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Encyclopedia > Frequency synthesiser

A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems, etc. Two digital voltmeters The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... Timebase was an amateur production company who released four unofficial, shoestring-budget Doctor Who stories from 2000 to 2003, all starring Rupert Booth as the Doctor with production by Philip T Robinson. ... An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. ... The word receiver has a number of different meanings: In communications and information processing, a receiver is the recipient (observer) of a message (information), which is sent from a source (object). ... Cellular redirects here. ... A radiotelephone is a communications device that allows two or more people to talk using radio. ... A walkie-talkie is a portable, bi-directional radio transceiver, first developed for military use. ... Citizens band radio (CB) is, in the United States, a system of short distance radio communication between individuals on a selection of 40 channels within the single 27 MHz (11 meter) band. ... The word receiver has a number of different meanings: In communications and information processing, a receiver is the recipient (observer) of a message (information), which is sent from a source (object). ... Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. ...


Prior to widespread use of synthesizers, radio and television receivers relied on manual tuning of a local oscillator. Some might remember the classic turret tuner commonly used in television receivers prior to the 1980s. Variations in temperature and aging of components caused frequency drift. Automatic frequency control (AFC) solves some of the drift problem, but manual retuning was often necessary. Since transmitter frequencies are well known and very stable, an accurate means of generating fixed, stable frequencies would solve the problem. The Superheterodyne receiver (or to give it its full name, The Supersonic Heterodyne Receiver – usually these days shortened to superhet) was invented by Edwin Armstrong in 1918. ... Automatic Frequency Control (AFC) is a method (or device) to automatically maintain a tuning of electromagnetic radiation (radio or microwave) signal to desired frequency. ...


A simple and effective solutions employs the use of many stable resonators or oscillators for each tuning frequency. Quartz crystals offer good stability and are often used for this purpose. This "brute force" technique is practical when only a handful of frequencies are required, but quickly becomes costly and impractical in many applications. For example, the FM radio band in many countries supports 100 individual frequencies from about 88 MHz to 108 MHz. Cable television can support even more frequencies or channels over a much wider band. A large number of crystals increases cost and requires greater space. Quartz is the most abundant mineral in the Earths continental crust. ... A crystal oscillator (sometimes abbreviated to XTAL on schematic diagrams) is an electronic circuit that uses the mechanical resonance of a physical crystal of piezoelectric material along with an amplifier and feedback to create an electrical signal with a very precise frequency. ... FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity broadcast radio sound. ... MegaHertz (MHz) is the name given to one million (106) Hertz, a measure of frequency. ... Channel, in communications (sometimes called communications channel), refers to the medium through which information is transmitted from a sender (or transmitter) to a receiver. ...


Many coherent and incoherent techniques have been devised over the years. Some approaches include phase locked loops, double mix, triple mix, harmonic, double mix divide, and direct digital synthesis (DDS). The choice of approach depends on several factors, such as cost, complexity, frequency step size, switching rate, phase noise, and spurious output. Coherence is a property of waves that measures the ability of the waves to interfere with each other. ... Many electronic systems use internal clocks which are required to be phase-aligned to and/or frequency multiples of some external reference clock. ... Direct Digital Synthesis is a method to digitally create arbitrary waveforms and frequencies from a single source fixed frequency. ... In an oscillator, phase noise is rapid, short-term, random fluctuations in the phase of a wave, caused by time domain instabilities. ...


Coherent techniques generate frequencies derived from a single, stable master oscillator. In most applications, crystal oscillator are common, but other resonators and frequency sources can be used. Incoherent techniques derive frequencies from a set of several stable oscillators, typically through frequency multiplication, division, and summing/differencing (mixing). The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost. A crystal oscillator (sometimes abbreviated to XTAL on schematic diagrams) is an electronic circuit that uses the mechanical resonance of a physical crystal of piezoelectric material along with an amplifier and feedback to create an electrical signal with a very precise frequency. ... In telecommunication, a mixer is a nonlinear circuit or device that accepts as its input two different frequencies and presents at its output (a) a signal equal in frequency to the sum of the frequencies of the input signals, (b) a signal equal in frequency to the difference between the...


Synthesizers used in commercial radio receivers are largely based on phase-locked loops or PLLs. Many types of frequency synthesiser are available as integrated circuits, reducing cost and size. High end receivers and electronic test equipment use more sophisticated techniques, often in combination. In electronics, a phase-locked loop (PLL) is a closed-loop feedback control system that maintains a generated signal in a fixed phase relationship to a reference signal. ... Optical Microscope image of an integrated circuit showing defects in the aluminium layer deposition. ...

Contents


Principle of PLL synthesizers

A phase locked loop does for frequency what the Automatic Gain Control does for voltage. It compares the frequencies of two signals and produces an error signal which is proportional to the difference between the input frequencies. The error signal is used to drive a voltage-controlled oscillator (VCO) which creates an output frequency. The output frequency is fed through a frequency divider back to the input of the system, producing a negative feedback loop. If the output frequency drifts, the error signal will increase, driving the frequency in the opposite direction so as to reduce the error. Thus the output is locked to the frequency at the other input. This input is called the reference and is derived from a crystal oscillator, which is very stable in frequency. The block diagram below shows the basic elements and arrangement of a PLL based frequency synthesiser. Automatic gain control (AGC) is an electronic system found in many types of devices. ... A voltage-controlled oscillator or VCO is an electronic circuit that uses amplification, feedback, and a resonant circuit to generate a repeating voltage waveform. ... This article needs to be cleaned up to conform to a higher standard of quality. ...


image:Freq_Synth.jpg Frequency Synthesiser block diagram - drawn by the author File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...


The key to the ability of a frequency synthesiser to generate multiple frequencies is the divider placed between the output and the feedback input. This is usually in the form of a digital counter, with the output signal acting as a clock signal. The counter is preset to some initial count value, and counts down at each cycle of the clock signal. When it reaches zero, the counter output changes state and the count value is reloaded. This circuit is straightforward to implement using flip-flops, and because it is digital in nature, is very easy to interface to other digital components or a microprocessor. This allows the frequency output by the synthesiser to be easily controlled by a digital system. This article is about the term counter used in electronics and computing. ... In synchronous digital electronics, such as most computers, a clock signal is a signal used to coordinate the actions of two or more circuits. ... |- ||0||0||0||0 |- ||0||0||1||1 |- ||0||1||X||0 |- ||1||0||X||1 |- ||1||1||0||1 |- ||1||1||1||0 |- |} The J-K flip-flop was named after Jack Kilby, the man who invented integrated circuits in 1958, for which he was awarded the Nobel Prize... A digital system is one that uses numbers, especially binary numbers, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (an analog system) or non-numeric symbols such as letters or icons. ... Microprocessors, including an Intel 80486DX2 and an Intel 80386 Microprocessor, here an AMD Athlon64 X2 4200+ A microprocessor (sometimes abbreviated µP or uP) is a digital electronic component with miniaturized transistors on a single semiconductor integrated circuit (IC). ...


Example

Suppose the reference signal is 100 kHz, and the divider can be preset to any value between 1 and 100. The error signal produced by the comparator will only be zero when the output of the divider is also 100 kHz. For this to be the case, the VCO must run at a frequency which is 100 kHz x the divider count value. Thus it will produce an output of 100 kHz for a count of 1, 200 kHz for a count of 2, 1 MHz for a count of 10 and so on. Note that only whole multiples of the reference frequency can be obtained.


Practical considerations

In practice this type of frequency synthesiser cannot operate over a very wide range of frequencies, because the comparator will have a limited bandwidth and may suffer from aliasing problems. This would lead to false locking situations, or an inability to lock at all. In addition, it is hard to make a high frequency VCO that operates over a very wide range. This is due to several factors, but the primary restriction is the limited capacitance range of varactor diodes. However, in most systems where a synthesiser is used, we are not after a huge range, but rather a finite number over some defined range, such as a number of radio channels in a specific band. Bandwidth is a measure of frequency range. ... In statistics, signal processing, and related disciplines, aliasing is an effect that causes different continuous signals to become indistinguishable (or aliases of one another) when sampled. ... A varicap diode, varactor diode or tuning diode is a type of diode used in electronic circuits. ... Types of diodes In electronics, a diode is a component that restricts the direction of movement of charge carriers. ...


Many radio applications require frequencies that are higher than can be directly input to the digital counter. To overcome this, the entire counter could be contructed using high-speed logic such as ECL, or more commonly, using a fast initial division stage called a prescaler which reduces the frequency to a manageable level. Since the prescaler is part of the overall division ratio, a fixed prescaler can cause problems designing a system with narrow channel spacings - typically encountered in radio applications. This can be overcome using a dual-modulus prescaler. In electronics, emitter coupled logic (or ECL) is a design which uses transistors to steer current through gates which compute logical functions (as does every logic family). ... The Dual modulus prescaler is an electronic circuit used in high frequency frequency synthesiser designs to overcome the problem of generating narrowly-spaced frequencies that are nevertheless too high to be passed directly through the feedback loop of the system. ...


Further practical aspects concern the amount of time the system can switch from channel to channel, time to lock when first switched on, and how much noise there is in the output. All of these are a function of the loop filter of the system, which is a low-pass filter placed between the output of the frequency comparator and the input of the VCO. Usually the output of a frequency comparator is in the form of short error pulses, but the input of the VCO must be a smooth noise-free DC voltage. (Any noise on this signal naturally causes frequency modulation of the VCO.). Heavy filtering will make the VCO slow to respond to changes, causing drift and slow response time, but light filtering will produce noise and other problems with harmonics. Thus the design of the filter is critical to the performance of the system and in fact the main area that a designer will concentrate on when building a synthesiser system. NOiSE is a one volume manga created by Tsutomu Nihei as a prequel to his much-acclaimed ten-volume work, Blame! It offers some rather sketchy information concerning the Megastructures origins and initial size, as well as the beginnings of silicon life ... Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave. ... In acoustics and telecommunication, the harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. ...


See also


  Results from FactBites:
 
Spartanburg SC | GoUpstate.com | Spartanburg Herald-Journal (1135 words)
A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator.
It compares the frequencies of two signals and produces an error signal which is proportional to the difference between the input frequencies.
The key to the ability of a frequency synthesizer to generate multiple frequencies is the divider placed between the output and the feedback input.
Fractional-division frequency synthesizer for digital angle-modulation - Patent 4492936 (2311 words)
A frequency synthesizer of the fractional-division type is already well-known and has the intended function of delivering an output signal of the form F.sub.S =(N+k)F.sub.R where N is the integral part of a number N+k and F.sub.R is the frequency of a reference signal.
When a frequency synthesizer of this type is intended to be associated with a digital-signal modulator for delivering a signal which is angle-modulated by digital data, it is a known practice to place the modulator in the control loop or at the output of the frequency synthesizer.
In the case of the frequency synthesizer according to the invention, the number G is derived from the sum of a number g and of a number dg, said sum being formed by an adder 80.
  More results at FactBites »

 
 

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