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[1] as found in the 1960s HP 5100A) and the more modern direct digital synthesizer (DDS) (table-look-up). The third type are routinely used as communication system IC building-blocks: indirect digital (PLL) synthesizers including integer-N and fractional-N.[2] The recently emerged TAF-DPS is also a direct approach. It directly constructs the waveform of each pulse in the clock pulse train.
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.[3]
Time-Average-Frequency Direct Period Synthesis (TAF-DPS)
Recently, a technique named Time-Average-Frequency Direct Period Synthesis (TAF-DPS) emerges as a new member to the frequency synthesizer family. It focuses on frequency generation for clock signal driving integrated circuit. Different from all other techniques, it uses a novel concept of Time-Average-Frequency.[4] Its aim is to address the two long-lasting problems in the field of on-chip clock signal generation: arbitrary-frequency-generation and instantaneous-frequency-switching.
Starting from a base time unit, TAF-DPS first creates two types of cycles TA and TB. These two types of cycles are then used in an interleaved fashion to produce the clock pulse train. As a result, TAF-DPS is able to address the problems of arbitrary-frequency-generation and instantaneous-frequency-switching more effectively. The first circuit technology of utilizing the TAF concept (although subconsciously) is the “Flying-Adder frequency synthesis architecture or“Flying-Adder PLL”, which is developed in late 1990s. Since the introduction of TAF concept in 2008, the development of a frequency synthesis technology that works on TAF formally kicks off. A detailed description of this technology can be found in those books[5][6] and this short tutorial. As development progresses, it gradually becomes clear that TAF-DPS is a circuit level enabler for system level innovation.[7] It can be used in many areas other than clock signal generation. Its impact is significant since clock signal is the most important signal in electronics, establishing the flow-of-time inside the electronic world. This profound influence is being seen in this directional change in Moore's Law from space to time.[8]
History
This section possibly contains original research. Confuses tuning of receiver LO and RF stage (receivers are still adjusted to different channels with variable capacitors); indirect synthesizers are often LC oscillators; there were crystal-controlled receivers (common in WW II); classic CB receiver used switched crystals (possibly partially populated); there were switched double-conversion crystal aircraft radios that tuned hundreds of channels; re "not very stable": there were stable VFO designs; "many orders of magnitude" is vague; for transmitters and receivers, the stability specs are not that demanding; microwave resonators used cavities. Please improve it by verifying the claims made and adding inline citations. Statements consisting only of original research should be removed.(February 2017) ( Learn how and when to remove this template message )
Prior to widespread use of synthesizers, in order to pick up stations on different frequencies, radio and television receivers relied on manual tuning of a local oscillator, which used a resonant circuit composed of an inductor and capacitor, or sometimes resonant transmission lines; to determine the frequency. The receiver was adjusted to different frequencies by either a variable capacitor, or a switch which chose the proper tuned circuit for the desired channel, such as with the turret tuner commonly used in television receivers prior to the 1980s. However the resonant frequency of a tuned circuit is not very stable; variations in temperature and aging of components caused frequency drift, causing the receiver to drift off the station frequency. Automatic frequency control (AFC) solves some of the drift problem, but manual retuning was often necessary. Since transmitter frequencies are stabilized, an accurate source of fixed, stable frequencies in the receiver would solve the problem.
Quartz crystal resonators are many orders of magnitude more stable than LC circuits and when used to control the frequency of the local oscillator offer adequate stability to keep a receiver in tune. However the resonant frequency of a crystal is determined by its dimensions and cannot be varied to tune the receiver to different frequencies. One solution is to employ many crystals, one for each frequency desired, and switch the correct one into the circuit. 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 channel frequencies from about 88 MHz to 108MHz; the ability to tune in each channel would require 100 crystals. 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.
The solution to this was the development of circuits which could generate multiple frequencies from a "reference frequency" produced by a crystal oscillator. This is called a frequency synthesizer. The new "synthesized" frequencies would have the frequency stability of the master crystal oscillator, since they were derived from it.
Many techniques have been devised over the years for synthesizing frequencies. 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, a crystal oscillator is common, but other resonators and frequency sources can be used. Incoherent techniques derive frequencies from a set of several stable oscillators.[9] 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 loops or PLLs. Many types of frequency synthesizer are available as integrated circuits, 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.[10] In the system design of a frequency synthesizer, states Manassewitsch, there are as many "best" design procedures as there are experienced synthesizer designers.[10]System analysis 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.[11][12] James A. Crawford says that these are mutually contradictive requirements.[12]
Influential early books on frequency synthesis techniques include those by Floyd M. Gardner (his 1966 Phaselock techniques)[13] and by Venceslav F. Kroupa (his 1973 Frequency Synthesis).[14]
Mathematical techniques analogous to mechanical gear-ratio relationships can be employed in frequency synthesis when the frequency synthesis factor is a ratio of integers.[14] This method allows for effective planning of distribution and suppression of spectral spurs.
Variable-frequency synthesizers, including DDS, are routinely designed using Modulo-N arithmetic to represent phase.
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.[15] 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.
Block diagram of a common type of PLL 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-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 synthesizer to be easily controlled by a digital system.
Example
Suppose the reference signal is 100kHz, 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 100kHz. For this to be the case, the VCO must run at a frequency which is 100kHz x the divider count value. Thus it will produce an output of 100kHz for a count of 1, 200kHz for a count of 2, 1MHz 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.[16]
Practical considerations
Philips TDA6651TT - 5 V mixer/oscillator and low noise PLL synthesizer for hybrid terrestrial tuner
In practice this type of frequency synthesizer 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 synthesizer 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.[16]
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 synthesizer system.[16]
Use as a frequency modulator
Many PLL frequency synthesizers can also generate frequency modulation (FM). The modulating signal is added to the output of the loop filter, directly varying the frequency of the VCO and the synthesizer output. The modulation will also appear at the phase comparator output, reduced in amplitude by any frequency division. Any spectral components in the modulating signal too low to be blocked by the loop filter end up back at the VCO input with opposite polarity to the modulating signal, thus cancelling them out. (The loop effectively sees these components as VCO noise to be tracked out.) Modulation components above the loop filter cutoff frequency cannot return to the VCO input so they remain in the VCO output.[17] This simple scheme therefore cannot directly handle low frequency (or DC) modulating signals but this is not a problem in the many AC-coupled video and audio FM transmitters that use this method. Such signals may also be placed on a subcarrier above the cutoff frequency of the PLL loop filter.
PLL frequency synthesizers can also be modulated at low frequency and down to DC by using two-point modulation to overcome the above limitation.[18] Modulation is applied to the VCO as before, but now is also applied digitally to the synthesizer in sympathy with the analog FM signal using a fast delta sigma ADC.
An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices ranging from simplest clock generators to digital instruments and complex computers and peripherals etc. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.
A Costas loop is a phase-locked loop (PLL) based circuit which is used for carrier frequency recovery from suppressed-carrier modulation signals and phase modulation signals. It was invented by John P. Costas at General Electric in the 1950s. Its invention was described as having had "a profound effect on modern digital communications". The primary application of Costas loops is in wireless receivers. Its advantage over other PLL-based detectors is that at small deviations the Costas loop error voltage is as compared to . This translates to double the sensitivity and also makes the Costas loop uniquely suited for tracking Doppler-shifted carriers, especially in OFDM and GPS receivers.
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a variable frequency oscillator and a phase detector in a feedback loop. The oscillator's frequency and phase are controlled proportionally by an applied voltage, hence the term voltage-controlled oscillator (VCO). The oscillator generates a periodic signal of a specific frequency, and the phase detector compares the phase of that signal with the phase of the input periodic signal, to adjust the oscillator to keep the phases matched.
A phase detector or phase comparator is a frequency mixer, analog multiplier or logic circuit that generates a signal which represents the difference in phase between two signal inputs.
A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned over some range. It is a necessary component in any tunable radio transmitter and in receivers that works by the superheterodyne principle. The oscillator controls the frequency to which the apparatus is tuned.
A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used for frequency modulation (FM) or phase modulation (PM) by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop. VCOs are used in synthesizers to generate a waveform whose pitch can be adjusted by a voltage determined by a musical keyboard or other input.
In electronic instrumentation and signal processing, a time-to-digital converter (TDC) is a device for recognizing events and providing a digital representation of the time they occurred. For example, a TDC might output the time of arrival for each incoming pulse. Some applications wish to measure the time interval between two events rather than some notion of an absolute time.
A tuner is a subsystem that receives radio frequency (RF) transmissions, such as FM broadcasting, and converts the selected carrier frequency and its associated bandwidth into a fixed frequency that is suitable for further processing, usually because a lower frequency is used on the output. Broadcast FM/AM transmissions usually feed this intermediate frequency (IF) directly into a demodulator that converts the radio signal into audio-frequency signals that can be fed into an amplifier to drive a loudspeaker.
Direct digital synthesis (DDS) is a method employed by frequency synthesizers used for creating arbitrary waveforms from a single, fixed-frequency reference clock. DDS is used in applications such as signal generation, local oscillators in communication systems, function generators, mixers, modulators, sound synthesizers and as part of a digital phase-locked loop.
In electronics, a frequency multiplier is an electronic circuit that generates an output signal and that output frequency is a harmonic (multiple) of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal. A subsequent bandpass filter selects the desired harmonic frequency and removes the unwanted fundamental and other harmonics from the output.
A prescaler is an electronic counting circuit used to reduce a high frequency electrical signal to a lower frequency by integer division. The prescaler takes the basic timer clock frequency and divides it by some value before feeding it to the timer, according to how the prescaler register(s) are configured. The prescaler values, referred to as prescales, that may be configured might be limited to a few fixed values, or they may be any integer value from 1 to 2^P, where P is the number of prescaler bits.
A direct-conversion receiver (DCR), also known as homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design that demodulates the incoming radio signal using synchronous detection driven by a local oscillator whose frequency is identical to, or very close to the carrier frequency of the intended signal. This is in contrast to the standard superheterodyne receiver where this is accomplished only after an initial conversion to an intermediate frequency.
In electronics, a delay-locked loop (DLL) is a pseudo-digital circuit similar to a phase-locked loop (PLL), with the main difference being the absence of an internal voltage-controlled oscillator, replaced by a delay line.
The "Wadley-drift-canceling-loop", also known as a "Wadley loop", is a system of two oscillators, a frequency synthesizer, and two frequency mixers in the radio-frequency signal path. The system was designed by Dr. Trevor Wadley in the 1940s in South Africa. The circuit was first used for a stable wavemeter.
In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers did not have to demodulate the radio signal, but just distinguish between the presence or absence of a radio signal, to reproduce the Morse code "dots" and "dashes". The device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.
A frequency divider, also called a clock divider or scaler or prescaler, is a circuit that takes an input signal of a frequency, , and generates an output signal of a frequency:
The harmonic mixer and subharmonic mixer are a type of frequency mixer, which is a circuit that changes one signal frequency to another. The ordinary mixer has two input signals and one output signal. If the two input signals are sinewaves at frequencies f1 and f2, then the output signal consists of frequency components at the sum f1+f2 and difference f1−f2 frequencies. In contrast, the harmonic and subharmonic mixers form sum and difference frequencies at a harmonic multiple of one of the inputs. The output signal then contains frequencies such as f1+kf2 and f1−kf2 where k is an integer.
Injection locking and injection pulling are the frequency effects that can occur when a harmonic oscillator is disturbed by a second oscillator operating at a nearby frequency. When the coupling is strong enough and the frequencies near enough, the second oscillator can capture the first oscillator, causing it to have essentially identical frequency as the second. This is injection locking. When the second oscillator merely disturbs the first but does not capture it, the effect is called injection pulling. Injection locking and pulling effects are observed in numerous types of physical systems, however the terms are most often associated with electronic oscillators or laser resonators.
A PLL multibit or multibit PLL is a phase-locked loop (PLL) which achieves improved performance compared to a unibit PLL by using more bits. Unibit PLLs use only the most significant bit (MSB) of each counter's output bus to measure the phase, while multibit PLLs use more bits. PLLs are an essential component in telecommunications.
The terms hold-in range, pull-in range, and lock-in range are widely used by engineers for the concepts of frequency deviation ranges within which phase-locked loop-based circuits can achieve lock under various additional conditions.
Xiu, Liming (2012), Nanometer Frequency Synthesis beyond Phase Locked Loop, Aug. 2012, John Wiley IEEE press (IEEE Press Series on Microelectronic Systems), ISBN978-1-118-16263-7.
Xiu, Liming (2015), From Frequency to Time-Average-Frequency: A Paradigm Shift in the Design of Electronic system, May 2015, John Wiley IEEE press (IEEE Press Series on Microelectronic Systems), ISBN978-1-119-02732-4.
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, ISBN0-471-52019-5
Hewlett-Packard 5100A (tunable, 0.01Hz-resolution Direct Frequency Synthesizer introduced in 1964; to HP, direct synthesis meant PLL not used, while indirect meant a PLL was used)
Hewlett-Packard (December 1965). Model 5100A Synthesizer(PDF). Operating and Service Manual.
Van Duzer, Victor E. (May 1964). "A 0-50 Mc Frequency Synthesizer with Excellent Stability, Fast Switching, and Fine Resolution"(PDF). Hewlett-Packard Journal. 15 (9): 1–6.. HP 5100A Direct synthesizer: comb generator; filter, mix, divide. Given 3.0bcdMHz, mix with 24MHz and filter to get 27.0bcdMHz, mix with 3.aMHz and filter to get 30.abcdMHz; divide by 10 and filter to get 3.0abcdMHz; feed to next stage to get another digit or mix up to 360.abcdMHz and start mixing and filtering with other frequencies in 1MHz (30–39MHz) and 10MHz (350–390MHz) steps. Spurious signals are -90dB (p.2).
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