Voltage controlled Oscillators
08/05/2021
The Basics of Voltage Controlled
Oscillators (VCOs)
Many electronic applications require that a signal’s frequency
be varied based on the amplitude of another signal.
The goal for designers is to determine how to perform this
function as efficiently and cost-effectively as possible, while
ensuring accuracy, reliability, and stability over time and
temperature.
This is the function of voltage controlled oscillators (VCOs).
These devices are designed to produce an output signal
whose frequency varies with the voltage amplitude of an
input signal over a reasonable range of frequencies. They are
used in PLLs(phase-locked loop or phase lock loop//is a
control mechanism), frequency and phase modulators, radar
and many other electronic systems.
What is Voltage Controlled Oscillator?
A voltage-controlled oscillator is an oscillator with
an output signal whose output can be varied over a
range, which is controlled by the input DC voltage.
It is an oscillator whose output frequency is directly
related to the voltage at its input.
The oscillation frequency varies from few hertz to
hundreds of GHz. By varying the input DC voltage,
the output frequency of the signal produced is
adjusted.
Frequency Control in Voltage
Controlled Oscillator
Many forms of frequency oscillators are
generally used. It can be of RC oscillator or multi
vibrator type or LC or crystal oscillator type.
However; if it is of RC oscillator type, the
oscillation frequency of output signal will be in
inversely proportional to capacitance as
In the case of LC oscillator, the oscillation
frequency of output signal will be
For VOCs
we can say that as the input voltage or control voltage increases,
the capacitance get reduced. Hence, the control voltage and
frequency of oscillations are directly proportional. That is, when
one increases, the other will increase.
The figure above represents the basic working of voltage
controlled oscillator. Here, we can see that at nominal
control voltage represented by VC(nom), the oscillator
works at its free running or normal frequency, fC(nom).
As the control voltage decreases from nominal voltage,
the frequency also decreases and as the nominal control
voltage increases, the frequency also gets higher.
The varactors diodes which are variable capacitance
diodes (available in different capacitance range) are
implemented for getting this variable voltage. For low
frequency oscillators, the charging rate of capacitors is
altered using voltage controlled current source to get the
variable voltage.
Types of Voltage Controlled Oscillator
The VCOs can be categorized based on the
output waveform:
• Harmonic Oscillators
• Relaxation Oscillators
Harmonic Oscillators
The output waveform produced by harmonic oscillators is sinusoidal.
This can often referred as linear voltage controlled oscillator.
The examples are LC and Crystal oscillators.
Here, the capacitance of the varactor diode is varied by the voltage
which is across the diode.
This in turns alters the capacitance of the LC circuit. Hence, the output
frequency will change.
Advantages are
frequency stability with reference to power supply,
noise and temperature, Accuracy in control of frequency.
The main drawback is this type of oscillators cannot be implemented
effortlessly on monolithic Ics(An integrated circuit or monolithic
integrated circuit).
Relaxation Oscillators
The output waveform produced by harmonic
oscillators is saw tooth. This type can give a large
range of frequency using reduced quantity of
components. Mainly it can be used in monolithic
ICs. The relaxation oscillators can possess the
following topologies:
• Delay-based ring VCOs
• Grounded capacitor VCOs
• Emitter-Coupled VCOs
Working Principle of Voltage
Controlled Oscillator (VCO)
VCO circuits can be designed by means of many
voltage control electronic components such as
varactor diodes, transistors, Op-amps etc. Here,
we are going to discuss about the working of a
VCO using Op-amps. The circuit diagram is
shown below.
Circuit operation
The output waveform of this VCO will be square wave.
As we know the output frequency is related to the control
voltage.
In this circuit the first Op-amp will function as an
integrator.
The voltage divider arrangement is implemented here.
Because of this, the half of the control voltage that is
given as input is given to the positive terminal of the Op-
amp 1.
The same level of voltage is maintained at the negative
terminal. This is to sustain the voltage drop across the
resistor, R1 as half of the control voltage.
When the MOSFET is in on condition, the current flowing
from the R1 resistor passes through the MOSFET.
The R2 have half the resistance, same voltage drop and
twice the current as that of R1.
So, the extra current charges the connected capacitor.
The Op-amp 1 should provide a gradually increasing
output voltage to supply this current.
When the MOSFET is in off condition, the current flowing
from the R1 resistor passes through the capacitor, get
discharged. The output voltage obtained from the Op-
amp 1 at this time will be falling. As a result, a triangular
waveform is generated as the output of Op-amp 1.
The Op-amp 2 will operate as Schmitt trigger.
The input to this Op-amp is triangular wave
which is the output of the Op-amp 1.
If the input voltage is higher than the threshold
level, the output from the Op-amp 2 will be VCC.
If the input voltage is less than the threshold
level, the output from the Op-amp 2 will be
zero. Therefore, the output of the Op-amp 2 will
be square wave.
Schmitt trigger
Assume the input voltage is lower than the reference voltage at the non-inverting pin
and the output is therefore high.
When the input goes above the reference voltage, the output goes low.
Applications of Voltage Controlled
Oscillator
• Function generator
• Phase Locked Loop-The phase-locked loop (PLL) block is a feedback
control system that automatically adjusts the phase of a locally generated signal to
match the phase of an input signal. PLLs operate by producing an oscillator frequency to
match the frequency of an input signal.
• Tone generator-an electronic device that artificially creates sounds
frequencies
• Frequency-shift keying
• Frequency modulation
• Inverters
• Frequency synthesizers, used in communication
circuits.
A Practical VCO – LM566
A practical example of a voltage-controlled oscillator
(VCO) is the LM566. The LM566 is a general-purpose VCO
that may be used to generate square wave and triangular
waveforms as a function input voltage.
The LM566 is specified for operation over 0˚C to 70˚C
temperature range. The frequency of which is a linear
function of a controlling voltage. The frequency is also
controlled by an external resistor and capacitor, whose
values control the free-running frequency.
IC Applications
• Function generator
• Tone generator
• FM modulation
• Frequency shift keying
• Clock generator
VOC circuit
Data sheet link
• https://www.ti.com/lit/ds/snosbq4e/snosbq4
e.pdf?ts=1620273354545&ref_url=https%253
A%252F%252Fwww.google.com%252F
