What is VCO in Electronics ?
The full form of VCO is Voltage Controlled Oscillator. The VCO can be classified based on their tuning methods and circuit elements. Classification based on tuning include linear tuning VCOs, exponential tuning VCOs, Octave VCOs and Switched Capacitor VCOs. Classification based on circuit elements include ring oscillator, LC tank VCOs, RC Oscillators, crystal oscillator, varactor tuned oscillator, Negative resistance oscillator and so on.
About VCO: It is an electrical circuit which generates oscillatory output voltage. VCO is oscillator whose output frequency changes in direct proportion to the applied input voltage. The VCO devices can be constructed which can oscillate from few Hz to hundreds of GHz. VCO are of many types based on its construction as explained later.
The VCO has a characteristic gain, which can be expressed as ratio of the VCO output frequency to the VCO input voltage. VCOs utilize variable control voltage input in order to give frequency output. The control voltage input can be tuned such that desired operational frequency can be produced by the VCO. There are different tuning methods for the VCO as explained below.
The figure-1 depicts one of the application of VCO in PLL circuit.
As shown PLL (Phase Locked Loop) consists of VCO, PD (phase detector), loop filter,
digital frequency divider by N and RF mixer. In steady state output frequency of VCO is expressed as
follows.
Fo = Fm + N*Fr
Where, Fo = Output Frequency of PLL
Fm is the frequency input to the RF mixer used in order to downconvert Fo to
some lower frequency input for the divider. Frequency Divider block further applies down conversion
by factor of N. The output of the divider is fed to the PD input.
Fr is the reference frequency for the PLL.
From the expression it is imperative that VCO output frequency depends on N, Fr, and Fm.
Usually frequency of the VCO can be controlled by various methods.
The popular among them is by varying Fm & N and keeping Fr constant. Refer RF synthesizer design using PLL.
Voltage Controlled Oscillator Applications
• Electronic jamming equipment.
• Function generator.
• Production of electronic music, for production of different types of noise.
• Phase locked loop (PLL).
• Frequency synthesizers or generators/analyzers, used in communication circuits.
VCO Types
Oscillators are classified into tuned oscillator and untuned oscillator. A tuned oscillator uses a frequency-selective or tuned-circuit in the feedback path and is generally sinusoidal. An untuned or oscillator uses nonlinear feedback and is generally non-sinusoidal. Figure-2 depicts classification of VCO types.
LC oscillator
These VCOs use LC tank circuits (inductor and capacitor combination) to generate oscillations. It is also known as colpitts oscillator.
The frequency of oscillation of LC oscillator depends on values of inductor and capacitor used in the circuit. Colpitts Oscillator➤
RC oscillator
These VCOs use resistors and capacitors in an RC network to control the oscillation frequency. It is known as wein bridge oscillator.
They are relatively simple and widely used. Wein Bridge Oscillator➤
Crystal oscillator
These VCOs use quartz crystals to provide stable and precise frequency references.
They are commonly used in applications requiring high frequency accuracy. Crystal Oscillator➤
Multivibrator
A multivibrator, on the other hand, is a two-stage amplifier circuit that has two stable states and can switch between them. It is used to generate square waves, pulse waves, or other digital waveforms. The term "multivibrator" comes from its ability to vibrate or switch between two distinct states.
There different types of multivibrators viz. astable, monostable and bistable. Figure depicts astable multivibrator. Refer Astable vs Monostable vs Bistable Multivibrator➤
Relaxation oscillator
A relaxation oscillator is a type of electronic oscillator that generates a non-sinusoidal waveform such as square wave, triangle wave or sawtooth wave. This is done by repeatedly charging and discharging a capacitor or other energy storage element through a nonlinear device.
The figure depicts relaxation oscillator. This type of oscillator relies on the concept of relaxation, in which energy storage element is alternately charged and discharged. This leads to a repetitive waveform.
Ring oscillator
It is a specific configuration of digital logic gates that forms a self sustaining oscillating circuit. It is simple and popular design used to generate clock signals, frequency references and test signals. The basic purpose of ring oscillator is to create closed loop of odd numbered digital inverters where delay through each inverter contributes to overall oscillation frequency.
The figure depicts 3 stage ring oscillator. The key features of ring vco are as follows.
• Consists of odd number of inverter stages in a loop
• Output of last inverter is fed back to the input of first inverter which creates positive feedback loop.
• Oscillation frequency of it is determined by propagation delay of each inverter stage. The cumulative delay determines
overall frequency.
• Frequency of ring oscillator can be controlled by adjusting biasing conditions of transistors within each stage.
• Frequency stability of ring oscillators depends on various factors such as temperature effects, process variations,
supply voltage fluctuations etc.
• Ring oscillators are relatively power hungry compared to other VCO types due to continuous switching of digital gates.
A ring oscillator is a versatile and widely used circuit configuration that provides a relatively simple way to generate oscillations. It is used for various applications such as clock generation, Built-in-self-test, frequency generation, PLLs, process monitoring etc.
VCO tuning methods
VCO is an oscillator whose frequency can be varied by voltage or current. This is referred as VCO tuning. VCO is used in local oscillator which is part of RF up conversion and RF down conversion chain. The VCO frequency can be varied over entire transmit or receive frequency range.
The VCO tuning variables are capacitance (varactor), current and power supply. The speed of VCO tuning is determined by bandwidth of PLL (Phase Locked Loop).
The figure above mentions different methods for tuning various types of VCOs. The figure shows that some parameters are to be controlled to determine frequency.
The choice of VCO type depends on the specific application requirements such as frequency range, linearity, stability and power consumption.