Electronic communication systems by wayne tomasi 5th edition pdf

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A portion of the oscillating tank circuit voltage is dropped across L,, and fed back to the base of Q,, where it is amplified. If excessive energy is fed back, the transistor saturates. Therefore, the position of the wiper on L, is adjusted until the amount of feedback energy is exactly what is required for a unity loop voltage gain and oscillations to continue.

A Clapp oscillator circuit is identical to the Colpitts os- cillator shown in Figure a except with the addition of a small capacitor Cy placed in se- ries with L,. The capacitance of C; is made smaller than Cy, or Cy, thus providing a large reactance. Frequency stability is often stated as eith short or long term. Short-term stability is affected predominantly by fluctuations in de crating voltages, whereas long-ferm stability is a function of component aging andl change in the ambient temperature and humidity.

Inthe LC tank-cireuit and RC phase shift oscil ators discussed previously, the frequency stability is inadequate for most radio communi cations applications because RC phase shift oscillators are susceptible to both short- a long-term variations. In addition, the Q-factors of the LC tank circuits are relatively low, lowing the resonant tank circuit to oscillate over a wide range of frequencies. The most obvious are those that rectly affect the value of the frequency-determining components, These include changes i inductance, capacitance, and resistance values due to environmental variations in temper ture and humidity and changes in the quiescent operating point of transistors and fiel effect transistors.

Stability is also affected by ac ripple in dc power supplies. The frequen stability of RC or LC oscillators can be greatly improved by regulating the de power st ply and minimizing the environmental variations. Therefore, it is important that all sources maintain their frequency of operatic within a specified tolerance. The crystal acts in a manner similar to the LC tank, except with several inherent advantages.

Crystals are sometimes called crystal res- onators, and they are capable of producing precise, stable frequencies for frequency counters, electronic navigation systems, radio transmitters and receivers, televisions, videocassette recorders VCRs , computer system clocks, and many other applications too numerous to list.

Crystallography is the study of the form, structure, properties, and classifications of crystals. The stress can be in the form of squeezing compression , stretching tension , twisting torsion , or shear- ing. If the stress is applied periodically, the output voltage will alternate. Conversely, when an alternating voltage is applied across a crystal at or near the natural resonant frequency of the erystal, the crystal will break into mechanical oscillations.

This process is called exciting a crystal into mechanical vibrations. The mechanical vibrations are called bulk acoustic waves BAWS and are directly proportional to the amplitude of the applied voltage. The piezoelectric effect is most pronounced in Rochelle salt, which is why itis the substance commonly used in crystal microphones.

Synthetic quartz, however, is used more often for frequency control in oscillators because of its permanence, low temperature co- efficient, and high mechanical Q.

Three sets of axes are associated with a crystal: optical, electrical, and mechanical. The longitudinal axis joining points atthe ends of the crystal is called the optical or Z-axis. Electrical stresses applied to the optical axis do not produce the piezo- electric effect.

The electrical or X-axis passes diagonally through opposite comers of the hexagon. Ifa thin flat section is cut from a crystal such thatthe flat sides are perpendicular to an electrical axis, mechanical stresses along the Y-axis will produce electrical charges on the flat sides.

As the stress changes from compression to tension and vice versa, the polarity of the charge is reversed. Conversely, if an alternating electrical charge is placed on the flat sides, a mechanical vibration is produced along the Y-axis, This is the piezoelectric effect andis also exhibited when mechanical forces are applied across the faces ofa crystal cut with its flat sides perpendicular to the Y-axis.

When a crystal wafer is cut parallel to the Z-axis with its faces perpendicular to the X-axis, itis called an X-cut crystal. When the faces are perpendicular to the Y-axis it is called a Y-cut crystal. A variety of cuts can be obtained by rotating the plane of the cut around one ot more axes. The AT cut is the most popular for high- frequency and very-high-frequency crystal resonators. CT and DT cuts exhibit low-frequency shear and are most useful in the t0 kHz range.

A crystal unit refers to the holder and the crystal itself. Because a crystals stability is somewhat temperature depender a crystal unit may be mounted in an oven to maintain a constant operating temperature. With this process, crystals with fundamental frequencies up to MHz are possible. This imposes an obvious physical limitation: The thinner the water, the more susceptible it is to damage and the less useful it becomes. In the overtone mode, harmonically related vibra- tions that occur simultaneously with the fundamental vibration are used.

The harmonics are called overtones because they are not true harmonics. Manufacturers can process crystals such that one overtone is enhanced more than the others. Using an overtone mode increases the usable limit of stan- dard crystal oscillators to approximately MHz. If the direction of the frequency change is the same as the temperature change. X-cut crystals are approximately 10 times more stable than Y-cut crystals.

Typical values of L from 0. Atextreme low frequencies, the series impedance of L, Cy, and R is very hi and capacitive —. This is shown in Figure c. At this frequency f, , the series impedance is minimum, ress tive, and equal to R. The parallel combination of L and C, causes the crystal toact as a parallel resonant circuit maximum impedance at resonance.

The relative steepness of the impedance curve shown in Figure b also attributes to the stability and accuracy of a crystal. However, forthe best frequency stability, the RLC half-bridge is the best choice. It uses relatively sim- ple circuitry requiring few components most medium-frequency versions require only one transistor. The Pierce oscillator design develops a high output signal power while dissi- ppating very lttle power in the crystal itself, Finally, the short-term frequency stability of the Pierce crystal oscillator is excellent because the in-circuit loaded Q is almost as high as the crystals internal Q.

The only drawback to the Pierce oscillator is that it requires a high- gain amplifier approximately Consequently, you must use a single high-gain transis- tor or possibly even a multiple-stage amplifier. Figure shows a discrete 1-MHz Pierce oscillator circuit. Q, provides all the gain necessary for self-sustained oscillations to occur. Unfortunately, C and C, introduce substantial losses and, consequently, the transistor must have a relatively high voltage gain; this is an obvious drawback.

Although it provides less frequency stability, it can be implemented using simple digital IC design and reduces costs substan- tially over conventional discrete designs. The: inal Meacham oscillator was developed in the s and used a full four-arm bridge negative-temperature-coefficient tungsten lamp. The circuit configuration shown in uses only a two-arm bridge and employs a negative-temperature-coefficient tor.

When oscillations begin, the signal amplitude increases gradually, decreasing the thermistor resistance until the bridge almost nulls. The amplitude of the oscillations stabi- lizes and determines the final thermistor resistance. The entire oscillator circuit is contained in a single metal can. A simplified schematic diagram for a Colpitts crystal oscillator mod- ule is shown in Figure a. X, is a crystal itself, and Q, is the active component for the amplifier.

C, is a shunt capacitor that allows the crystal oscillator frequency to be varied over a narrow range of operating frequencies. VC; is a voltage-variable capacitor varicap or varactor diode. A varactot diode is a specially constructed diode whose in- ternal capacitance is enhanced when reverse biased, and by varying the reverse-bias volt- age, the capacitance of the diode can be adjusted.

A varactor diode has a special deple- tion layer between the p- and n-type materials that is constructed with various degrees and types of doping material the term graded junction is often used when describing vvaractor diode fabrication. Figure b shows the capacitance versus reverse-bias volt- age curves for a typical varactor diode.

The varactor diode, in conjunction with a temperature-compensating module, provides instant fre- quency compensation for variations caused by changes in temperature. The compen tion module includes a buffer amplifier Q, and a temperature-compensating netw Z.

T; is a negative-temperature-coefficient thermistor. When the temperature falls low the threshold value of the thermistor, the compensation voltage increases. The pensation voltage is applied to the oscillator module, where it controls the capacitance the varactor diode. In many of these applications, commercial monolithic integrated-circuit oscillators and function generators are available that provide the circuit designer with a low-cost alternative to their noninte- grated-cireuit counterparts.

The basic operations required for waveform generation and shaping are well suited to monolithic integrated-circuit technology. In fact, monolithic linear integrated circuits LICS have several inherent advantages over discrete circuits, such as the availability of a large number of active devices on a single chip and close matching and thermal tracking of component values.

It is now possible to fabricate integrated-circuit waveform generators that provide a performance comparable to that of complex discrete generators at only a frac- tion of the cost. Atypical waveform generator consists of four basic sections: 1 an oscillator to generate the basic periodic waveform, 2 a waveshaper, 3 an optional AM modulator, and 4 an out- put buffer amplifier to isolate the oscillator from the load and provide the necessary drive current, Figure shows a simplified block diagram of an integrated-circuit waveform generator circuit showing the relationship among the four sections.

Each section has been built separately in monolithic form for several years; therefore, fabrication of all four sections onto a single monolithic chip was a natural extension of a preexisting tech- nology.

The oscillator section generates the basic oscillator frequency, and the wave- shaper circuit converts the output from the oscillator to either a sine-, square-, triangular-, or ramp-shaped waveform.

Integrated-circut waveform generator ALE AE Va-Va FIGURE Simplified integrated-circuit waveform generator: a schematic diagram: 6 waveforms amplitude-modulated signals, and the output buffer amplifier isolates the oscillator from its load and provides a convenient place to add de levels to the output waveform.

The sync output can be used either as a square-wave source of as a synchronizing pulse for external timing circuitry. A typical IC oscillator circuit utilizes the constant-current charging and discharging of external timing capacitors. The circuit oper- ates as follows, When transistor Q, and diode D, are conducting, transistor Q, and diode Dz are off and vice versa, This action alternately charges and discharges capacitor C,, from.

The voltage across D, and D2 is a symmetrical square wave with 1 peak-to-peak amplitude of 2Vpp. Output Vat is identical to V,4 0 , except it is delayed by ahalf-cycle. Figure b shows the output voltage waveforms typically available. The output waveforms from the XR can be both amplitude and frequency modulated by an external modulating signal, and the frequency of operation can be selected externally over a range from 0.

The XR is, ideally suited to-communications, instrumentation, and function generator applications re- quiring sinusoidal tone, AM, or FM generation. The output from a VCO is a frequency, and its input isa bias or control signal that can be either a dc or an ac voltage. The VCO actually produces an output frequency that is pro- portional to an input current that is produced by a resistor from the timing terminals either pin 7 of 8 to ground.

The current switches route the current from one of the timing pins to the VCO. The current selected depends on the voltage level on the frequency shift keying input pin pin 9. Therefore, two discrete output frequencies can be independently pro- duced.

If pin 9 is open circuited or connected to a bias voltage 22 V, the current passing through the resistor connected to pin 7 is selected. Similarly ifthe voltage level at pin 9 is 1 V, the current passing through the resistor connected to pin 8 is selected. Thus, the out- put frequency can be keyed between f, and f, by simply changing the voltage on pin 9. Frequency varies linearly with current over a range of current values between 1 pA to 3 pA.

The frequency of oscillation is related Ve by BL Monolithie voltage-controlled oscillators. The XR is a monolith yoltage-controlled oscillator VCO integrated circuit featuring excellent frequency stabil ity and a wide tuning range.

The duty cycle of the tiangul and square-wave outputs can be varied from 0. Two binary input pins pins 8 and 9 determine which of the four timing. These currents are set by resistors to ground from each of the four timing input terminals pins 4 through 7. The XR is a monolithic vari- able-frequency oscillator circuit featuring excellent temperature stability and a wide lin- ear sweep range. The circuit provides simultaneous triangle- and square-wave outputs, and the frequency is set by an external RC product.

The oscillator frequency is set by an external capacitor and timing resistor. The frequency of operation for the XR is proportional to the timing current drawn from the timing pin. This current can be modulated by applying a control voltage, Vo. Con- versely, if Vois higher than the voltage on pin 4, the frequency of oscillation is decreased. PLLs are used in transmitters and receivers using both analog and digital modulatia and with the transmission of digital pulses.

PLLs were first used in for synchronous detection and demodulation of radio si nals, instrumentation circuits, and space telemetry systems. However, with the advent of large-scale integration, PLLs can now pre vide reliable, high-performance operation and at the same time be extremely small and ea to use and dissipate litle power. The basic block diagram for a phase-locked Jo circuit is shown in Figure As the figure shows, a PLL consists of four primary blo 1 a phase comparator or phase detector, 2 a low-pass filter LPF , 3 a low-gain op tional amplifier, and 4 a VCO.

The four circuits are modularized and placed on an grated circuit, with each circuit provided external input and output pins, allowing use interconnect the circuits as needed and to set the break frequency of the low-pass filter gain of the amplifier, and the frequency of the VCO. However, before lock can occur, a PLL must be frequency locked. After frequency lock has occurred phase comparator produces an output voltage that is proportional to the difference in pl between the VCO output frequency and the external input frequency.

When there is no external input signal or when the feed- back loop is open, the VCO operates at a preset frequency called its natural or free- running frequency f,. The VCO's natural frequency is determined by external components. As previously stated, before a PLL can perform its intended function, frequency lock must occur. When an external input signal f is initially applied to the PLL, the phase com- parator compares the frequency of the external input signal to the frequency of the VCO output signal.

The phase comparator produces an error voltage v. The error voltage is filtered, ampli- fied, and then applied to the input to the VCO. If the frequency of the external input sig- nal f is sufficiently close to the VCO natural frequency f, , the feedback nature of the PLL causes the VCO to synchronize or lock onto the external input signal. When in the free-running state, the VCO oscillates at its natural frequency deter- mined by external components.

To be in the capture state, there must be an external in- put signal, and the feedback loop must be complete. When in the capture state, the PLL is in the process of acquiring frequency lock.

In the fock state, the VCO output frequency is locked onto equal to the frequency of the external input signal.

When in the lock state, the VCO output frequency tracks follows changes in the frequency of the external in- put signal. As the VCO output frequency changes, the amplitude and frequency of te beat frequency changes proportionately.

After several cycles around the loop, the VCO output frequency equals the external input frequency, and the loop is said to have acquired frequency lock.

Once frequency lock has occurred, the beat frequency at the output of the LPF is 0 Hz a de voltage , and its magnitude and polarity is proportional to the difference in phase between the external input signal and the VCO output signal. Sheriff Lowe. Linus Antonio. Eugine Ajetan. She San. Sai Prasad Iyer J.

Mark Stephen de Jesus. Sheehan Kayne De Cardo. Hector Ledesma III. Raymond Cruzin. Manish Kumar Verma. Vinnie Segovia. Anonymous gm3jsNA. Yui Phantomhive. More From romelle. Fessenden transmits first human speech through radio waves.

Marchese Guglielmo Marconi transmits telegraphic radio messages from Cornwall, England, to Newfoundland. First successful transatlantic transmission of radio signals. John Fleming invents the two-electrode vacuum-tube rectifier. Fessenden invents amplitude modulation AM. First radio program of voice and music broadcasted in the United States by Reginald A.

Lee DeForest invents the triode three-electrode vacuum tube. Fessenden invents a high-frequency electric generator that produces radio waves with a frequency of kHz. Reeves invents binary-coded pulse-code modulation PCM. First use of two-way radio communications using walkie-talkies. Texas Instruments becomes the first company to commercially produce silicon transistors. Citizens band CB radio first used.

If two powers are expressed in the same units e. Because P2 is in the denominator of Equation , it is the reference power, and the dB value is for power Px with respect to power P2. When used in electronic circuits to measure a power gain or loss, Equation can be rewritten as. Absolute Ratio logtio [ratio] 10 log 10 [ratio]. Since Pw is the reference power, the power gain is for Pout with respect to Pm.

An absolute power gain can be converted to a dB value by simply taking the log of it and multiplying by Instead, the dB represents the ratio of the signal level at one point in a circuit to the signal level at another point in a circuit. Decibels can be positive or negative, depending on which power is larger. The sign associated with a dB value indicates which power in Equation is greater the denominator or the numerator. A negative - dB value indicates that the output power is less than the input power, which indicates a power loss.

A power loss is sometimes called attenuation. This is sometimes referred to as a unity power gain. Examples of absolute power ratios equal to or greater than 1 i. Although Tables and list absolute ratios that range from 0. From Tables and , it can be seen that the dB indicates compressed values of absolute ratios, which yield much smaller values than the original ratios. This is the essence of the decibel as a unit of measurement and what makes the dB easier to work with than absolute ratios or absolute power levels.

Properties of exponents correspond to properties of logarithms. Example Convert the absolute power ratio of to a power gain in dB. Solution Substituting into Equation gives. Equation l-4d can be used to determine power gains in dB but only when the input and output resistances are equal.

However, Equation l-4d can be used to represent the dB voltage gain of a device regardless of whether the input and output resistances are equal. Voltage gain in dB is expressed mathematically as.

With dBm, the reference level is 1 mW i. One milliwatt was chosen for the reference because it equals the average power produced by a telephone transmitter. The decibel was originally used to express sound levels acoustical power.

It was later adapted to electrical units and defined as 1 mW of electrical power measured across a ohm load and was intended to be used on telephone circuits for voice-frequency measurements. Today, the dBm is the measurement. As the tables show, a power level of 1 mW equates to 0 dBm, which means that 1 mW is 0 dB above or below 1 mW.