project - frequency synthesizer

8/6/2019 PROJECT - Frequency Synthesizer

1/10

FREQUENCY SYNTHESIZER

BY

1.INDRAJIT DUTTA

2.KUSHAL GHOSH

3.DEBDIP CHOWDHURY

4.ABHISHEK CHOUDHURY

Guided by

PROFESSOR B .SOM

B. P. PODDAR INSTITUTE OF MANAGEMENT AND TECHNOLOGY

137, V.I.P. ROAD

KOLKATA 700 052

A Project Report submitted for the partial fulfillment for the Degree of Bachelor of

Technology in Electronics and Communication Engineering Department.


2/10

A Voltage-Controlled Oscillator or VCO is an electronic oscillator designed to becontrolled in oscillation frequencyby a voltage input. The frequency of oscillation is

varied by the applied DC voltage, while modulatingsignals may also be fed into the VCO

to cause frequency modulation (FM) orphase modulation (PM); a VCO with digital pulseoutput may similarly have its repetition rate (FSK, PSK) or pulse width modulated

(PWM).

Types of VCOs

VCOs can be generally categorized into two groups based on the type of waveformproduced: 1) harmonic oscillators, and 2) relaxation oscillators.

Harmonic oscillators generate a sinusoidal waveform. They consist of an amplifier that

provides adequate gain and a resonant circuit that feeds back signal to the input.Oscillation occurs at the resonant frequency where a positive gain arises around the loop.

Some examples of harmonic oscillators are crystal oscillators and LC-tank oscillators.

When part of the resonant circuit's capacitance is provided by a varactor diode, the

voltage applied to that diode varies the frequency.

Relaxation oscillators can generate a sawtooth or triangular waveform. They are

commonly used in monolithic integrated circuits (ICs). They can provide a wide range of

operational frequencies with a minimal number of external components. Relaxation

oscillator VCOs can have three topologies: 1) grounded-capacitor VCOs, 2) emitter-coupled VCOs, and 3) delay-based ring VCOs. The first two of these types operate

similarly. The amount of time in each state depends on the time for a current to charge ordischarge a capacitor. The delay-based ring VCO operates somewhat differentlyhowever. For this type, the gain stages are connected in a ring. The output frequency is

then a function of the delay in each of stages.

Harmonic oscillator VCOs have these advantages over relaxation oscillators.

Frequency stability with respect to temperature, noise, and power supply is muchbetter for harmonic oscillator VCOs.

They have good accuracy for frequency control since the frequency is controlled

by a crystal or tank circuit.

A disadvantage of harmonic oscillator VCOs is that they cannot be easily implemented inmonolithic ICs. Relaxation oscillator VCOs are better suited for this technology.

Relaxation VCOs are also tunable over a wider range of frequencies.

Control of frequency in VCOs
http://en.wikipedia.org/wiki/Electronic_oscillatorhttp://en.wikipedia.org/wiki/Oscillationhttp://en.wikipedia.org/wiki/Oscillationhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Frequency_modulationhttp://en.wikipedia.org/wiki/Phase_modulationhttp://en.wikipedia.org/wiki/Pulse-width_modulationhttp://en.wikipedia.org/wiki/Electronic_oscillatorhttp://en.wikipedia.org/wiki/Oscillationhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Frequency_modulationhttp://en.wikipedia.org/wiki/Phase_modulationhttp://en.wikipedia.org/wiki/Pulse-width_modulation


3/10

A voltage-controlled capacitor is one method of making an LC oscillator vary its

frequency in response to a control voltage. Any reverse-biased semiconductordiode

displays a measure of voltage-dependent capacitance and can be used to change thefrequency of an oscillator by varying a control voltage applied to the diode. Special-

purpose variable capacitancevaractordiodes are available with well-characterized wide-

ranging values of capacitance. Such devices are very convenient in the manufacture ofvoltage-controlled oscillators[note 1] For low-frequency VCOs, other methods of varying

the frequency (such as altering the charging rate of a capacitor by means of a voltage

controlled current source) are used. See Function generator.

The frequency of a ring oscillatoris controlled by varying either the supply voltage or thecapacitive loading on each stage.

Voltage-controlled crystal oscillators

A voltage-controlled crystal oscillator(VCXO) is used when the frequency of

operation needs to be adjusted only finely. The frequency of a voltage-controlled crystaloscillator can be varied only by typically a few tens of parts per million (ppm), because

the high Q factorof the crystals allows "pulling" over only a small range of frequencies.

There are two reasons for using a VCXO:

To adjust the output frequency to match (or perhaps be some exact multiple of) anaccurate external reference.

Where the oscillator drives equipment that may generate radio-frequency

interference, adding a varying voltage to its control input can disperse theinterference spectrum to make it less objectionable. See spread-spectrum clockgeneration.

Phase-locked loop

A phase-locked loop orphase lock loop (PLL) is a control system that tries to generate

an output signal whosephase is related to the phase of the input "reference" signal. It is

anelectronic circuitconsisting of a variable frequency oscillatorand aphase detector.This circuit compares the phase of the input signal with the phase of the signal derived

from its output oscillator and adjusts the frequency of its oscillator to keep the phasesmatched. The signal from the phase detector is used to control the oscillator in a feedbackloop.

Frequency is the derivativeof phase. Keeping the input and output phase in lock step

implies keeping the input and output frequencies in lock step. Consequently, a phase-

locked loop can track an input frequency, or it can generate a frequency that is a multiple
http://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Varactorhttp://en.wikipedia.org/wiki/Varactorhttp://en.wikipedia.org/wiki/Voltage-controlled_oscillator#cite_note-0http://en.wikipedia.org/wiki/Current_sourcehttp://en.wikipedia.org/wiki/Function_generatorhttp://en.wikipedia.org/wiki/Ring_oscillatorhttp://en.wikipedia.org/wiki/Crystal_oscillatorhttp://en.wikipedia.org/wiki/Crystal_oscillatorhttp://en.wikipedia.org/wiki/Q_factorhttp://en.wikipedia.org/wiki/Spread_spectrum#Spread-spectrum_clock_signal_generationhttp://en.wikipedia.org/wiki/Spread_spectrum#Spread-spectrum_clock_signal_generationhttp://en.wikipedia.org/wiki/Control_systemhttp://en.wikipedia.org/wiki/Signal_(electrical_engineering)http://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Electronic_circuithttp://en.wikipedia.org/wiki/Electronic_circuithttp://en.wikipedia.org/wiki/Electronic_circuithttp://en.wikipedia.org/wiki/Electronic_oscillatorhttp://en.wikipedia.org/wiki/Phase_detectorhttp://en.wikipedia.org/wiki/Feedback_loophttp://en.wikipedia.org/wiki/Feedback_loophttp://en.wikipedia.org/wiki/Derivativehttp://en.wikipedia.org/wiki/Derivativehttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Varactorhttp://en.wikipedia.org/wiki/Voltage-controlled_oscillator#cite_note-0http://en.wikipedia.org/wiki/Current_sourcehttp://en.wikipedia.org/wiki/Function_generatorhttp://en.wikipedia.org/wiki/Ring_oscillatorhttp://en.wikipedia.org/wiki/Crystal_oscillatorhttp://en.wikipedia.org/wiki/Q_factorhttp://en.wikipedia.org/wiki/Spread_spectrum#Spread-spectrum_clock_signal_generationhttp://en.wikipedia.org/wiki/Spread_spectrum#Spread-spectrum_clock_signal_generationhttp://en.wikipedia.org/wiki/Control_systemhttp://en.wikipedia.org/wiki/Signal_(electrical_engineering)http://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Electronic_circuithttp://en.wikipedia.org/wiki/Electronic_oscillatorhttp://en.wikipedia.org/wiki/Phase_detectorhttp://en.wikipedia.org/wiki/Feedback_loophttp://en.wikipedia.org/wiki/Feedback_loophttp://en.wikipedia.org/wiki/Derivative


4/10

of the input frequency. The former property is used fordemodulation, and the latter

property is used for indirect frequency synthesis.

Most frequency synthesizers are based around a phase locked loop or PLL. The PLL usesthe idea of phase comparison as the basis of its operation. From the block diagram of a

basic loop shown below, it can be seen that there are three basic circuit blocks, a phasecomparator, voltage controlled oscillator, and loop filter. A reference oscillator is

sometimes included in the block diagram, although this is not strictly part of the loopitself even though a reference signal is required for its operation.

Block diagram of a basic phase locked loop (PLL)

The phase locked loop, PLL, operates by comparing the phase of two signals. The signals

from the voltage controlled oscillator and reference enter the phase comparator Here a

third signal equal to the phase difference between the two input signals is produced.

The phase difference signal is then passed through the loop filter. This performs anumber of functions including the removal of any unwanted products that are present on

this signal. Once this has been accomplished it is applied to the control terminal of the

voltage controlled oscillator. This tune voltage or error voltage is such that it tries toreduce the error between the two signals entering the phase comparator. This means that

the voltage controlled oscillator will be pulled towards the frequency of the reference,

and when in lock there is a steady state error voltage. This is proportional to the phase

error between the two signals, and it is constant. Only when the phase between twosignals is changing is there a frequency difference. As the phase difference remains

constant when the loop is in lock this means that the frequency of the voltage controlled

oscillator is exactly the same as the reference.

Phase-locked loops are widely used inradio,telecommunications, computers and otherelectronic applications. They may generate stable frequencies, recover a signal from a

noisy communication channel, or distribute clock timing pulses in digital logic designssuch as microprocessors. Since a single integrated circuit can provide a complete phase-

locked-loop building block, the technique is widely used in modern electronic devices,

with output frequencies from a fraction of a hertz up to many gigahertz
http://en.wikipedia.org/wiki/Demodulationhttp://en.wikipedia.org/wiki/Demodulationhttp://en.wikipedia.org/wiki/Frequency_synthesishttp://en.wikipedia.org/wiki/Frequency_synthesishttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Computerhttp://en.wikipedia.org/wiki/Microprocessorhttp://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/wiki/Demodulationhttp://en.wikipedia.org/wiki/Frequency_synthesishttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Computerhttp://en.wikipedia.org/wiki/Microprocessorhttp://en.wikipedia.org/wiki/Integrated_circuit


5/10

A phase detector compares two input signals and produces an error signal which is

proportional to their phase difference. The error signal is then low-pass filtered and used

to drive a VCO which creates an output phase. The output is fed through an optionaldivider back to the input of the system, producing a negative feedback loop. If the output

phase drifts, the error signal will increase, driving the VCO phase in the opposite

direction so as to reduce the error. Thus the output phase is locked to the phase at theother input. This input is called the reference.

Analog phase locked loops are generally built with an analog phase detector, low pass

filter and VCO placed in a negative feedbackconfiguration. A digital phase locked loop

uses a digital phase detector; it may also have a divider in the feedback path or in thereference path, or both, in order to make the PLL's output signal frequency a rational

multiple of the reference frequency. A non-integer multiple of the reference frequency

can also be created by replacing the simple divide-by-N counter in the feedback path witha programmablepulse swallowing counter. This technique is usually referred to as a

fractional-N synthesizer.

The oscillator generates a periodic output signal. Assume that initially the oscillator is at

nearly the same frequency as the reference signal. If the phase from the oscillator fallsbehind that of the reference, the phase detector changes the control voltage of the

oscillator so that it speeds up. Likewise, if the phase creeps ahead of the reference, the

phase detector changes the control voltage to slow down the oscillator. Since initially theoscillator may be far from the reference frequency, practical phase detectors may also

respond to frequency differences, so as to increase the lock-in range of allowable inputs.

Depending on the application, either the output of the controlled oscillator, or the control

signal to the oscillator, provides the useful output of the PLL system.

A phase locked loop is a feedback control system. It compares the phases of two inputsignals and produces an error signalthat is proportional to the difference between their

phases.[11] The error signal is then low pass filtered and used to drive a voltage-controlled

oscillator(VCO) which creates an output frequency. The output frequency is fed throughafrequency dividerback to the input of the system, producing anegative feedbackloop.

If the output frequency drifts, the phase error signal will increase, driving the frequency

in the opposite direction so as to reduce the error. Thus the output is lockedto thefrequency at the other input. This other input is called the reference and is usually

derived from a crystal oscillator, which is very stable in frequency.

PLL frequency synthesizer basics

A phase locked loop, PLL, needs some additional circuitry if it is to be converted into a

frequency synthesizer. This is done by adding a frequency divider between the voltage

controlled oscillator and the phase comparator as shown below.
http://en.wikipedia.org/wiki/Negative_feedback_loophttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Rational_numberhttp://en.wikipedia.org/wiki/Pulse_swallowing_counterhttp://en.wikipedia.org/wiki/Fractional-N_synthesizerhttp://en.wikipedia.org/w/index.php?title=Error_signal&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Error_signal&action=edit&redlink=1http://en.wikipedia.org/wiki/Frequency_synthesizer#cite_note-10http://en.wikipedia.org/wiki/Voltage-controlled_oscillatorhttp://en.wikipedia.org/wiki/Voltage-controlled_oscillatorhttp://en.wikipedia.org/wiki/Frequency_dividerhttp://en.wikipedia.org/wiki/Frequency_dividerhttp://en.wikipedia.org/wiki/Frequency_dividerhttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Negative_feedback_loophttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Rational_numberhttp://en.wikipedia.org/wiki/Pulse_swallowing_counterhttp://en.wikipedia.org/wiki/Fractional-N_synthesizerhttp://en.wikipedia.org/w/index.php?title=Error_signal&action=edit&redlink=1http://en.wikipedia.org/wiki/Frequency_synthesizer#cite_note-10http://en.wikipedia.org/wiki/Voltage-controlled_oscillatorhttp://en.wikipedia.org/wiki/Voltage-controlled_oscillatorhttp://en.wikipedia.org/wiki/Frequency_dividerhttp://en.wikipedia.org/wiki/Negative_feedback


6/10

A programmable divider added into a phase locked loop, PLL, enables the

frequency to be changed.

Programmable dividers or counters are used in many areas of electronics, including many

radio frequency applications. They take in a pulse train like that below, and give out a

slower train. In a divide by two circuit only one pulse is given out for every two that are

fed in and so forth. Some are fixed, having only one division ratio. Others areprogrammable and digital or logic information can be fed into them to set the division

ratio.

Operation of a programmable divider

When the divider is added into the circuit the phase locked loop, PLL, still tries to reduce

the phase difference between the two signals entering the phase comparator. Again

when the circuit is in lock both signals entering the comparator are exactly the same infrequency. For this to be true the voltage controlled oscillator must be running at a

frequency equal to the phase comparison frequency times the division ratio.

It can be seen that if the division ratio is altered by one, then the voltage controlled

oscillator will have to change to the next multiple of the reference frequency. This meansthat the step frequency of the synthesizer is equal to the frequency entering the

comparator.

Most synthesizers need to be able to step in much smaller increments if they are to be ofany use. This means that the comparison frequency must be reduced. This is usually

accomplished by running the reference oscillator at a frequency of a megahertz or so, and

then dividing this signal down to the required frequency using a fixed divider. In this way

a low comparison frequency can be achieved.


7/10

Comparison frequency reduced by adding a fixed divider after the reference

oscillator

Analogue PLL frequency synthesizers

Placing a digital divider is not the only method of making a synthesizer using a phase

locked loop, PLL. It is also possible to use a mixer in the loop. Using this technique

places an offset into the frequency generated by the loop.

Phase comparator

A phase detector orphase comparator is a frequency mixer,analog multiplierorlogic

circuit that generates a voltage signal which represents the difference in phase betweentwo signal inputs. It is an essential element of thephase-locked loop(PLL).
http://en.wikipedia.org/wiki/Frequency_mixerhttp://en.wikipedia.org/wiki/Frequency_mixerhttp://en.wikipedia.org/wiki/Analog_multiplierhttp://en.wikipedia.org/wiki/Digital_logichttp://en.wikipedia.org/wiki/Digital_logichttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Frequency_mixerhttp://en.wikipedia.org/wiki/Analog_multiplierhttp://en.wikipedia.org/wiki/Digital_logichttp://en.wikipedia.org/wiki/Phase-locked_loop


8/10

Detecting phase differences is very important in many applications, such as motor

control, radarand telecommunication systems, servo mechanisms, anddemodulator

Phase detectors forphase-locked loop circuits may be classified in two types.[1]A Type Idetector is designed to be driven by analog signals or square-wave digital signals and

produces an output pulse at the difference frequency. The Type I detector alwaysproduces an output waveform, which must be filtered to control the phase-locked loop

voltage-controlled oscillator(VCO). A type II detector is sensitive only to the relativetiming of the edges of the input and reference pulses, and produces a constant output

proportional to phase difference when both signals are at the same frequency. This output

will tend not to produce ripple in the control voltage of the VCO.

Frequency divider

A frequency divider, also called a clock divider orscaler orprescaler, is a circuit thattakes an input signal of a frequency, fin, and generates an output signal of a frequency:

where n is an integer. Phase-locked loopfrequency synthesizers make use of frequencydividers to generate a frequency that is a multiple of a reference frequency. Frequency

dividers can be implemented for bothanalog and digital applications.

Crystal oscillator

A crystal oscillator is an electronic oscillatorcircuit that uses the mechanicalresonance

of a vibrating crystal ofpiezoelectric materialto create an electrical signal with a veryprecise frequency. This frequency is commonly used to keep track of time (as in quartz

wristwatches), to provide a stable clock signal fordigitalintegrated circuits, and to

stabilize frequencies forradio transmitters and receivers. The most common type ofpiezoelectric resonator used is thequartz crystal, so oscillator circuits designed around

them became known as "crystal oscillators. The crystal oscillator circuit sustains

oscillation by taking a voltage signal from the quartz resonator, amplifying it, and feedingit back to the resonator. The rate of expansion and contraction of the quartz is theresonant frequency, and is determined by the cut and size of the crystal. When the energy

of the generated output frequencies matches the losses in the circuit, an oscillation can be

sustained.

An oscillator crystal has two electrically conductive plates, with a slice or tuning fork of

quartz crystal sandwiched between them. During startup, the circuit around the crystal
http://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Servomechanismhttp://en.wikipedia.org/wiki/Demodulatorhttp://en.wikipedia.org/wiki/Demodulatorhttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Phase_detector#cite_note-0http://en.wikipedia.org/wiki/Phase_detector#cite_note-0http://en.wikipedia.org/wiki/Voltage-controlled_oscillatorhttp://en.wikipedia.org/wiki/Circuithttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Frequency_synthesizerhttp://en.wikipedia.org/wiki/Analog_electronicshttp://en.wikipedia.org/wiki/Analog_electronicshttp://en.wikipedia.org/wiki/Digitalhttp://en.wikipedia.org/wiki/Electronic_oscillatorhttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Piezoelectricity#Materialshttp://en.wikipedia.org/wiki/Piezoelectricity#Materialshttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Quartz_clockhttp://en.wikipedia.org/wiki/Quartz_clockhttp://en.wikipedia.org/wiki/Clock_signalhttp://en.wikipedia.org/wiki/Digitalhttp://en.wikipedia.org/wiki/Digitalhttp://en.wikipedia.org/wiki/Digitalhttp://en.wikipedia.org/wiki/Integrated_circuitshttp://en.wikipedia.org/wiki/Radio_transmitterhttp://en.wikipedia.org/wiki/Radio_receiverhttp://en.wikipedia.org/wiki/Radio_receiverhttp://en.wikipedia.org/wiki/Quartz_crystalhttp://en.wikipedia.org/wiki/Quartz_crystalhttp://en.wikipedia.org/wiki/Resonatorhttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Servomechanismhttp://en.wikipedia.org/wiki/Demodulatorhttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Phase_detector#cite_note-0http://en.wikipedia.org/wiki/Voltage-controlled_oscillatorhttp://en.wikipedia.org/wiki/Circuithttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Phase-locked_loophttp://en.wikipedia.org/wiki/Frequency_synthesizerhttp://en.wikipedia.org/wiki/Analog_electronicshttp://en.wikipedia.org/wiki/Digitalhttp://en.wikipedia.org/wiki/Electronic_oscillatorhttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Piezoelectricity#Materialshttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Quartz_clockhttp://en.wikipedia.org/wiki/Quartz_clockhttp://en.wikipedia.org/wiki/Clock_signalhttp://en.wikipedia.org/wiki/Digitalhttp://en.wikipedia.org/wiki/Integrated_circuitshttp://en.wikipedia.org/wiki/Radio_transmitterhttp://en.wikipedia.org/wiki/Radio_receiverhttp://en.wikipedia.org/wiki/Quartz_crystalhttp://en.wikipedia.org/wiki/Resonatorhttp://en.wikipedia.org/wiki/Resonance


9/10

applies a random noiseAC signal to it, and purely by chance, a tiny fraction of the noise

will be at the resonant frequency of the crystal. The crystal will therefore start oscillating

in synchrony with that signal. As the oscillator amplifies the signals coming out of thecrystal, the signals in the crystal's frequency band will become stronger, eventually

dominating the output of the oscillator. The narrow resonance band of the quartz crystal

filters out all the unwanted frequencies.

The output frequency of a quartz oscillator can be either the fundamental resonance or amultiple of the resonance, called anovertone frequency. High frequency crystals are

often designed to operate at third, fifth, or seventh overtones.

PRINCIPLE OF OPERATION

The heart of the unit is a voltage controlled oscillator.The VCO actually generates the 6

Mhz ( or 8 or 12 Mhz) output frequency.This signal is fed to a switched buffer amplifierand then into the frequency multipliers needed to generate the correct crystal frequency.

Two sets of multiplier are usually needed, one set for transmitting frequencies and onefor receiving frequencies. All the rest of the synthesizer is used as an automatic frequency

control to keep the VCO on frequency.The correct terminology is actually to call it a

phase-locked loop (PLL).This is done by a feedback loop that compares the VCO outputfrequency with a crystal generated reference frequency in the phase (and frequency )

comparator.It is used here as a frequency comparator, but since it works by actually

comparing phases, it is called a phase comparator.The crystal frequency is divided by a

reference frequency divider circuit.The VCO frequency is divided by a programmabledivider by a value n.If after comparison the values are same nothing happens.If different,

say the value of the output of the programmable divider is too low, phase comparatorforces VCO to a high value and vice versa.
http://en.wikipedia.org/wiki/Random_noisehttp://en.wikipedia.org/wiki/Random_noisehttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Electronic_filterhttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Overtonehttp://en.wikipedia.org/wiki/Overtonehttp://en.wikipedia.org/wiki/Random_noisehttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Electronic_filterhttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Overtone


10/10

.

project - frequency synthesizer

Documents