Hartley Oscillator

Aim:

To study the operation of Hartley Oscillator

Apparatus:

1.     Analog board AB68

2.   DC Power Supplies +12V from external source or ST2612 analog lab

3.   Oscilloscope 20 MHz, Caddo 802 or equivalent

4.   2 mm patch cords

Theory:

The Hartley oscillator is one of the simplest and best known oscillators and is used extensively in circuits, which work at radio frequencies.  The transistor  is in voltage divider bias which sets up Q-point of the circuit. The output voltage is fed back to the base and sustains oscillations developed across the tank circuit, provided there is enough voltage gain at the oscillation frequency. The resonant frequency of the Colpitt oscillator can be calculated from the tank circuit used. We can calculate the approximately resonant frequency as

1

ResonantFrequency(f_r) =

2π√L_T C

Here, the Inductor used is the equivalent Inductance. In Hartley oscillator the circulating current passes through the series combination of L1 and L2. Therefore equivalent Inductance is,

L_T = L₁ + L₂ + 2M

√

Where, M is the mutual inductance between two inductors.

M = K    L₁L₂

Where, K is the coefficient of coupling, lies between 0 to 1. The coefficient of coupling gives the extent to which two inductors are couple.

Circuit Diagram:

Procedure:

1.     Connect +12V dc power supplies at their indicated position from external source or ST2612 Analog Lab.

2.   Connect a patch cord between points a and b and another patch cord between point d and ground.

3.   Switch ON the power supply.

4.   Connect oscilloscope between Vout and ground on AB68 board.

5.   Record the value of output frequency on oscilloscope.

6.   Calculate the resonant frequency using equation mentioned.

7.    Compare measured frequency with the theoretically calculated value.

8.   Switch off the supply.

9.   Remove the patch chord connected between points a and b and connect it between points a and c.

10.   Remove the patch cord connected between points d and ground and con- nect it between point e and ground.

11.     Follow the procedure from point 4 to 7.

Result:

•    When patch cords are connected across a and b.

Practically calculated Output frequency (on CRO): 1.055 MHz Theoretically calculated values:

L_T : 2.078 x 10⁻⁵ H

Resonant frequency (f_r): 1.1038 MHz

•    When patch cords are connected across a and c

Practically calculated Output frequency (on CRO): 310.6 KHz Theoretically calculated values:

L_T : 2.278 x 10⁻⁴ H

Resonant frequency (f_r): 334 KHz