There are some great pages on the internet about Hartley oscillators, such as the Wikipedia article, so there is no need to reproduce that information here.

Instead one or two bits of practical information will be offered. Generally speaking, a Hartley or Colpitts oscillator will be used for higher (radio) frequencies. For lower frequencies (eg audio) a resistor-capacitor based oscillator or "RC" oscillator may be more suitable.

In common with other LC oscillators the Hartley oscillator can drift in frequency with changing temperature of the components. It is not uncommon for the oscillator to be constructed inside a temperature controlled enclosure often referred to as an "oven" to counteract this effect. Another effect is that any mechanical vibration (including sound) can influence the inductance value if the inductor is not mechanically robust. This is sometimes referred to as "microphony". Again this may cause frequency instability.

Another issue that is not often mentioned is that the output from the oscillator is rarely a very good sine wave. This is because the amplifying components (transistor) must be designed with sufficient power gain to overcome losses in the LC tank circuit and ensure reliable startup. This has the undesirable effect of causing the oscillations to increase in amplitude on applying power until they eventually "clip". This causes a mishapen sine wave with odd harmonic frequency components. For this reason, it is usually good practice to follow the Hartley oscillator with a low-pass filter stage. This restores the sine wave shape.

It is also common practice to follow this type of oscillator with a "buffer" stage, such as an emitter follower. This ensures that any circuit connected to the oscillator does not affect its frequency of oscillation.

Another good practice is to supply the oscillator from a voltage regulator, again to prevent unwanted changes in operating frequency.

The Hartley oscillator uses a parallel LC circuit. The 'L' part of this tuned circuit is made up of a tapped inductor. The LC tuned circuit is used as the input to an amplifier, and the output of the amplifier is fed to the tap on the inductor.

The circuit below is a practical example of a Hartley oscillator. The components L1 and VC1 form the LC tuned circuit. The capacitor C1 couples this to the amplifier formed by Q1. The output of the amplifier is taken from the emitter of Q1 and is fed to the tap on L1. A follower stage would be necessary in practice to prevent loading on the oscillator.

Hartley Oscillator

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