Crystal Oscillators

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

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.