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DESIGN OF VARIABLE FREQUENCY

OSCILLATOR

Abstract -This project explores robust design of colpitts oscillator. Considering the difficulties faced by designer to select the values of components like resistors, capacitors, inductors etc., this project shows the development of a program that shall remove this problem. The circuit upon which program has been developed is basic colpitts oscillator circuit. The program will calculate the appropriate values of all the required parameters needed for the circuit as per an algorithm explained later and the oscillator will produce the desired frequency response . Although we have set maximum and minimum values of frequency of operation but the program can be used even for higher and lower frequency ranges.

I. INTRODUCTION

Colpitts and Hartley oscillators are widely used in the communication and computer systems. The Colpitts circuit, like other LC oscillators, consists of a gain device (such as a bipolar junction transistor, field-effect transistor, operational amplifier, or vacuum tube) with its output connected to its input in a feedback loop containing a parallel LC circuit (tuned circuit), which functions as a band pass filter to set the frequency of oscillation.

Usually while designing the oscillator we need to first choose an input frequency. After setting the frequency we need to choose proper values of inductances and capacitances for the tank circuit. The calculations required for designing the amplifier part has also to be done beforehand. After that to produce the sinusoidal curve of the desired frequency is also a tedious task. The program developed here eliminates the need to calculate all such values. The program itself shall calculate the values of all required parameters and produce the desired frequency response. Hence user has to input only the required frequency. Simulation and experimental results will verify the above mentioned results.

II. COLPITTS OSCILLATOR

Oscillators are circuits that generate a continuous voltage output waveform at a required frequency with the values of the inductors, capacitors or resistors forming a frequency selective LC resonant tank circuit and feedback network. The frequency of the oscillatory voltage depends upon the value of the inductance and capacitance in the LC tank circuit.

A.Basic Colpitts Oscillator Circuit

It consists of a tank circuit which is an LC resonance sub circuit made of two series capacitors connected in parallel to an inductor and frequency of oscillations can be determined by using the values of these capacitors and inductor of the tank circuit. This oscillator is almost similar to Hartley oscillator in all aspects; hence, it is termed as electrical dual of Hartley oscillator and is designed for the generation of high frequency sinusoidal oscillations with the radio frequencies typically ranging from 10 KHz to 300MHz. The major difference between these two oscillators is that it uses tapped capacitance,

whereas the Hartley oscillator uses tapped inductance.

III. DEVELOPMENT OF NGSPICE CODE

Ngspice is a general-purpose circuit simulation program for nonlinear and linear analyses. Circuits may contain resistors, capacitors, inductors, mutual inductors,

independent or dependent voltage and current sources, loss-less and lossy

transmission	lines,		switches,	uniform	distributed
RC lines, and the		five most common semiconductor
devices:	diodes,		BJTs,	JFETs,	MESFETs,

and MOSFETs.

A. Tank Circuit

The frequency is determined by the tank circuit and is varied by varying the values of the variable inductor L and keeping the values of capacitor C1 and C2 constant or vice versa. Here we have assumed C1=C2=C. The formula for calculating the frequency is given as follows:

ƒr=1/(2П√(L1*C T ))

f_r=frequency of oscillation

L=Value of inductance

C_T=Value of effective capacitance

C_T= C1C2/(C1+C2)

Ngspice Code

*PARAM: Parametric netlists

Ngspice allows for the definition of parametric attributes in the netlists. This is an enhancement

of the ngspice front-end that adds arithmetic functionality to the circuit description language.

Syntax:

.param <ident> =<expr>…..

First of all appropriate capacitance is selected for the desired frequency response as calculated

from relevant design equation.

The calculations were done on project and final values of each component is given in the following table

TABLE I

Values of Various components involved in oscillator design

Symbols	Components		Calculated values
Vcc	Power supply		6V
Vb	Bias voltage		1.2V
Ib	Base current		12uA
Ie	Collector current		1.2mA
R1	Pot.	Divider	40k ohm
	Resistance 1
R2	Pot.	Divider	10k ohm
	Resistance 1
Re	Emitter resistance		1k ohm
Cb	Coupling Capacitor		0.02nF
Cc	Coupling Capacitor		3.2nF
Ce	Bypass capacitor		1.6nF