Each have their own characteristics of voltage and current gain as well as input and output impedance. Clean up take your circuit s apart and put components away. It takes the original signal that has both sidebands and a carrier signal, and then modulates it so that only the. The lc modulator is a circuit that is resonant at the frequency that is to be phase modulated. Frequency modulation methods electronics tutorials. Q1, along with its associated circuitry, is the oscillator. Aug 15, 20 the circuit above is essentially the same thing.
Fm modulation system fm transmitter, reactance modulator. In the stabilize reactance modulator afc system, a. Larger reactance leads to smaller currents for the same voltage applied. This may be caused by static charge exceeding the maximum reverse voltage of the diode. The alternative to an ic interface is a discrete design. 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. The rc circuit now appears to be purely capacitive. Fet balanced modulator for ssb under repositorycircuits. Find out information about reactancetube modulator. Leakages of this level put the burden of circuit performance on clean, solderresin free, low leakage circuit layout. 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. When using the mosfet as a switch we can drive the. Eg vcolinear integrated circuit direct fm modulator highfrequency deviations and high modulation indices. 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.
Explain how and why we use capacitive coupling of the signal at the fet gates. 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. Eg vcolinear integratedcircuit direct fm modulator highfrequency deviations and high modulation indices. Sma connectors were used for the rf input and output. Either the mobile or the fixed base station can have power outputs as high as 150 w. Reactance is similar to electric resistance, but it differs in several respects reactance is used to compute amplitude and phase changes of sinusoidal. Generation of frequency modulation varactor diode modulator. Therefore, the value of reactance introduced into the tuned circuit varies directly with the transconductance of the reactance tube. Fet reactance modulator 3 terminal reactance that can be connected across tank circuit.
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. 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. A balanced modulator is a device that modifies a signal, usually in the form of an amplitude modulated am radio signal. We here briefly consider i the reactance tube modulation ii reactance modulation using bjt and iii modulator using varactor diode. The trimcapacitors were used to tune out the internal reactance of the device. 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 output can be used to simulate the response of lc and tank circuits. Before the invention of transistor, reactance tube modulation was popularly used to produce frequency modulation. Figure 2 illustrates the basic construction and operating principles of a simple nchannel jfet. Notes two signal generators are used in this circuit, one. The isolation is maintained by ensuring that the diode junction remains reverse biased when operated within the circuit. Ppt frequency modulation fm powerpoint presentation free. 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. Fm reactance modulator a direct fm modulator using a jfet.
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. The value of this reactance is proportional to the trasconductance of the device. With the switch open, only a small leakage current flows between source and drain. The varactor diode modulator circuit is shown in fig5. Fieldeffect transistor circuits, may 1967 electronics. 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. 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. Jul 22, 2019 with the switch open, only a small leakage current flows between source and drain. The lecture describes the working of fet reactance modulator for fm generation. In the stabilized reactance modulator afc system, a. 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. How does one select the values of components used in amplitude modulation circuit using a transistor common emitter amplifier circuit. Ppt frequency modulation fm powerpoint presentation.
The base band signal is referred to as the modulating signal and the output of the modulation process is called as the modulation signal. Resistors in the reactance modulator circuit will be precision types with close tolerance. Explain why the modulator modulates, and how the demodulator recovers the original signal. How can you use a varactor diode in the generation of fm wave. Let the modulating and carrier signals be denoted as m t and c t a c cos. Jan 06, 2018 fet reactance modulator 3 terminal reactance that can be connected across tank circuit.
We have presented six simple, lowcost circuits that illustrate many of the principles of fet operation. The transistor reactance modulator shown in the figure is to be used in an fm transmitter. 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. This circuit uses two signal generators to simulate an amplitude modulated rf carrier wave. How can you use a varactor diode in the generation of fm. Troubleshooting of fm transmitter engineering projects. Switching modulator is similar to the square law modulator. Figure 212 shows a typical frequencymodulated oscillator stage operated as a reactance modulator. Some contain automatic circuitry to shut the transmitter. A reactance modulator is a circuit that uses a transistor amplifier that acts like either a variable capacitor or an inductor. 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.
The circuit that i designed ends up giving a weird waveform but i am pretty sure my professor. Fets control the flow of current by the application of a voltage to the gate. Am modulator this circuit uses two signal generators to simulate an amplitude modulated rf carrier wave. 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. Some of these transmitters can be damaged if not operated into the proper load impedance, usually 50ohm. 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. Varactor diode is arranged in reverse bias to offer junction capacitance effect. The practice has shown that the input diode byv1040, easily gets defective when connecting the supply voltage the diode gets an internal short circuit. Find out information about reactance tube modulator. Any one of these capacitors could cause low output.
Explain, for your writeup, exactly how this whole circuit works. For low fm signal outputs check capacitors c1, c2, c5, c6 and c7 with a capacitor checker. There for there is no current provided to the drain of the gaas fet when there is no negative bias on the gate. And again a class a bias circuit is used for this device. 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. 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. Advantages of direct method simplicity of the modulator circuit low cost disadvantages of direct method this type of modulator is not stable enough. Modulation is defined as the process by which some characteristics of a carrier signal isvaried in accordance with a modulating signal. 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. The circuit shown is the basic circuit of fet reactance modulator which behaves as a three terminal reactance that may be connected across the. Fm reactance modulator a direct fm modulator using a jfet as. Reactance modulator circuit operation and troubleshooting. The jfet or junction fet is a form of fet where the gate is formed by using a diode junction onto the channel.
May 26, 2017 viewing fm frequency modulation signal shape. 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. Fm reactance modulator a direct fm modulator using a jfet as the active device. The practice has shown that the input diode byv1040, easily gets defective when connecting the supply voltage the diode gets an internal short. The capacitive reactance of c at the oscillator frequency is made five to ten times larger than the value of series resistance. 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. Fieldeffect transistor circuits, may 1967 electronics world. Reactancetube modulator article about reactancetube. Fet circuit configurations are the common source, common gate, and common drain formats. In electric and electronic systems, reactance is the opposition of a circuit element to the flow of current due to that elements inductance or capacitance. Fieldeffect transistor reactance circuits by gary dale clark captain, united states marine corps b.
Mutual conductance g m varies in accordance with the modulating voltage. The varicap can only appear as a capacitance that becomes part of the frequency determining branch of the oscillator. Indirect method this method of fm generation is also known as armstrong method of fm generation. Varactor diodes are used along with reactance modulator to provide automatic frequency correction for an fm transmitter. Hoag a reactance modulator changes the frequency of the tank circuit of the oscillator by changing its reactance. 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 choice of the fet circuit configuration or topology is one of the key design parameters on which the overall circuit design is based. It is a function of various parameters such as temperature, device ageing etc. It is a key requirement of the fet circuit design to ensure the junction remains reverse biased for satisfactory. 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. The fieldeffect transistor fet is a type of transistor which uses an electric field to control the flow of current. This is accomplished by a combination of a resistor, a condenser, and a vacuum tube. It can be made inductive or capacitive by a simple component change. Hence keeping c and r constant, the effective capacitance c e offered by the reactance tube varies.
By applying a suitable drive voltage to the gate of an fet, the resistance of the drainsource channel, r dson can be varied from an offresistance of many hundreds of k. Clean up take your circuits apart and put components away. The j fet or junction fet is a form of fet where the gate is formed by using a diode junction onto the channel. Direct pm angle modulation in which the frequency of the carrier is deviated indirectly by the modulating signal. The modulating voltage v m is applied at the control grid of the reactance tube as shown in figure 1. An electrontube circuit, used to produce phase or frequency modulation, in which the reactance is varied in accordance with the instantaneous amplitude of.