus forming a pn junction.
Linear ic vco and function generators can generate a direct fm output waveform that is relatively stable, accurate, and directly proportional to the input modulating signal. The only difference is that in the square law modulator, the diode is operated in a nonlinear mode, whereas, in the switching modulator, the diode has to operate as an ideal switch. Advantages of direct method simplicity of the modulator circuit low cost disadvantages of direct method this type of modulator is not stable enough. Reactancetube modulator article about reactancetube. The capacitive reactance of c at the oscillator frequency is made five to ten times larger than the value of series resistance. In the stabilize reactance modulator afc system, a. The circuit that i designed ends up giving a weird waveform but i am pretty sure my professor. Fet circuit applications polycarbonate dielectric tlh67911 sample and hold with offset adjustment the 2n4339 jfet was selected because of its low lgss k100 pa, verylow ldoff k50 pa and low pinchoff voltage. How can you use a varactor diode in the generation of fm wave. The varicap can only appear as a capacitance that becomes part of the frequency determining branch of the oscillator. It consists of a bar of ntype semiconductor material with a drain terminal at one end and a source terminal at the other. If the signal plus dc bias is connected through a resistor to the source of a fet, and a square wave at the carrier frequency is connected to the gate, then you will get multiplication by a square wave. Fets control the flow of current by the application of a voltage to the gate.
1262 1443 760 1008 416 282 714 1029 700 219 352 978 547 1012 52 221 107 430 1472 185 1538 816 1221 640 1415 834 194 1037 194 1207 314 1345 399 1391 90 1120 424 723 1297 364 1340 483 145 1493 1447 285 1260 334 518 112 725