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VCO types-LC, RC, Crystal, Multivibrator, Relaxation, Ring and VCO tuning

This page covers basics of VCO types and mentions VCO tuning methods. The VCO types include LC oscillator, RC oscillator, crystal oscillator, multivibrator, relaxation oscillator and ring oscillator.The difference between these VCO types are also covered.

About VCO: The VCO is the short form of Voltage Controlled Oscillator. 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.

VCO in PLL

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.

Applications of VCO

• Electronic jamming equipment.
• Function generator.
• Production of electronic music, for production of different types of noise.
• Phase locked loop.
• Frequency synthesizers, used in communication circuits.

VCO Types

VCO types

Oscillators are classied 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

It is made of L and C components. Figure depicts colpitts oscillator. colpitts oscillator
Colpitts Oscillator➤

RC oscillator

It is made of R and C components. The oscillator is known as wein bridge oscillator. wein bridge oscillator
Wein Bridge Oscillator➤

Crystal oscillator

The figure depicts crystal oscillator. crystal oscillator
Crystal Oscillator➤

Multivibrator

There different types of multivibrators viz. astable, monostable and bistable. Figure depicts astable multivibrator. astable multivibrator
Astable vs Monostable vs Bistable Multivibrator➤

Relaxation oscillator

relaxation oscillator

The figure depicts relaxation oscillator.

Ring oscillator

3-stage ring oscillator

The figure depicts 3 stage ring oscillator.

VCO tuning methods

VCO tuning

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).

VCO tuning methods

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.

VCO RELATED LINKS

VCO Frequency Pushing vs Frequency Pulling
What is VCO
Z-Comm VCO
RF Synthesizer

RF and Wireless Terminologies


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