Inverter oscillator - can be crystal controlled

Not another TTL squarewave gen- erator?! Surely, there are plenty of them in other issues of Elektor? Yes, but this is an oscillator with a difference: unlike most of its counterparts its frequency is vari- able. In fact it may be adjusted over a wide range. The circuit shown here consists of two inverters with one or two ex- ternal components. Resistors R1 and R2 and the trimming capacitor C1 set the frequency. With the given component values, the oscil- lator frequency may be adjusted from 800 kHz to 12 MHz. The R3 resistors set the frequency in just about the right region, whereas C1 provides the fine ad- justment. The resistor values are not really critical; just make sure that they are both the same. The circuit is also suitable as a stable crystal oscillator. All you have to do is replace the trimming capacitor with a crystal with the corresponding frequency. Sup- posing, for instance, the oscillator frequency is to be 1 MHz, then the crystal will have to be a 1 MHz type. in...