Frequency synthesizer
Encyclopedia
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, radiotelephone
s, walkie-talkie
s, CB radios, satellite receivers, GPS systems, etc. A frequency synthesizer can combine frequency multiplication
, frequency division, and frequency mixing
(the frequency mixing process generates sum and difference frequencies) operations to produce the desired output signal.
). The third type are routinely used as communication system IC
building-blocks: indirect digital (PLL) synthesizers including integer-N and fractional-N.
, such as with the 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.
A simple and effective solution employs the use of many stable resonators or oscillators, one 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.
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.
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. The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost.
Synthesizers used in commercial radio receivers are largely based on phase-locked loop
s or PLLs. Many types of frequency synthesiser are available as integrated circuit
s, reducing cost and size. High end receivers and electronic test equipment use more sophisticated techniques, often in combination.
of a frequency synthesizer involves output frequency range (or frequency bandwidth or tuning range), frequency increments (or resolution or frequency tuning), frequency stability (or phase stability, compare spurious outputs), phase noise performance (e.g., spectral purity), switching time
(compare settling time
and rise time
), and size, power consumption, and cost. James A. Crawford says that these are mutually contradictive requirements
This began to change with the works of Floyd M. Gardner
(his 1966 Phaselock techniques) and Venceslav F. Kroupa (his 1973 Frequency Synthesis). Manassewitsch calls this the Brute-force approach. Techniques and formulae have been provided by Dean Banerjee.
relationships can be employed in frequency synthesis when the frequency synthesis factor is composed of multiplicative integers in the numerator and denominator. This method allows for effective planning of distribution and suppression of spectral spurs.
are routinely designed using this method.
A phase locked loop is a feedback control system. It compares the phases of two input signals and produces an error signal that is proportional to the difference between their phases. 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 through a frequency divider back to the input of the system, producing a negative feedback
loop. 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 locked to the frequency 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. The block diagram below shows the basic elements and arrangement of a PLL based frequency synthesizer.
The key to the ability of a frequency synthesizer 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-flop
s, 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 synthesizer to be easily controlled by a digital system.
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 diode
s. 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.
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 constructed 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
.
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 harmonic
s. 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.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
system for generating any of a range of frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
from a single fixed timebase
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...
or oscillator
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...
. They are found in many modern devices, including radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
, mobile telephones, radiotelephone
Radiotelephone
A radiotelephone is a communications system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the...
s, walkie-talkie
Walkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...
s, CB radios, satellite receivers, GPS systems, etc. A frequency synthesizer can combine frequency multiplication
Frequency multiplier
In electronics, a frequency multiplier is an electronic circuit that generates an output signal whose output frequency is a harmonic of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal...
, frequency division, and frequency mixing
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
(the frequency mixing process generates sum and difference frequencies) operations to produce the desired output signal.
Types
Three types of synthesizer can be distinguished. The first and second type are routinely found as stand-alone architecture: Direct Analog Synthesis (also called a mix-filter-divide architecture as found in the 1960s HP 5100A) and by comparison the more modern Direct Digital Synthesizer (DDS) (Table-Look-UpLookup table
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...
). The third type are routinely used as communication system IC
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
building-blocks: indirect digital (PLL) synthesizers including integer-N and fractional-N.
Digiphase Synthesizer
It is in some ways similar to a DDS, but it has architectural differences. One of its big advantages is to allow a much finer resolution than other types of synthesizers with a given reference frequency.History
Prior to widespread use of synthesizers, radio and television receivers relied on manual tuning of a local oscillatorLocal oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
, such as with the turret tuner commonly used in television receivers prior to the 1980s. Variations in temperature and aging of components caused frequency drift
Frequency drift
In electrical engineering, and particularly in telecommunications, frequency drift is an unintended and generally arbitrary offset of an oscillator from its nominal frequency. Causes may include changes in temperature, which can alter the piezoelectric effect in a quartz crystal, or problems with...
. Automatic frequency control (AFC)
Automatic frequency control
In radio equipment, Automatic Frequency Control is a method to automatically keep a resonant circuit tuned to the frequency of an incoming radio signal...
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.
A simple and effective solution employs the use of many stable resonators or oscillators, one for each tuning frequency. Quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
crystals
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
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
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
over a much wider band. A large number of crystals increases cost and requires greater space.
Many coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
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
Direct digital synthesis
Direct Digital Synthesizer is a type of frequency synthesizer used for creating arbitrary waveforms from a single, fixed-frequency reference clock...
(DDS). The choice of approach depends on several factors, such as cost, complexity, frequency step size, switching rate, phase noise
Phase noise
Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities...
, and spurious output.
Coherent techniques generate frequencies derived from a single, stable master oscillator. In most applications, crystal oscillator
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
are common, but other resonators and frequency sources can be used. Incoherent techniques derive frequencies from a set of several stable oscillators. The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost.
Synthesizers used in commercial radio receivers are largely based on phase-locked loop
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
s or PLLs. Many types of frequency synthesiser are available as integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s, reducing cost and size. High end receivers and electronic test equipment use more sophisticated techniques, often in combination.
System analysis and design
A well-thought-out design procedure is considered to be the first significant step to a successful synthesizer project. In the system design of a frequency synthesizer, states Manassewitsch, there are as many "best" design procedures as there are experienced synthesizer designers. System analysisSystem analysis
System analysis in the field of electrical engineering characterizes electrical systems and their properties. System Analysis can be used to represent almost anything from population growth to audio speakers, electrical engineers often use it because of its direct relevance to many areas of their...
of a frequency synthesizer involves output frequency range (or frequency bandwidth or tuning range), frequency increments (or resolution or frequency tuning), frequency stability (or phase stability, compare spurious outputs), phase noise performance (e.g., spectral purity), switching time
Switching time
For a frequency synthesizer, the switching time or more colloquially the switching speed is the amount of time from when the command for the next frequency is requested until the time that the synthesizer's output becomes usable and meets the specified requirements. Such requirements will vary...
(compare settling time
Settling time
The settling time of an amplifier or other output device is the time elapsed from the application of an ideal instantaneous step input to the time at which the amplifier output has entered and remained within a specified error band, usually symmetrical about the final value.Settling time includes a...
and rise time
Rise time
In electronics, when describing a voltage or current step function, rise time refers to the time required for a signal to change from a specified low value to a specified high value...
), and size, power consumption, and cost. James A. Crawford says that these are mutually contradictive requirements
Trial-and-error superseded by calculation and control theory
The trial and error method was once the work-horse for designers of frequency synthesizers.This began to change with the works of Floyd M. Gardner
Floyd M. Gardner
Floyd M. Gardner is well-known expert and author in the area of PLL's. The first, second, and third editions of his book Phaselock Techniques have been highly influential and remains a well-recognized reference among electrical engineers specializing in areas involving PLL's.-Some well-cited...
(his 1966 Phaselock techniques) and Venceslav F. Kroupa (his 1973 Frequency Synthesis). Manassewitsch calls this the Brute-force approach. Techniques and formulae have been provided by Dean Banerjee.
Gearbox approach
Surprisingly sophisticated mathematical techniques analogous to mechanical gear ratioGear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...
relationships can be employed in frequency synthesis when the frequency synthesis factor is composed of multiplicative integers in the numerator and denominator. This method allows for effective planning of distribution and suppression of spectral spurs.
Modulo-N approach
Variable frequency synthesizers including DDSDirect digital synthesis
Direct Digital Synthesizer is a type of frequency synthesizer used for creating arbitrary waveforms from a single, fixed-frequency reference clock...
are routinely designed using this method.
Principle of PLL synthesizers
- See main article: Phase-locked loopPhase-locked loopA phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
A phase locked loop is a feedback control system. It compares the phases of two input signals and produces an error signal that is proportional to the difference between their phases. The error signal is then low pass filtered and used to drive a voltage-controlled oscillator
Voltage-controlled oscillator
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation or phase...
(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
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
loop. 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 locked to the frequency 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. The block diagram below shows the basic elements and arrangement of a PLL based frequency synthesizer.
The key to the ability of a frequency synthesizer 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
Clock signal
In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...
. 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-flop
Flip-flop (electronics)
In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic...
s, and because it is digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
in nature, is very easy to interface to other digital components or a microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
. This allows the frequency output by the synthesizer to be easily controlled by a digital system.
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 with the simplest integer N dividers. Fractional N dividers are readily available .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 aliasingAliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
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 diode
Varicap
In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals....
s. 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.
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 constructed using high-speed logic such as ECL
Emitter coupled logic
In electronics, emitter-coupled logic , is a logic family that achieves high speed by using an overdriven BJT differential amplifier with single-ended input, whose emitter current is limited to avoid the slow saturation region of transistor operation....
, 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
Dual-modulus prescaler
A dual modulus prescaler is an electronic circuit used in high-frequency synthesizer 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. The modulus of a prescaler is its frequency...
.
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
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical 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
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
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 harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
s. 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.
See also
- superheterodyne receiverSuperheterodyne receiverIn electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
- digitally controlled oscillator
- Dual-modulus prescalerDual-modulus prescalerA dual modulus prescaler is an electronic circuit used in high-frequency synthesizer 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. The modulus of a prescaler is its frequency...
Further reading
- Ulrich L. Rohde "Digital PLL Frequency Synthesizers - Theory and Design ", Prentice-Hall, Inc., Englewood Cliffs, NJ, January 1983
- Ulrich L. Rohde " Microwave and Wireless Synthesizers: Theory and Design ", John Wiley & Sons, August 1997, ISBN 0-471-52019-5
External links
- Hewlett-Packard 5100A (tunable, 0.01 Hz-resolution Direct Frequency Synthesizer introduced in 1964; to HP, direct synthesis meant PLL not used, while indirect meant a PLL was used)
- FREQUENCY SYNTHESIZER U.S. Patent 3,555,446, Braymer, N. B., (1971, January 12)