1.1 TRANSMITTER FUNDAMENTALS
The AM transmitter generates such high power that it’s prime requirement is efficiency. Amplitude modulation can be generated at any point after the radio frequency source as a matter of fact, even a crystal oscillator could be amplitude modulated except that this would be an unnecessary interference with its frequency stability. If the output stage in a transmitter is plate modulated the system is called high-level modulation.
An AM transmitter which may be either low level or high level modulated have a stable RF source and buffer amplifier followed by RF power amplifier. The audio voltage is processed or filtered so as to occupy the correct bandwidth and compressed somewhat of reduce the ration of maximum to minimum amplitude. In IM transmitter, the prime requirement of an FM system is a variable output frequency, with the variation proportional to the instantaneous amplitude or the modulation voltage.
The power and auxiliary stages of FM transmitter are similar to those in AM transmitter except that FM has an advantage, since it is a constant amplitude modulation system all the power amplifier can be operated in class c and that is very efficient.
1.2 MODULATION (ANALOG AND DIGITAL)
Modulation is the systematic transformation of a carrier wave in accordance with the message signal. To a large extent the success of a communication system in any given mission depends on the modulation so much so that the type of modulation is a pivotal decision to system design. There are two basic types of modulating techniques the analog and digital modulation. Analogue modulation uses sinusoidal waveform as the carrier signal while digital modulation uses a discrete or pulse train as the carrier signal. Analogue modulation being a continuous process is obviously suited to signal that are continually varying with time. The carriers are at a frequency much higher modulating signal. The modulation process is thus characterized by frequency translation. Pulse modulation is discontinuous or discrete process in the sense that the pulses are present only at a certain interval of time. In the past, analogue modulation methods have been very largely exploited and still are because of the capital investment in existing systems and theirs basic simplify. The two most important methods of analogue modulation are amplitude modulation.
1.3 AMPLITUDE MODULATION (AM)
In amplitude modulation the amplitude of a carrier the modulating voltage whose frequency is invariably lower than that of the carrier varies signal. AM is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage. Let the carrier voltage and the modulating voltage Vc and Vm respectively be represented by
Vc = VC Sin Wct
Vm = Vm Sin Wmt
Note that phase angle has been ignored in both expressions since it is unchanged by the amplitude modulation process from the definition of AIG amplitude vc. Of the unmodulated carrier will have to be made proportion to the instantaneous modulating voltage vm sin wmt where the carrier is amplitude modulated.
1.4 FREQUENCY MODULATION
Frequency modulation is a system in which the amplitude of the carrier is made constant whereas its frequency is varied about its unmodulated frequency in a way and manner determined by the amplitude of the modulating signal. When the information signal is positive the carrier frequency is increased above its unmodulated value. The increase in carrier frequency varies linearly with the instantaneous value of the information reaching a maximum when the modulating signal reaches its peak value. The converse applies when the modulating signals is negative ie the instantaneous carrier. Frequency being decreased in proportion to the instantaneous value the modulating or information signal. The illustration is shown below:
0 Fig 1.4a
0 Fig 1.4b
V Frequency modulated signal
1.5 METHOD OF FREQUENCY MODULATION
There are two methods namely direct and indirect. In direct method, frequency modulation is obtained by varying the frequency of an oscillator. If either the capacitance or inductance of an L C oscillator tank is varied FM of some form will result and is the variation is made directly proportional to the voltage supplied by the modulation circuits true FM will be obtained.
The direct modulator has the disadvantages of being based on an LC oscillator which is not stable enough for broadcast purposes. And if the variation is made directly proportional to the voltage supplied by the modulation circuits true FM will be obtained. The direct modulators have the disadvantages of being based on an LV oscillator, which is not stable enough for broadcast purpose. This requires stabilization of the modulator with attendant circuit complexity: this method is called INDIRECT METHOD.
1.6 MODULATOR OVERVIEW
Of the various methods of providing a voltage variable reactions which can be connected across the tank circuits of an oscillator the most common are the reactance modulator and vibratory diode. These will now be discussed as below:
1.6.1 BASIC REACTANCE MODULATOR
Provide that certain simple condition are met, the impedance Z as seen at the input terminal A-A of figure is almost entirely reactive. 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. It can be made inductive or capacitive by a simple component change. The value of this reactance is proportional to the Tran conductance of the device, which can be made to depend on the gate bias and its variations.
1.6.2 THEORY OF REACTION MODULATOR
In order to determine Z, a voltage V is applied to the terminals A-A between which the impedance is to be measured, and the resulting current i is calculated. The applied voltage is then divided by this current giving the impedance seen when looking into the terminals. In order for this impedance to be a pure reactance (it is capacitive here). Two requirements must be fulfilled.
The first is that the bias network current ib must be negligible compared to the drain current. The impedance of the bias network must be large enough to be ignored. The second requirement is that the drain – to- gate impedance (Xc here) must be greater than the gate to source impedance (R in this case) preferably by more than 5:1. the following analysis may then be applied.
Vg = ibR = RV
FET drain current is
i = gmVg = gmRv
:. Impedance at A – A
Z = V = V – gmRv R – jXc
i R-jXc gmR = I . (I jXc)
Z = -j Xc
X eq = Xc = I I .
gmR 2 fgmRC = 2 Fceq
Ceq = gmRc
Xc = 1
Wc = nR
Ceq = gmRc = gmR
ceq = gm
1.7 TYPES OF REACTANCE MODULATOR
There are four different arrangement of the reactance modulator (including the one initially discussed), which will yield useful results. Their data are shown in table below. The general prerequisite for all of them is that drain current must be much greater than bias network current. It is seen that two of the arrangement gave a capacitive reactance and the other two gave an inductive reactance.
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. 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. The oscillator used must not be one that requires two tuned circuits for its operation such as the tuned-base tuned- collector oscillator. The hartly and copitts (or clap-Gouriet) oscillators are most commonly used and each should be isolated with a buffer. RF chokes in the circuit shown are used to isolate various points of the circuit for alternating current while still providing a dc path.
TABLE 1.7 reactance modulator elements
NAME Zgd Zgs Condition Reactance Formula A
RC Capacitive C R Xc> > R Ceq =gmRC
RC Inductive R C R> > Xc Leq =RC
RL Iductive L R XL > >R Leq = L
RL Capacitive R L R> > XL Ceq = gmL
1.8 VARACTOR DIODE MODULATOR
A varactor diode is a semiconductor diode whose junction capacitance varies linearly with the applied voltage when the diode is reverse biased. It may also be used to produce frequency modulation. Varactor diode is certainly employed frequently, together with a reactance modulator to provide automatic frequency correction for an FM transmitter. The circuit below shows such a modulator. It is seen that the diode has been back biased to provide the junction capacitance effect and since this bias is varied by the modulating voltage which is in series with it, the junction capacitance will also vary, causing the oscillator frequency to change accordingly. Although this is the simplest reactance modulator circuit. It does have the disadvantage of using a two terminal device, its applications are some what limited. However it is other used for automatic frequency control and remote turning.
Cb (Rf) AF in
fig 1.8 Varactor diode modulator Vb.
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