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Communication Principles - 7

The CW transmitter can be greatly improved by simply adding a power am...

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Electronics Engineering (CR 061)

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Samar State University

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LESSON CONTENT

Transmitter Configurations - Simplest transmitter – is a single-transistor oscillator connected directly to an antenna. o Oscillator generates the carrier and can be switched off and on by a telegraph key to produce the dots and dashes of the International Morse Code. o This type of information transmission is referred to as continuous-wave (CW) transmission. o Rarely used since Morse code is nearly extinct and the oscillator power is too low for reliable communication. o Nowadays transmitters such as this are built only by amateur (ham) radio operators for what is called QRP or low-power operation for personal hobby communication. o The CW transmitter can be greatly improved by simply adding a power amplifier to it, as illustrated on the figure below.

High-Level AM Transmitters

o An oscillator, in most applications a crystal oscillator, generates the final carrier frequency. o The carrier signal is then fed to a buffer amplifier whose primary purpose is to isolate the oscillator from the remaining power amplifier stages. o The buffer amplifier usually operates at the class A level and provides a modest increase in power output. o The main purpose of the buffer amplifier is simply to prevent load changes in the power amplifier stages or in antenna from causing frequency variations in the oscillator. o The signal from the buffer amplifier is applied to a class C driver amplifier designed to provide an intermediate level of power amplification.  The purpose of this circuit is to generate sufficient output power to drive the final power amplifier stage.

o The final power amplifier, normally just referred to as the final, also operates at the class C level at very high power. o All the RF circuits in the transmitter are usually solid state  They are implemented with either bipolar transistors or metal-oxide semiconductor field-effect transistors (MOSFETs).  Bipolar Transistors are by far the most common type.  MOSFETs increase in use because they are now capable of handling high power at high frequencies. o Transistors are also typically used in the final as long as the power level does not exceed several hundred watts. o Individual RF power transistors can handle up to about 800 W. o It can be connected in parallel or in push-pull configurations to increase the powerhandling capability to many kilowatts. o For higher power levels, vacuum tubes are still used in some transmitters, but rarely in new designs.  Vacuum tubes function into the VHF and UHF ranges, with power level of 1kW or more.

Low-Level FM Transmitters o In low-level modulation, modulation is performed on the carrier at low power level, and the signal is then amplified by power amplifiers.

o Indirect method of FM generation is used. o A stable crystal oscillator is used to generate the carrier signal, and a buffer amplifier is used to isolate it from the remainder of the circuitry. o The carrier signal is then applied to a phase modulator. o The voice input is amplified and processed to limit the frequency range and prevent overdeviation. o The output of the modulator is the desired FM signal. o Most FM transmitters are used in the VHF and UHF range. o Crystal oscillators are not available for generating those frequencies directly, the carrier is usually generated at a frequency considerably lower than the final output frequency. o One or more frequency multiplier stages are used to achieve the desired output frequency. o A frequency multiplier is a class C amplifier whose output frequency is some integer multiple of the input frequency.  Most frequency multipliers increase the frequency by a factor of 2, 3, 4, or 5.  Most frequency multipliers also provide a modest amount of power amplification. o The final amplifier stages in FM broadcast transmitters typically use large vacuum tubes class C amplifiers. o In FM transmitters operating in the microwave range, klystrons, magnetrons, and traveling-wave tubes are used to provide the final power amplification.

The serial data representing the data to be transmitted is sent to the DSP, which then generates two data streams that are then converted to RF for transmission.
The data paths from the DSP chip are sent to DACs where they are translated to equivalent analog signals.
The analog signals are filtered in a low-pass filter (LPF) and then applied to mixers that will upconvert them to the final output frequency.
The mixers receive their second inputs from an oscillator or a frequency synthesizer that selects the operating frequency.
Note that the oscillator signals are in quadrature o One is shifted 90 0 from the other o One is a sine wave, and the other is a cosine wave
The upper signal is referred to as the in-phase (I) signal and the other as the quadrature (Q) signal.
The output signals from the mixers are then added, and the result is amplified and transmitted by the power amplifier (PA).
Two quadrature signals are needed at the receiver to recover the signal and demodulate it in a DSP chip.

Carrier Generators The starting point for all transmitters is carrier generation. Once generated, the carrier can be modulated, processed in various ways, amplified, and finally transmitted. The source of most carriers in modern transmitters is a crystal oscillator. PLL frequency synthesizers in which a crystal oscillator is the basic stabilizing reference are used in applications requiring multiple channels of operation.

Crystal Oscillators - The only oscillator capable of meeting the precision and stability demanded by the FCC - Crystal o Is a piece of quartz that has been cut and ground into a thin, flat wafer and mounted between two metal plates. - Piezoelectric Effect o When crystal is excited by an ac signal across its plates, it vibrates. o The frequency of vibration is determined primarily by the thickness of the crystal. - Other factors influencing frequency are the cut of the crystal o The place and angle of cut made in the base quartz rock from which the crystal was derived, and the size of the crystal wafer. - Crystal frequencies range from as low as 30 kHz to as high as 150 MHz. - As the crystal vibrates or oscillates, it maintains a very constant frequency. - Greater stability can be achieved by mounting the crystal in sealed, temperature-controlled chambers known as crystal ovens. o These devices maintain an absolute constant temperature, ensuring a stable output frequency - The crystal acts as an LC tuned circuit. It can emulate a series or parallel LC circuit with a Q as high as 30,000. - The crystal is simply substituted for the coil and capacitor in a convenient oscillator circuit. - The precision, or stability, of a crystal is usually expressed in parts per million (ppm).

For example, to say that a crystal with a frequency of 1 MHz has a precision of 100 ppm means that the frequency of the crystal can vary from 999,900 to 1,000,100 Hz.
Most crystals have a tolerance and stability values in the 10- to 1000-ppm.

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The feedback comes from the capacitor voltage divider C 1 -C 2.
The output is taken from the emitter, which is untuned.
Most oscillators of this type operate as class A amplifiers with a sine wave output.
JFETs are also widely used in discrete component amplifiers.
Occasionally you will see a capacitor in series or in parallel with the crystal (not both). - These capacitors can be used to make minor adjustments in the crystal frequency. o The capacitors are called crystal pulling capacitors, and the whole process of fine0tuning a crystal is sometimes referred to as rubbering. o When the pulling capacitor is a varactor, FM or FSK can be produced. o The analog or binary modulating signal varies the varactor capacitance that shifts the crystal frequency.

Overtone Oscillators - Main problem with crystals is that their upper frequency operation is limited. - The higher the frequency, the thinner the crystal must be to oscillate at that frequency. - At an upper limit of about 50 MHz, the crystal is so fragile that it becomes impractical to use. - One way to achieve VHF, UHF, and even microwave frequencies using crystals is by employing frequency multiplier circuits. - The carrier frequency oscillator operates on a frequency less than 50 MHz, and multipliers raise the frequency to the desired level. o Example: if the desired operating frequency is 163 MHz and the frequency multipliers multiply by a factor of 24, the crystal frequency must be 163/24 = 6. MHz - Another way to achieve crystal precision and stability at frequencies above 50 MHz is to use overtone crystals. - Overtone Crystals – is cut in a special way so that it optimizes its oscillation at an overtone of the basic crystal frequency. - Overtone – is like a harmonic as it is usually some multiple of the fundamental vibration frequency. However, the term harmonic is usually applied to electric signals, and the term overtone refers to higher mechanical vibration frequencies. o Usually some integer multiple of the base vibration frequency. o Most overtones are slightly more or slightly less than the integer value. - In a crystal, the second harmonic is the first overtone, the third harmonic is the second overtone, and so on. o Example: a crystal with a fundamental frequency of 20 MHz would have a second harmonic or first overtone of 40 MHz, and a third harmonic or second overtone of 60 MHz.

The term overtone is often used as a synonym for harmonic. Most manufacturers refer to their third overtone crystals as third harmonic crystals.
The odd overtones are far greater in amplitude than the even overtones. Most overtone crystals oscillate reliably at the third and fifth overtone of the frequency at which the crystal is originally ground.
There are also seventh-overtone crystals.
Versions of packaged crystal oscillators o Basic crystal oscillator (XO) – has stability in the tens of ppm. o Voltage-controlled crystal oscillator (VCXO) – uses a varactor in series or parallel with the crystal to vary the crystal frequency over a narrow range with an external DC voltage. o Temperature-compensated crystal oscillator (TCXO) – improved stability, which uses a feedback network with a thermistor to sense temperature variations, which in turn controls a voltage variable capacitor (VVC) or varactor to pull the crystal frequency to some desired value. It can achieve stability values of ±0 to ±2 ppm o Oven-controlled crystal oscillator (OCXO) – packages the crystal and its circuit in a temperature-controlled oven that holds the frequency stable at the desired frequency. A thermistor sensor feedback network varies the temperature of a heating element in the oven. Stabilities in the ±1 x 10-8 or better can be obtained.

Frequency Synthesizers - Are variable-frequency generators that provide the frequency stability of crystal oscillators but with the convenience of incremental tuning over a broad frequency range. - Usually provide an output signal that varies in fixed frequency increments over a wide range. - In transmitter, it provides basic carrier generation for channelized operation. - Also used in receivers as local oscillators and perform the receiver tuning function. - Most frequency synthesizers nowadays us some variation of the phase-locked loop (PLL). A newer technique called digital signal synthesis (DSS) is becoming more popular as integratedcircuit technology has made high-frequency generation practical.

Phase-Locked Loop Synthesizers

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Note that the VCO output is not connected directly back to the phase detector but applied to a frequency divider first.

A frequency divider is a circuit whose output frequency is some integer submultiple of the input frequency. o A divide-by-10 frequency synthesizer produces an output frequency that is one-tenth of the input frequency. o Can be easily implemented with digital circuits to provide any integer value of frequency division.
A more complex PLL synthesizer, a circuit that generates VHF and UHF frequencies over the 100- to 500-MHz range is shown in Fig 8-9.
This circuit uses an FET oscillator to generate the carrier frequency directly.
No frequency multiplier needed.
The output of the frequency synthesizer can be connected directly to the driver and power amplifiers in the transmitter.
This synthesizer has an output frequency in the 390-MHz range, and the frequency can be varied in 30-kHz increments above and below that frequency.
The VCO circuit for the synthesizer in Fig 8-9 is shown in Fig 8-10.

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The frequency of this LC oscillator is set by the values of L 1 , C 1 , C 2 and capacitances of the varactor diodes D 1 and D 2 , Ca and Cb, respectively.
The DC applied to the varactors changes the frequency. Two varactors are connected back to back, and thus the total effective capacitance of the pair is less than either individual capacitance.
It is equal to the series capacitance Cs, where Cs = CaCb/(Ca + Cb). o If D 1 and D 2 are identical, Cs = Ca/
A negative voltage with respect to ground is required to reverse-bias the diodes.
Increasing negative voltage increases the reverse bias and decreases the capacitance, therefore increasing the oscillator frequency.
Using two varactors allows the oscillator to produce higher RF voltages without the problem of the varactors becoming forward-biased. o If a varactor, which is a diode, becomes forward-biased, it is no longer a capacitor.
Fractional N divider PLL o The VCO output is applied to a special variable-modulus prescaler divider. o It is made of emitter-coupled logic or CMOS circuits. o It is designed to have two divider ratios, M and M + 1.

Direct Digital Synthesis - A DDS synthesizer generates a sine wave output digitally. - The output frequency can be varied in increments depending upon a binary value supplied to the unit by a counter, a register, or an embedded microcontroller.

A read-only memory (ROM) is programmed with the binary representation of a sine wave.
These values that would be generated by an analog-to-digital (A/D) converter if an analog sine wave were digitized and stored in the memory.
If these binary values are fed to a digital-to-analog (D/A) converter, the output of the D/A converter will be a stepped approximation of the sine wave.
A low-pass filter is used to remove the high-frequency content near the clock frequency, thereby smoothing the ac output into a nearly perfect since wave.
To operate the circuit: o A binary counter is used to supply the address word to the ROM. o A clock signal steps the counter that supplies a sequentially increasing address to ROM. o The binary numbers stored in ROM are applied to the D/A converter, and the stepped sine wave is generated. o The frequency of the clock determines the frequency of the sine wave.
To illustrate this concept: o Assume a 16-word ROM in which each storage location has a 4-bit address o The addresses are supplied by a 4-bit binary counter that counts from 0000 through 1111 and recycles. o Stored in ROM are binary numbers representing values that are the sine of particular angles of the sine wave to be generated. o Since sine wave is 360 0 in length, and since the 4-bit counter produces 16 addresses or increments, the binary values represent the sine values at 360/16 = 22 0 increments. o Assume further that these sine values are represented with 8 bits of precision. o The 8-bit binary sine values are fed to the D/A converter, where they are converted to a proportional voltage.
Accumulator – the combination of the register and adder. o This circuit is arranged so that upon the occurrence of each clock pulse, the constant C is added to the previous value of the register content and the sum is re-stored in the address register.
The higher the constant value C, the fewer the samples used to reconstruct the output sine wave.

Phase Noise - Is the minor variation in the amplitude and phase of the signal generator output. - The noise comes from natural semiconductor sources, power supply variations, or thermal agitation in the components. - The phase variations manifest themselves as frequency variations.

The average noise power is referred to as the spectral power density o ((((((((((((((() = 𝐀⁄⁄⁄⁄⁄⁄⁄⁄⁄⁄⁄⁄⁄⁄⁄𝐀  )((((((((((((((( = phase noise

o However, FM signals do not vary in amplitude and can therefore be amplified with more efficient nonlinear class C amplifiers. o Makes a good frequency multiplier as harmonics are generated in the amplification process.

Switching amplifiers o Act like on/off or digital switches. o The effectively generate a square wave output. o Such a distorted output is undesirable; however, by using high-q tuned circuits in the output, the harmonics generated as part of the switching process can be easily filtered out. o Designated class D, E, F, and S

Linear Amplifiers - Used primarily in AM and SSB transmitters, and both low- and high-power versions are used.

Class A Buffers

This type of amplifier is used between the carrier oscillator and the final power amplifier to isolate the oscillator from the power amplifier load, which can change the oscillator frequency.
- It provides a modest power increase to provide the driving power required by the final amplifier.

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Usually provide milliwatts of power and rarely more than 1 W.

The carrier oscillator signal is capacitively coupled to the input.
The bias is derived from R 1 , R 2 and R3.
The emitter resistor R 3 is bypassed to provide maximum gain.
The collector is tuned with a resonant LC circuit at the operating frequency. - An inductively coupled secondary loop transfers power to the next stage.

High-Power Linear Amplifiers

A power MOSFET may also be used in this circuit with a few modifications.
Base bias is supplied by a constant-current circuit that is temperature-compensated.
The RF input from a 50-ohm source is connected to the base via an impedance-matching circuit made up of C 1 , C 2 and L 1.
The output is matched to a 50-ohm load by the impedance-matching network made up of L 2 , L 3 , C 3 and C 4.
When connected to a proper heatsink, the transistor can generate up to 100 W of power up to about 200 MHz.
Class A amplifiers have a maximum efficiency of 50 percent. Thus only 50 percent of the dc power is converted to RF, with the remaining 50 percent being dissipated in the transistor.
For 100-W RF output, the transistor dissipates 100 W. - Efficiencies of less than 50 percent are typical.

Class B Push-Pull Amplifiers - The RF driving signal is applied to Q 1 and Q 2 through input transformer T 1. - It provides impedance-matching and base drive signals Q 1 and Q 2 that are 180o out of phase. - An output transformer T 2 couples the power to the antenna or load. - Bias is provided by R 1 and D 1. - For class B operation Qi and Q 2 must be biased right at the cutoff point. - The emitter-base junction of a transistor will not conduct until about 0 to 0 V of forward bias is applied because of the built-in potential barrier. - This effect causes the transistor to be naturally biased beyond cutoff, not right at it. A forwardbiased silicon diode D 1 has about 0 V across it, and this is used to put Q 1 and Q 2 right on the conduction threshold. - On the positive half-cycle of the RF input, the base of Q 1 is positive and the base of Q 2 is negative. - The Q 2 us cut off, but Q 1 conducts, linearly amplifying the positive half-cycle. Collector current flows in the upper half of T 2 , which induces an output voltage in the secondary.

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Communication Principles - 7

Course: Electronics Engineering (CR 061)

95 Documents

Students shared 95 documents in this course

University: Samar State University

Was this document helpful?

LESSON CONTENT

Transmitter Configurations

- Simplest transmitter – is a single-transistor oscillator connected directly to an antenna. o

Oscillator generates the carrier and can be switched off and on by a telegraph key to produce

the dots and dashes of the International Morse Code.

oThis type of information transmission is referred to as continuous-wave (CW)

transmission. o Rarely used since Morse code is nearly extinct and the oscillator

power is too low for reliable communication. o Nowadays transmitters such as this

are built only by amateur (ham) radio operators for what is called QRP or low-power

operation for personal hobby communication. o The CW transmitter can be greatly

improved by simply adding a power amplifier to it, as illustrated on the figure below.

- High-Level AM Transmitters

o An

oscillator, in most applications a crystal oscillator, generates the final carrier frequency. o

The carrier signal is then fed to a buffer amplifier whose primary purpose is to isolate the

oscillator from the remaining power amplifier stages.

oThe buffer amplifier usually operates at the class A level and provides a modest

increase in power output.

oThe main purpose of the buffer amplifier is simply to prevent load changes in the

power amplifier stages or in antenna from causing frequency variations in the

oscillator.

oThe signal from the buffer amplifier is applied to a class C driver amplifier designed to

provide an intermediate level of power amplification.

The purpose of this circuit is to generate sufficient output power to drive the

final power amplifier stage.

Communication Principles - 7

Electronics Engineering (CR 061)

Samar State University

Comments

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Preview text

LESSON CONTENT

-

-

-

-

-

Communication Principles - 7

Course: Electronics Engineering (CR 061)

University: Samar State University

Students also viewed

Related documents