VOLTAGE CONTROLLED
OSCILLATOR USING ADC
INTRODUCTION
A 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 (FM) or phase modulation
(PM).
There are various ways in which one can implement a VCO. The current starved
VCO is the most common. We have used a different approach here to achieve the
same operation.
Our design contains:
Ring oscillator
Multiplexer
Analog to Digital Converter (ADC)
RING OSCILLATOR
The basic structure that generates oscillations is the ring oscillator. It is nothing
but a series of odd number of inverters whose output is fed back as the input.
Here, we use 9 inverters out of which the first one is a NAND gate. This is used
for facilitating simulation in DSCH3. When enable is 0, the output of the series is 1
and this is fed back. Without enable, the output will be in high impedance state.
When enable is 1, the NAND gate acts as an inverter.
The principle behind generating different frequencies is varying the number of
inverter stages generating the oscillations. Lesser the number of stages, lesser is
the delay and higher is the frequency.
The selection of different number of stages is done using a multiplexer.
MULTIPLEXER
A multiplexer is a circuit which chooses different inputs, based on its select
lines.
We have used a 4-to-1 MUX and it has 2 select lines. Thus, 4 different inputs can
be selected.
0 0 a
0 1 b
1 0 c
1 1 d
S1 S0 out
Thus when select inputs are 00 the longest series is selected (lowest frequency).
When select inputs are 11 the shortest series is selected (highest frequency).
The multiplexer is a digitally controlled circuit. But dc voltage is analog in nature.
Thus, we use an ADC to convert the DC voltage to digital.
ADC
The ADC has three parts:
Voltage divider
Comparator (open loop op-amp)
Encoder
Voltage divider : We use transmission gates
that are always ON instead of resistances to
divide the voltage. By trial and error, a
voltage divider with a combination of pass
transistors and transmission gates was
constructed. As we are using 0.12
micrometer technology, Vdd is 1.2V and this
is divided into 4 approximately equal steps-
0.25V, 0.55V, 0.85V and 1.2V
The transistor level of open
loop opamp is shown. Our
analog input signal is given to
the inverting terminal of the
opamp and the reference
voltage from the voltage
divider network is given to the
non-inverting terminal.
Thus, when input voltage is
lesser than the reference,
comparator output is HIGH.
The outputs of the opamps are encoded using priority encoder.
Vin C1 C2 C3 S1 S0
<V3 1 1 1 0 0
V3<Vin<V2 1 1 0 0 1
V2<Vin<V1 1 0 0 1 0
V1<Vin<Vdd 0 0 0 1 1
V3<V2<V1
All the components are connected together and the final VCO is shown below.
INPUT VOLTAGE (V) FREQUENCY (GHz)
0 to 0.22 3.51
0.23 to 0.49 3.81
0.5 to 0.79 4.44
0.8 to 1.2 5.66
RANGE OF INPUT VOLTAGE AND CORRESPONDING FREQUENCIES
LAYOUT OF VCO
SNAPSHOTS OF SIMULATION IN MICROWIND
Vin = 0V to 0.22V
Vin = 0.23V to 0.49V
Vin = 0.5V to 0.79V
Vin = 0.8V to 1.2V
The frequencies may vary depending upon the
software and components used.
VCOs are used in:
Electronic jamming equipment
Function Generators
The production of electronic music, to generate variable tones,
Phase-locked loops
Frequency synthesizers used in communication equipment.
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 digital signal for
transmission over a long distance, since the frequency will not drift or be affected
by noise. VCOs may have sine and/or square wave outputs. 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 for them. Low-frequency VCOs are used in analog music
synthesizers. For these, sweep range, linearity, and distortion are often most
important specs. Since music synthesis is nowadays done digitally, the market for
audio-frequency VCOs has largely disappeared.
APPLICATIONS
ACKNOWLEDGEMENTS
Behind every achievement lies an unfathomable sea of
gratitude to those who actuated it, without whom it would never
have come into existence. To them we lay the words of
gratitude imprinted not just in paper but also deep in our hearts.
We express our sincere gratitude towards Mrs.
A.R.Priyarenjini, Professor, Department of Electronics and
Communication, MSRIT, for constantly supporting us and
without whose invaluable guidance, the successful completion
of the project would not have been possible.
REFERENCES
The Designers Guide to Jitter in Ring Oscillators by John A. McNeill and David S.
Ricketts
www.microwind.net
