Simple Oscillator Design Procedure
27 Following is a simple step-by-step procedure for the design of a Colpitts oscillator. The Colpitts can work over a wide frequency range. (A Hartley can be designed using a similar set of steps.)
By following this procedure, you can design an oscillator that works in the majority of cases. There is a procedure you can use that guarantees that the oscillator will work, but it is far more complex.
Follow these steps:
1. Choose the frequency of the oscillator output signal.
2. Choose a suitable coil. This step presents the greatest practical difficulty. Some values of coil are often not available, so you must use whatever is readily available. Fortunately, you can use a wide range of inductance values and still obtain the desired resonance frequency by adjusting the value of the capacitor.
3. If you know the value of the inductance, calculate the capacitor value using this formula:
Use this value of capacitor for C1 in the next steps.
4. If you don't know the inductance value, choose any value of capacitance and call this C1. This may produce a frequency considerably different from what you require. However, at this stage, the main thing is to get the circuit oscillating. You can adjust values later.
Choose a capacitor C2
that is between 3 and 10 times the value of C1
. Figure 9.36
shows the two capacitors and the coil connected in a ...