Ans reactance modulator is direct fm, where as armstrong method is indirect fm. Fm reactance modulator a direct fm modulator using a jfet as the active device from engineerin. For nrz format we can see that, the pulse does not return to zero on its own. 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. Reactance modulator cannot operate on crystal controlled oscillator. Series r, l, and c reactance and impedance r, l, and. The first step is to determine the reactance in ohms for the inductor and the capacitor the next step is to express all resistances and reactances in a mathematically common form. A modulator that produces a full carrier signal 59. Therefore frequency stability is better than reactance modulator. Capacitive reactances capacitive reactance is an opposition to ac, so series or parallel reactances are combined in the same way as resistances.
Upload a corrupt or damaged pdf and we will try to fix it. Direct pm angle modulation in which the frequency of the carrier is deviated indirectly by the modulating signal. 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. This causes the collector voltage of q2 to vary at the same modulating rate. What type of circuit is used in many fm receivers to convert signals coming from the if amplifier to audio. It is basically a standard commonemitter class a amplifier. Hi, im trying to make a simulation of a reactance fm modulator.
As the instantaneous value of modulating voltage changes, the reactance offered by the transistor or fet will change proportionally. A circuit that acts as a variable resistance or capacitance to produce fm signals b. The reactance circuit operates on the master oscillator to cause its resonance frequency to shift down depending on the modulating signal being applied. Why armstrong method of fm is superior to reactance modulator. 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. Lets take the following example circuit and analyze it. This is our assignment in chapter frequency modulation. Furthermore, this video explain briefly about the operation of reactance modulator. Phase modulation differs from, it through a phase modulator you get fm, and if the modulating wave were put through a differentiator before a frequency modulator you get a phase modulated wave. As you know, the induced voltage in a conductor is proportional to the rate at which magnetic lines of force cut the conductor. However, to understand the basic theory of frequency modulation, this project makes use of only transistors to form the heart of. Fm reactance modulator a direct fm modulator using a jfet as the active device. Reactance is the property of resisting or impeding the flow of ac current or ac voltage in inductors and capacitors. The modulator active device and the rc network are both in parallel with the resonant circuit of the carrier oscillator.
Fm reactance modulator 44 a complete fm modulator on a single 8pin dip integrated circuit chip. Calculation of the input impedance of reactance modulator. Recover content and data from corrupt files with ease. Inductive reactance because it is the reaction of the inductor to the changing value of alternating current.
Variable reactance means for frequency modulating an. Also if anyone has a great tutorial for making one, it is always welcome. Inductive reactance is measured in ohms and its symbol is x l. Fm reactance modulator a direct fm modulator using a jfet. Create pdf files without this message by purchasing novapdf printer. When the reactance tube is connected across the tank circuit with no modulating voltage applied, it will affect the frequency of the oscillator. A reactance modulator detector radio frequency modulation. Troubleshooting the reactance modulator the fm output signal from coil l2 can be lost due to open bias resistors, open rf chokes rfc1 and rfc2, or an open winding at coil l1. Introduction chapter 6 notes american radio relay league. Define carriers bandwidth for transmission of fm or pm signal. Hence crystal oscillators can be used in armstrong method.
Reactance modulator circuit operation and troubleshooting. In this case, the reactance looks like a variable capacitor in the oscillators tank circuit. Reactance modulator operates on the tank circuit of an lc. This device is connected across the tuned circuit of an oscillator. Amplitude modulation of a sine or cosine carrier results in a variation of the carrier am plitude that is. The alc automatic level control is a selfcontrolling transmitter circuit that attempts to maintain a constant level of output power by automatically adjusting the gain of the final amplifier, to prevent it from overloading and damaging the final stage by excessive. Communication engineering electrical and electronics. Mc76 45 can generate a direct fm output waveform that is relatively stable, accurate, and directly proportional to the input modulating signal. An electrontube circuit, used to produce phase or frequency modulation, in which the reactance is varied in accordance with the instantaneous amplitude of. Capacitive or inductive reactance calculator is an online tool for electrical and electronic circuits to measure the electrical resistance of the capacitor and inductor. Larger reactance leads to smaller currents for the same voltage applied. Fm reactance modulators icbased modulators direct fm indirect fm. Part 2, reactance this is part 2 of a 3part series on resistance, reactance and impedance. Fm modulation system fm transmitter, reactance modulator.
The use of ganged inductors and capacitors can also be easily found in modern radio set. Resistors r1 and r2 from a voltage divider to bias the transistor. This invention relates to a circuitarrangement for acting upon the natural frequency of an oscillatory circuit by means of a reactance included in the oscillatory circuit, the value of which can be changed by means of a modulating current or voltage. Note particularly we speak of alternating current only ac, which expression includes audio af and radio frequencies rf. The mc44cc373 mc44cc374 cmos family of rf modulators is the latest generation.
When the modulating signal is applied to the base of q2, the emittertobase bias varies at the modulation rate. The voltage across the oscillator tank circuit l1 and c1 is also in parallel with the series network of r1 and c7. The transistor reactance modulator shown in the figure is to be used in an fm transmitter. Combining capacitive reactances is opposite to the way capacitances are combined. To study and perform pulse width modulation and demodulation. For calculations you can choose which formula to use for inductive reactance, either 2l or. The exact details of modulation format used to transmit the video signal. 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. Ive got a schematic but i dont really know which values i should use for the components. A balanced modulator provides a double sideband, suppressed carrier signal. The interplay between resistance and reactance will form the basis of many effects in the later modules.
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. Q1, along with its associated circuitry, is the oscillator. Generation of frequency modulation varactor diode modulator. 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. L, but 2l is more commonly used, one reason being that scientific.
Resistance, reactance and impedance a primer douglas brooks, phd ultracad design, inc. Fm reactance modulator a direct fm modulator using a jfet as the. Both are similar to resistance and are measured in ohms, but resistance remains the same, whatever the frequency. Modulated fm signal, fm modulators and demodulators, approximately compatible. Rearrange individual pages or entire files in the desired order.
The frequency multiplier section, which features several frequency multipliers. But it must have the stability of a crystal oscillator if it is to be the part of a commercial transmitter achieved by stabilized reactance modulator corrects any drift in the average frequency of the master oscillator. The l 1 and c 1 represent the tuned circuit of the carrier oscillator. Phase deviation of the output is multiple of phase deviation of the modulator.
Figure 212 shows a typical frequencymodulated oscillator stage operated as a reactance modulator. The transmit audio or microphone gain is typically adjusted for proper alc setting on an amateur single sideband transceiver. Reactancetube modulator article about reactancetube. The reactance circuit appears capacitive in nature to the master oscillator.
Since coupling capacitor c8 has negligible reactance at the. The value of c 2 is made very large at the operating frequency so that its reactance is very low. Frequency multiplier antenna a u d i o i n p u t power section buffer amplifier acts as a constant high impedance load on the oscillator to help stabilize the oscillator frequency. Most of us are familiar with resistance at least we think we are. Ans reactance modulator is direct fm, where as armstrong method is. The theory behind the reactance modulator shown in figure a is to add a phaseshifting network consisting of a series r and c. Provided that certain simple conditions are met, the impedance z, as seen at the input terminals a a of figure 511, is almost entirely reactive. Inductive reactance rises with an increase in frequency, and capacitive reactance falls. Introduction to communication system introduction, orientation introduction to communication system, need for modulation, amplitude modulation, time domain and frequency domain description, power relations in am wave, square law modulator, switching. The capacitive reactance of c at the oscillator frequency is made five to ten times larger than the value of series resistance. Now, this modulating audio is applied to the varactor diode in the reactance modulator causing the frequency of the. The audio modulation frequency range is 1 khz to 10 khz. Suppose the output of a balanced modulator has a center frequency of 10 mhz.
Modulation index is a term used to describe the amount of amplitude change present in an. The exciter section contains the carrier oscillator, reactance modulator and the buffer amplifier. 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. Reactance is similar to electric resistance, but it differs in several respects reactance is used to compute amplitude and phase changes of sinusoidal alternating. The passive components capacitors and inductors are the most widely used in electrical and electronic circuits. As a result, when c 2 is connected in series with the lower capacitance of d 1.