This oscillator is very similar to the Colpitts except that it has a split inductance. It is represented in a similar way to the Colpitts, as seen in Fig. 1.14. It may be designed using a similar approach to the Colpitts but it has the disadvantages of mutual inductance between the coils, which causes unpredictable frequencies, and also the inductance is more difficult to vary. 
When two coils are placed in close proximity to one another the flux due to the magnetic field of one interacts with the other. Hence an induced voltage is applied to the second coil due to the rate of change of flux. Similarly, flux due to the magnetic field of the second coil may cut the first coil, also inducing a voltage in it. 
This is referred to as mutual induction, in contrast to self-inductance which is caused by lines of magnetic force cutting a single coil. Hence the rate of change of flux in one coil affects the other.

Splitting a single coil causes similar effects and mutual inductance exists between the two parts. As can be seen from equations (1.14), (1.15) and (1.16), the gain and frequency are dependent on the mutual inductance, and these parameters may be difficult to achieve as the tapping point has to be precise.

Two practical circuits are shown in Fig. 1.15. In both circuits the frequency is given by

where LT = L1 + L2 + 2M as both coils are virtually in series; note that M is the mutual inductance. The β factor and gain are

The remarks made earlier concerning loading and Q factors also apply here. While the Hartley and Colpitts oscillators have a similar design, the Hartley is easier to tune while the Colpitts requires two ganged capacitors. 

An advantage of using a Colpitts oscillator is the reduction in low-capacitance paths which can cause spurious oscillations at high frequencies. This is mainly due to the inter-electrode capacitance of the semiconductors. 

The Hartley oscillator, on the other hand, can produce several LC combinations due to the capacitance between the turns of the coil and thus cause spurious oscillations. It is for this reason that the Colpitts oscillator is often used as the local oscillator in receivers.

Hartley Oscillator Diagram

1 comment:

  1. where is audio input of the circuit, if it is use as FM modulator.


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