the design of crystal oscillators

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Crystal Oscillators


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THE DESIGN PRINCIPLES OF CRYSTAL OSCILLATORS

Crystal Oscillators are usually, fixed frequency oscillators where stability and accuracy are the primary considerations. For example it is almost impossible to design a stable and accurate LC oscillator for the upper HF and higher frequencies without resorting to some sort of crystal control. Hence the reason for crystal oscillators.

I won't be discussing frequency sythesisers and direct digital synthesis (DDS) here. They are particularly interesting topics to be covered later.

A PRACTICAL EXAMPLE OF A CRYSTAL OSCILLATOR

This image is copyrighted © by Ian C. Purdie VK2TIP - crystal oscillators schematic
Fig 1.

This is a typical example of the type of crystal oscillators which may be used for say converters. Some points of interest on crystal oscillators..

The transistor could be a general purpose type with an Ft of at least 150 Mhz for HF use. A typical example would be a 2N2222A.

The turns ratio on the tuned circuit depicts an anticipated nominal load of 50 ohms. This allows a theoretical 2K5 ohms on the collector. If it is followed by a buffer amplifier (recommended) I would simply maintain the typical 7:1 turns ratio. I have included a formula for determining L and C in the tuned circuits of crystal oscillators in case you have forgotten earlier tutorials. Personally I would make L a reactance of around 250 ohms. In this case I'd make C a smaller trimmer in parallel with a standard fixed value.

You can use an overtone crystal for the crystal and set L * C for the odd particular multiple of overtone wanted in your crystal oscillators.

More later.

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