When the reactance tube is connected across the tank circuit with no modulating voltage applied, it will affect the frequency of the oscillator. Before the invention of transistor, reactance tube modulation was popularly used to produce frequency modulation. Varactor diode and transistor direct pm modulator two common methods for. Dec 15, 2019 the transistor 2n3904 used in fm transmitter circuit. Fm reactance modulator a direct fm modulator using a jfet as the.
We have written the ngspice code for the modulator and. The lc modulator is a circuit that is resonant at the frequency that is to be phase modulated. In the telecommunication, the frequency modulation fm transfers the information by varying the frequency of carrier wave according to the message signal. Any changes or mistake please leave it in the comment below. Considering the difficulties faced by the designer in terms of hardware for designing such systems we have used a system that uses less hardware for this process. 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. Reactancetube modulator article about reactancetube. The frequency of the modulation signal determines the rate of the frequency change from the center freque. Provided that the correct component values are employed any reactance modulator may be connected across the tank circuit of any lc oscillator not crystal with one provision. If the current is passed through l1 inductor and the variable capacitor, the fm transmitter circuit will start oscillating with the resonant to that of the carrier frequency i.
Transistor basics emitter to base junction is forward biased normally collector to base junction is reverse biased normally transistors are current operated devices, so kcl should be applied first. The threeterminal ones include the reactance fieldeffect transistor fet, the bipolar transistor and the tube. Expired lifetime application number us705723a inventor montgomery. This causes the collector voltage of q2 to vary at the same modulating rate. The capacitive reactance of c at the oscillator frequency is made five to ten times larger than the value of series resistance.
Aug 20, 2017 transistor q 1 q 2 are reverse biased. Resistors r2 and r3 establish a voltage divider network that biases q1. The carrier signal should be a 1mhz 1vpp sinusoidal wave from one of the nice digital function generators. Q1, along with its associated circuitry, is the oscillator. It has a number of disadvantages, such as instability of its parameters with temperature changes and a current drain in the controlvoltage circuit, which have limited its applications. 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. Generation of frequency modulation varactor diode modulator. The amplitude of the modulation signal determines the amount of the frequency change from the center frequency. Fm modulators and transmitters sonoma state university. Fm modulation system fm transmitter, reactance modulator. We have only 3 of these in the lab at this time, but this signal will be set to a. Typical applications include suppressed carrier and amplitude modulation, synchronous detection, fm detection, phase detection, and chopper. Aug 15, 20 the circuit above is essentially the same thing.
Use of transistors and semiconductor diodes in frequency modulation techniques has been reported ever. The transistor reactance modulator shown in the figure is to be used in an fm transmitter. Transistor q1 makes up the reactance modulator circuit. Frequency modulation methods electronics tutorials.
The transistor circuit receives its input via the baseemitter side of the device and outputs the signal on the collectoremitter side, with the emitter terminal being common to both input and output circuits. Oct 19, 2018 it may also be used to produce generation of frequency modulation. The rc circuit now appears to be purely capacitive. One of the following is an in direct way of generating fm. Transistor transmitters for the amateur american radio history. Reactance transistor article about reactance transistor by. Block diagram of fm transmitter and receiver and its explanation. Vee 230 radio communications college of micronesia fsm. Furthermore, this video explain briefly about the operation of reactance modulator. University of north carolina at charlotte department of. Fm transmitter circuit working and its applications.
Fieldeffect transistor reactance circuits by gary dale clark captain, united states marine corps b. Reactance modulator circuit operation and troubleshooting. The reactance transistor is a variation on the reactance tube. Emitter resistance and the transistor emitter resistor. Fm reactance modulator a direct fm modulator using a jfet as the active device. In the reactance modulator shown below an rc capacitive transistor reactance modulator, quite a common one in use operates on the tank circuit of a clap gouriet oscillator. The voltage across the oscillator tank circuit l1 and c1 is also in parallel with the series network of r1 and c7. Fm transmitter fm transmitter block diagram direct method. 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. For class b operation, the transistor is biased such that when both the carrier and modulating signals are zero, the dc voltage at the transistor base will be 0. Frequency stabilised reactance modulator crossby modulator. The reactance circuit appears capacitive in nature to the master oscillator. We here briefly consider i the reactance tube modulation ii reactance modulation using bjt and iii modulator using varactor diode. The theory behind the reactance modulator shown in figure a is to add a phaseshifting network consisting of a series r and c.
Generally, the fm transmitter uses vhf radio frequencies of 87. Q2 is the modulator and is connected to the circuit so that its collectorto emitter capacitance c ce is in parallel with a portion of the rf oscillator coil, l1. 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 triode unit is useful as a reactance modulator. The transistor has resistors connected between the emitter and base thereof. Synchronous modulation and demodulation photo optical modulator. L, assignor to radio corporation of america, a corporation of delaware application may 3, 1954, serial n 0. In the reactance modulator shown in figure 512, an rc capacitive transistor reactance modulator, quite a common one in use, operates on the tank circuit of a clappgouriet oscillator. The transistor amplifier is the most commonly used circuit with nonlinear element. Fm reactance modulator a direct fm modulator using a jfet as. Pdf 92cm9806 92cm9807 gridn22 92cm98ii 92cm9803 reactance modulator 9806. The frequency multiplier section, which features several frequency multipliers. Hunter ereunnmnn v1 b w 7 0 0v 12 i attorney united states patent transistorreactance circuit hunter c.
Apr 14, 2015 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. When the modulating signal is applied to the base of q2, the emittertobase bias varies at the modulation rate. Us4303894a fm modulator using variable transistor capacitance. Emitter resistance connected to the emitter terminal of a transistor amplifier can be used to increases the amplifiers bias stabilisation the aim of an ac signal amplifier circuit is to stabilise the dc biased input voltage to the amplifier and thus only amplify the required ac signal. The transistor configuration used in the circuit is a commonemitter design. Abstractthis project explores the robust design of amplitude modulation and demodulation using bjt amplifier and diode detector. In this case, the reactance looks like a variable capacitor in the oscillators tank circuit. A reactance modulator changes the frequency of the tank circuit of the oscillator by changing its reactance. It is a npn transistor which is basically used for signal and voltage amplification purposes.
The theory behind the reactance modulator shown in figure a is to add a phase shifting network consisting of a series r and c. A reactance modulator detector radio frequency modulation. Fm reactance modulator a direct fm modulator using a jfet as the active. Provided that the correct component values are employed, any reactance modulator may be connected across the tank circuit. In the stabilize reactance modulator afc system, a. The exciter section contains the carrier oscillator, reactance modulator and the buffer amplifier. For direct generation of fm signal reactance modulator and varactor diode modulator used. The varicap can only appear as a capacitance that becomes part of the frequency determining branch of the oscillator circuit. Figure 212 shows a typical frequencymodulated oscillator stage operated as a reactance modulator. Find out information about reactance tube modulator. Its resistance increases, hence the output developed across it increases. Apr 25, 2015 this is our assignment in chapter frequency modulation. Describe a transistor collector am modulator circuit.