Loyce Trisha M. BelosoCatherine Laryli S. Perez

TCOM121.2July 28, 2010

Voltage Controlled Oscillator

LTSpice Mini-tutorial

• Voltage Divider Circuit

LTSpice Mini-tutorial

• Simulation

LTSpice Mini-tutorial

• To simulate output voltage…

LTSpice Mini-tutorial

• To simulate current flow…

Voltage Controlled Oscillator

Literature

http://www.falstad.com/circuit/e-vco.html

Modified Design

VCO: DEFINITION

• A voltage-controlled oscillator (VCO) is a circuit which can output a signal of a certain waveform (eg. triangular, sinusoidal).

• 2 kinds:

Harmonic oscillator – generates sinusoidal waveforms

Relaxation oscillator – generates a sawtooth or triangular waveform

VCO: DEFINITION

• A VCO circuit is voltage controllable over a frequency range.

• The frequency of oscillation can be controlled by the DC voltage applied.

Circuit basics: Definition

1st OPAMP – INTEGRATOR – Output is the integral of the input voltage.

– Since the input is DC, the integrated output will be a sloping signal.

Circuit basics: Defintion

2nd OPAMP – SCHMITT TRIGGER– Works like a comparator circuit

– It compares the input to certain reference voltages, then outputs a signal based on the comparison.

HOW THE CIRCUIT WORKS• 1st opamp: 2 input voltages (V+ and

V–)

• At first, the two input voltages will attempt to have the same voltage.

• A dc input voltage will be integrated by the 1st opamp.

• It will output a positive slope signal.

HOW THE CIRCUIT WORKS• The Schmitt trigger takes this output

as its input voltage.

• Schmitt trigger will compare the input voltage to two reference voltages 0V and 20/3 V.

• In this case, it will compare this to 0V since other nodes are grounded and I+ is initially 0.

• Since the input to the s. trigger is above 0V, it will output 20V peak.

HOW THE CIRCUIT WORKS• Consequently, it can be seen that

the gate of the MOSFET is connected to the output of the 2nd opamp.

• When the output of the trigger is high, it will cause a current to flow through the MOSFET causing it to be on and discharging the capacitor.

As a result, there will be current flowing in the drain causing V- to be greater than V+ in the first op amp. Thus, the integrated input signal will have a negative slope.

HOW THE CIRCUIT WORKS• The trigger will now compare this to

20/3 V and will output low (0V).

• As long as it is lower than the threshold voltage, it will continuously output low.

• When the output of the trigger is low, it will cause the MOSFET to be off, hence, no current will pass through and it will just be the same as the 1st state. The process will repeat over and over again.

Graph: 1V Vin

Graph: 2V Vin

Graph: 5V Vin

Varying Vin Graphs

Vin = 1V Vin = 2V

Vin = 5V

0s:3V||150ms:1V||260ms:6V||390m:1V

Equations• For triangular Waves

• For Square Waves

Time Lag

C= 10 nF

C= 20 nF

C= 30 nF

Varying Capacitor

C = 10 nF C = 20 nF

C = 30 nF

Spectral AnalysisV(n004)= Vin || V(n005) = Vout

Analysis• The control voltage is directly proportional to the

frequency of the output of the circuit.

• The time lag is related to the value of the capacitor used.

• The frequency is inversely proportional to the value of the capacitor used.

Sinusoidal Input Voltage• DC offset: 5, Amplitude: 12, Frequency: 5Hz

Sinusoidal Input Voltage• Positive Input Voltage

Sinusoidal Input Voltage• Negative Input Voltage

VCO With Low Pass Filter

New Schematic Diagram

Graph: Vout and vin

Spectral Analysis

Sources• http://www.falstad.com/circuit/e-vco.html

• R. L. Boylesyad and L. Nashelsky, Eletronic Devices and Circuit Theory, 7th ed., New Jersey: Prentice-Hall, pp. 735.

• **Special thanks to Mr. Lee Dong Woo for helping us out with the project