Variable-frequency oscillator
Encyclopedia
A variable frequency oscillator (VFO) in electronics
is an oscillator whose frequency
can be tuned (i.e. varied) over some range. It is a necessary component in any tunable radio
receiver or transmitter that works by the superheterodyne principle, and controls the frequency
to which the apparatus is tuned.
radio receiver, the incoming radio frequency signal (at frequency
) from the antenna
is mixed
with the VFO output signal tuned to
, producing an intermediate frequency
(IF) signal that can be processed downstream to extract the modulated
information. The IF signal frequency is chosen to be either the sum of the two frequencies at the mixer inputs (up-conversion),
or more commonly, the difference frequency (down-conversion), 
, depending on the receiver design.
In addition to the desired IF signal and its unwanted image (the mixing product of opposite sign above), the mixer output will also contain the two original frequencies,
and 
and various harmonic
combinations of the input signals. These undesired signals are rejected by the IF filter
. If a double balanced mixer is employed, the input signals appearing at the mixer outputs are greatly attenuated, reducing the required complexity of the IF filter.
The advantage of using a VFO as a hetrodyning oscillator is that only a small portion of the radio receiver (all sections before the mixer such as the preamplifier) need to have a wide bandwidth. The rest of the receiver can be finely tuned to the IF frequency.
In a direct-conversion receiver
, the VFO is tuned to the same frequency as the incoming radio frequency and
Hz. Demodulation takes place at baseband using low-pass filter
s and amplifier
s.
In a radio frequency (RF) transmitter
, VFOs are often used to tune the frequency of the output signal, often indirectly through a hetrodyning process similar to that described above. Other uses include chirp
generators for radar systems where the VFO is swept rapidly through a range of frequencies, timing signal generation for oscilloscope
s and time domain reflectometers, and variable frequency audio generators used in musical instruments and audio test equipment.
There are two main types of VFO in use: analog and digital
.
where the value of at least one of the passive components is adjustable under user control so as to alter its output frequency.
The passive component whose value is adjustable is usually a capacitor
, but could be a variable inductor
.
. Adjustment of this capacitor is sometimes facilitated by a mechanical step-down gearbox to achieve fine tuning.
A reversed-biased semiconductor
diode
exhibits capacitance. Since the width of its non-conducting depletion region
depends on the magnitude of the reverse bias voltage, this voltage
can be used to control the junction capacitance. The varactor bias voltage may be generated in a number of ways and there may need to be no significant moving parts in the final design.
Varactors have a number of disadvantages including temperature drift and aging, electronic noise, low Q factor
and non-linearity.
The advantages including smaller designs, lack of moving parts, and the ease with which preset frequencies can be stored and manipulated in the digital computer that is usually embedded
in the design for other purposes.
It is also possible for the radio to become extremely frequency-agile in that the control computer could alter the radio's tuned frequency many tens, thousands or even millions of times a second.
This capability allows communications receivers effectively to monitor many channels at once, perhaps using digital selective calling (DSC
) techniques to decide when to open an audio output channel and alert users to incoming communications.
Pre-programmed frequency agility also forms the basis of some military radio encryption and stealth techniques.
Extreme frequency agility lies at the heart of spread spectrum
techniques that have gained mainstream acceptance in computer wireless networking such as Wi-Fi
.
There are disadvantages to digital synthesis such as the inability of a digital synthesiser to tune smoothly through all frequencies, but with the channelisation of many radio bands, this can also be seen as an advantage in that it prevents radios from operating in between two recognised channels.
Digital frequency synthesis relies on stable crystal controlled
reference frequency sources. Crystal controlled oscillators are more stable than inductively and capacitively controlled oscillators. Their disadvantage is that changing frequency (more than a small amount) requires changing the crystal, but frequency synthesizer techniques have made this unnecessary in modern designs.
Direct Digital Synthesis (DDS): Enough data points for a mathematical sine
function are stored in digital memory. These are recalled at the right speed and fed to a digital to analog converter where the required sine wave is built up.
Direct Frequency Synthesis: Early channelized communication radios had multiple crystals - one for each channel on which they could operate. After a while this thinking was combined with the basic ideas of heterodyning and mixing described under purpose above. Multiple crystals can be mixed in various combinations to produce various output frequencies.
Phase Locked Loop (PLL): Using a varactor-controlled or voltage-controlled oscillator (VCO) (described above in varactor under analog VFO techniques) and a phase detector, a control-loop can be set up so that the VCO's output is frequency-locked to a crystal controlled reference oscillator. The phase detector's comparison is made between the outputs of the two oscillators after frequency division by different divisors. Then by altering the frequency-division divisor(s) under computer control, a variety of actual (undivided) VCO output frequencies can be generated.
The PLL technique dominates most radio VFO designs today.
and spectral purity. All of these factors tend to be inversely proportional to the tuning circuit's
Q factor
. Since in general, the tuning range is also inversely proportional to Q, these performance factors generally degrade as the VFO's frequency range is increased.
to a stable reference oscillator. PLLs use negative feedback
to correct for the frequency drift of the VFO allowing for both wide tuning range and good frequency stability.
frequency synthesizer have less stringent requirements since the system is as stable as the crystal-controlled reference frequency.
ally related. Each of these peaks can potentially mix
with some other incoming signal and produce a spurious response. These spurii (sometimes spelled spuriae) can result in increased noise or two signals detected where there should only be one. Additional components can be added to a VFO to suppress high-frequency parasitic oscillations, should these be present.
In a transmitter, these spurious signals are generated along with the one desired signal. Filtering may be required to ensure the transmitted signal meets regulations for bandwidth and spurious emissions.
s' in the signal's timing will mean that the peak is sitting on 'skirts' of phase noise
at frequencies either side of the desired one.
These are also troublesome in crowded bands. They allow through unwanted signals that are fairly close to the expected one, but because of the random quality of these phase-noise 'skirts', the signals are usually unintelligible, appearing just as extra noise in the received signal. The effect is that what should be a clean signal in a crowded band can appear to be a very noisy signal, because of the effects of strong signals nearby.
The effect of VFO phase noise on a transmitter is that random noise is actually transmitted either side of the required signal. Again, this must be avoided for legal reasons in many cases.
-based alternatives. They tend to be more stable, more repeatable, have fewer and lower harmonics and lower noise than all the alternatives in their cost-band. This in part explains their huge popularity in low-cost and computer-controlled (i.e. PLL and synthesizer-based) VFOs.
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...
is an oscillator whose frequency
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...
can be tuned (i.e. varied) over some range. It is a necessary component in any tunable 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...
receiver or transmitter that works by the superheterodyne principle, and controls the frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
to which the apparatus is tuned.
Purpose
In a simple superhetSuperheterodyne receiver
In 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...
radio receiver, the incoming radio frequency signal (at frequency
) from the antennaAntenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
is mixed
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 -...
with the VFO output signal tuned to
, producing an intermediate frequencyIntermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(IF) signal that can be processed downstream to extract the modulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
information. The IF signal frequency is chosen to be either the sum of the two frequencies at the mixer inputs (up-conversion),
or more commonly, the difference frequency (down-conversion), , depending on the receiver design.In addition to the desired IF signal and its unwanted image (the mixing product of opposite sign above), the mixer output will also contain the two original frequencies,
and and various harmonicHarmonic
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...
combinations of the input signals. These undesired signals are rejected by the IF filter
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
. If a double balanced mixer is employed, the input signals appearing at the mixer outputs are greatly attenuated, reducing the required complexity of the IF filter.
The advantage of using a VFO as a hetrodyning oscillator is that only a small portion of the radio receiver (all sections before the mixer such as the preamplifier) need to have a wide bandwidth. The rest of the receiver can be finely tuned to the IF frequency.
In a direct-conversion receiver
Direct-conversion receiver
A direct-conversion receiver , 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...
, the VFO is tuned to the same frequency as the incoming radio frequency and
Hz. Demodulation takes place at baseband using low-pass filterLow-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
s and amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s.
In a radio frequency (RF) transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
, VFOs are often used to tune the frequency of the output signal, often indirectly through a hetrodyning process similar to that described above. Other uses include chirp
Chirp
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...
generators for radar systems where the VFO is swept rapidly through a range of frequencies, timing signal generation for oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
s and time domain reflectometers, and variable frequency audio generators used in musical instruments and audio test equipment.
There are two main types of VFO in use: analog and 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...
.
Analog VFO
An analog VFO is an electronic oscillatorElectronic 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...
where the value of at least one of the passive components is adjustable under user control so as to alter its output frequency.
The passive component whose value is adjustable is usually a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, but could be a variable inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
.
Tuning Capacitor
The variable capacitor is a mechanical device in which the separation of a series of interleaved metal plates is physically altered to vary its capacitanceCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
. Adjustment of this capacitor is sometimes facilitated by a mechanical step-down gearbox to achieve fine tuning.
Varactor
See varactor and voltage controlled oscillator.A reversed-biased semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
exhibits capacitance. Since the width of its non-conducting depletion region
Depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...
depends on the magnitude of the reverse bias voltage, this voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
can be used to control the junction capacitance. The varactor bias voltage may be generated in a number of ways and there may need to be no significant moving parts in the final design.
Varactors have a number of disadvantages including temperature drift and aging, electronic noise, low Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
and non-linearity.
Digital Crystal VFO
Modern radio receivers and transmitters usually use some form of digital frequency synthesis to generate their VFO signal.The advantages including smaller designs, lack of moving parts, and the ease with which preset frequencies can be stored and manipulated in the digital computer that is usually embedded
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
in the design for other purposes.
It is also possible for the radio to become extremely frequency-agile in that the control computer could alter the radio's tuned frequency many tens, thousands or even millions of times a second.
This capability allows communications receivers effectively to monitor many channels at once, perhaps using digital selective calling (DSC
Global Maritime Distress Safety System
The Global Maritime Distress and Safety System is an internationally agreed-upon set of safety procedures, types of equipment, and communication protocols used to increase safety and make it easier to rescue distressed ships, boats and aircraft....
) techniques to decide when to open an audio output channel and alert users to incoming communications.
Pre-programmed frequency agility also forms the basis of some military radio encryption and stealth techniques.
Extreme frequency agility lies at the heart of spread spectrum
Spread spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
techniques that have gained mainstream acceptance in computer wireless networking such as Wi-Fi
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
.
There are disadvantages to digital synthesis such as the inability of a digital synthesiser to tune smoothly through all frequencies, but with the channelisation of many radio bands, this can also be seen as an advantage in that it prevents radios from operating in between two recognised channels.
Digital frequency synthesis relies on stable crystal controlled
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...
reference frequency sources. Crystal controlled oscillators are more stable than inductively and capacitively controlled oscillators. Their disadvantage is that changing frequency (more than a small amount) requires changing the crystal, but frequency synthesizer techniques have made this unnecessary in modern designs.
Digital Frequency Synthesis
The electronic and digital techniques involved in this include:Direct Digital Synthesis (DDS): Enough data points for a mathematical sine
Sine
In mathematics, the sine function is a function of an angle. In a right triangle, sine gives the ratio of the length of the side opposite to an angle to the length of the hypotenuse.Sine is usually listed first amongst the trigonometric functions....
function are stored in digital memory. These are recalled at the right speed and fed to a digital to analog converter where the required sine wave is built up.
Direct Frequency Synthesis: Early channelized communication radios had multiple crystals - one for each channel on which they could operate. After a while this thinking was combined with the basic ideas of heterodyning and mixing described under purpose above. Multiple crystals can be mixed in various combinations to produce various output frequencies.
Phase Locked Loop (PLL): Using a varactor-controlled or voltage-controlled oscillator (VCO) (described above in varactor under analog VFO techniques) and a phase detector, a control-loop can be set up so that the VCO's output is frequency-locked to a crystal controlled reference oscillator. The phase detector's comparison is made between the outputs of the two oscillators after frequency division by different divisors. Then by altering the frequency-division divisor(s) under computer control, a variety of actual (undivided) VCO output frequencies can be generated.
The PLL technique dominates most radio VFO designs today.
Performance
The quality metrics for a VFO include frequency stability, phase noisePhase 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 spectral purity. All of these factors tend to be inversely proportional to the tuning circuit's
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
. Since in general, the tuning range is also inversely proportional to Q, these performance factors generally degrade as the VFO's frequency range is increased.
Stability
Stability is the measure of how far a VFO's output frequency drifts with time and temperature. To mitigate this problem, VFOs are generally "phase locked"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...
to a stable reference oscillator. PLLs use 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 :...
to correct for the frequency drift of the VFO allowing for both wide tuning range and good frequency stability.
Repeatability
Ideally, for the same control input to the VFO, the oscillator should generate exactly the same frequency. A change in the calibration of the VFO can change receiver tuning calibration; periodic re-alignment of a receiver may be needed. VFO's used as part of a phase-locked loopPhase-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...
frequency synthesizer have less stringent requirements since the system is as stable as the crystal-controlled reference frequency.
Purity
A plot of a VFO's amplitude vs. frequency may show several peaks, probably harmonicHarmonic
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...
ally related. Each of these peaks can potentially mix
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 -...
with some other incoming signal and produce a spurious response. These spurii (sometimes spelled spuriae) can result in increased noise or two signals detected where there should only be one. Additional components can be added to a VFO to suppress high-frequency parasitic oscillations, should these be present.
In a transmitter, these spurious signals are generated along with the one desired signal. Filtering may be required to ensure the transmitted signal meets regulations for bandwidth and spurious emissions.
Phase noise
When examined with very sensitive equipment, the pure sine-wave peak in a VFO's frequency graph will most likely turn out not to be sitting on a flat noise-floor. Slight random 'jitterJitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
s' in the signal's timing will mean that the peak is sitting on 'skirts' of 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...
at frequencies either side of the desired one.
These are also troublesome in crowded bands. They allow through unwanted signals that are fairly close to the expected one, but because of the random quality of these phase-noise 'skirts', the signals are usually unintelligible, appearing just as extra noise in the received signal. The effect is that what should be a clean signal in a crowded band can appear to be a very noisy signal, because of the effects of strong signals nearby.
The effect of VFO phase noise on a transmitter is that random noise is actually transmitted either side of the required signal. Again, this must be avoided for legal reasons in many cases.
Crystal control
In all performances cases, crystal controlled oscillators are better behaved than the semiconductor- and LCLC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
-based alternatives. They tend to be more stable, more repeatable, have fewer and lower harmonics and lower noise than all the alternatives in their cost-band. This in part explains their huge popularity in low-cost and computer-controlled (i.e. PLL and synthesizer-based) VFOs.
See also
- Voltage-controlled oscillatorVoltage-controlled oscillatorA 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...
- Numerically controlled oscillator
- RadioRadioRadio 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...
- Oscillator
- ResonanceResonanceIn physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
- TunerTunerTuner may refer to someone or something which adjusts or configures a mechanical, electronic, or musical device.- Electronic :* Antenna tuner, a device to adjust the resonance frequency of an antenna or transmission line...
- FeedbackFeedbackFeedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
- frequency division
- MicrocontrollerMicrocontrollerA microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...