Voltage-controlled oscillator
From Wikipedia, the free encyclopedia





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.

A voltage-to-frequency converter (VFC) is a special type of VCO designed to
be very linear in frequency control over a wide range of input control
voltages.[1][2][3]

	
	
A microwave (12–18 GHz) voltage-controlled oscillator




Types
VCOs can be generally categorized into two groups based on the type of
waveform produced.[4]

Linear or harmonic oscillators generate a sinusoidal waveform. Harmonic
oscillators in electronics usually consist of a resonator with an amplifier that
replaces the resonator losses (to prevent the amplitude from decaying) and 
isolates the resonator from the output (so the load does not affect the
resonator). Some examples of harmonic oscillators are LC oscillators and crystal
oscillators.
Relaxation oscillators can generate a sawtooth or triangular waveform.
They are commonly used in integrated circuits (ICs). They can provide a wide
range of operational frequencies with a minimal number of external components.



Frequency control




A voltage-controlled capacitor is one method of making an LC oscillator vary
its frequency in response to a control voltage. Any reverse-biased semiconductor
diode displays a measure of voltage-dependent capacitance and can be used to 
change the frequency of an oscillator by varying a control voltage applied to 
the diode. Special-purpose variable-capacitance varactor diodes are available 
with well-characterized wide-ranging values of capacitance. A varactor is used 
to change the capacitance (and hence the frequency) of an LC tank. A varactor
can also change loading on a crystal resonator and pull its resonant frequency.

For low-frequency VCOs, other methods of varying the frequency (such as altering
the charging rate of a capacitor by means of a voltage-controlled current 
source) are used (see function generator).

The frequency of a ring oscillator is controlled by varying either the supply
voltage, the current available to each inverter stage, or the capacitive 
loading on each stage.

	
	
Schematic of an audio-frequency voltage-controlled oscillator




Phase-domain equations
VCOs are used in analog applications such as frequency modulation and
frequency-shift keying. The functional relationship between the control voltage
and the output frequency for a VCO (especially those used at radio frequency)
may not be linear, but over small ranges, the relationship is approximately 
linear, and linear control theory can be used. A voltage-to-frequency converter
(VFC) is a special type of VCO designed to be very linear over a wide range of
input voltages.

Modeling for VCOs is often not concerned with the amplitude or shape (sinewave,
triangle wave, sawtooth) but rather its instantaneous phase. In effect, the
focus is not on the time-domain signal A sin(ωt+θ0) but rather the argument of
the sine function (the phase). Consequently, modeling is often done in the
phase domain.

The instantaneous frequency of a VCO is often modeled as a linear relationship
with its instantaneous control voltage. The output phase of the oscillator is
the integral of the instantaneous frequency.

	

f(t) is the instantaneous frequency of the oscillator at time t
(not the waveform amplitude)
f_{0} is the quiescent frequency of the oscillator (not the waveform amplitude)
K_{0} is called the oscillator sensitivity, or gain. Its units are
hertz per volt.
f ( τ ) is the VCO's frequency
θ ( t ) is the VCO's output phase
v in ( t ) is the time-domain control input or tuning voltage of the VCO

For analyzing a control system, the Laplace transforms of the above signals are
useful.
	



Design and circuits
Tuning range, tuning gain and phase noise are the important characteristics
of a VCO. Generally, low phase noise is preferred in a VCO. Tuning gain and
noise present in the control signal affect the phase noise; high noise or
high tuning gain imply more phase noise. Other important elements that
determine the phase noise are sources of flicker noise (1/f noise) in the
circuit,[5] the output power level, and the loaded Q factor of the
resonator.[6] (see Leeson's equation). The low frequency flicker noise
affects the phase noise because the flicker noise is heterodyned to the
oscillator output frequency due to the non-linear transfer function of
active devices. The effect of flicker noise can be reduced with negative
feedback that linearizes the transfer function (for example, emitter
degeneration).

VCOs generally have lower Q factor compared to similar fixed-frequency
oscillators, and so suffer more jitter. The jitter can be made low enough
for many applications (such as driving an ASIC), in which case VCOs enjoy
the advantages of having no off-chip components (expensive) or on-chip
inductors (low yields on generic CMOS processes).



LC oscillators
Commonly used VCO circuits are the Clapp and Colpitts oscillators. The more
widely used oscillator of the two is Colpitts and these oscillators are very
similar in configuration. (Pics are hyperlinks)

 
 








Clapp
Colpits 1
Colpits 2




Crystal oscillators




A voltage-controlled crystal oscillator (VCXO) is used for fine adjustment
of the operating frequency. The frequency of a voltage-controlled crystal
oscillator can be varied a few tens of parts per million (ppm) over a
control voltage range of typically 0 to 3 volts, because the high Q factor
of the crystals allows frequency control over only a small range of
frequencies.
A 26 MHz TCVCXO

A temperature-compensated VCXO (TCVCXO) incorporates components that
partially correct the dependence on temperature of the resonant frequency of
the crystal. A smaller range of voltage control then suffices to stabilize
the oscillator frequency in applications where temperature varies, such as
heat buildup inside a transmitter.

Placing the oscillator in a crystal oven at a constant but higher-than
-ambient temperature is another way to stabilize oscillator frequency. High
stability crystal oscillator references often place the crystal in an oven
and use a voltage input for fine control.[7] The temperature is selected to
be the turnover temperature: the temperature where small changes do not
affect the resonance. The control voltage can be used to occasionally adjust
the reference frequency to a NIST source. Sophisticated designs may also
adjust the control voltage over time to compensate for crystal
aging.[citation needed]

	
	
27 MHz VCXO clock generator IC (TLSI T73227), used in a DVB-T set-top box.




Clock generators




A clock generator is an oscillator that provides a timing signal to
synchronize operations in digital circuits. VCXO clock generators are used
in many areas such as digital TV, modems, transmitters and computers. Design
parameters for a VCXO clock generator are tuning voltage range, center
frequency, frequency tuning range and the timing jitter of the output
signal. Jitter is a form of phase noise that must be minimised in
applications such as radio receivers, transmitters and measuring equipment.

When a wider selection of clock frequencies is needed the VCXO output can be
passed through digital divider circuits to obtain lower frequencies or be
fed to a phase-locked loop (PLL). ICs containing both a VCXO (for external
crystal) and a PLL are available. A typical application is to provide clock
frequencies in a range from 12 kHz to 96 kHz to an audio digital-to-analog
converter.

	
	




Frequency synthesizers
A frequency synthesizer generates precise and adjustable frequencies based
on a stable single-frequency clock. A digitally controlled oscillator based
on a frequency synthesizer may serve as a digital alternative to analog
voltage controlled oscillator circuits.



Applications




VCOs are used in function generators, phase-locked loops including frequency
synthesizers used in communication equipment and the production of
electronic music, to generate variable tones in synthesizers.

Function generators are low-frequency oscillators which feature multiple
waveforms, typically sine, square, and triangle waves. Monolithic function
generators are voltage-controlled.

Analog phase-locked loops typically contain VCOs. High-frequency VCOs are
usually used in phase-locked loops for radio receivers. Phase noise is the
most important specification in this application.[citation needed]

	
	
The Korg Monologue is a monophonic synthesizer with two VCOs.


Audio-frequency VCOs are used in analog music synthesizers. For these, sweep
range, linearity, and distortion are often the most important
specifications. Audio-frequency VCOs for use in musical contexts were
largely superseded in the 1980s by their digital counterparts, digitally
controlled oscillators (DCOs), due to their output stability in the face of
temperature changes during operation. Since the 1990s, musical software has
become the dominant sound-generating method.

Voltage-to-frequency converters are voltage-controlled oscillators with a
highly linear relation between applied voltage and frequency. They are used
to convert a slow analog signal (such as from a temperature transducer) to a
signal suitable for transmission over a long distance, since the frequency
will not drift or be affected by noise. Oscillators in this application may
have sine or square wave outputs.

Where the oscillator drives equipment that may generate radio-frequency
interference, adding a varying voltage to its control input, called
dithering,[8][9][10][11][12][13][excessive citations] can disperse the
interference spectrum to make it less objectionable (see spread spectrum
clock).
See also

 Low-frequency oscillation (LFO)
 Modular synthesizer
 Numerically-controlled oscillator (NCO)
 Variable-frequency oscillator (VFO)
 Variable-gain amplifier
 Voltage-controlled filter (VCF)