# Can we make Colpitts Oscillator adjustable?

Collpits oscillator is one of the most widely used oscillators.

It's frequency equation is like this:

According to the equation, we can change its frequency by changing its LC tank parameters L and C. However, making adjustable capacitors and inductors are not much realistic. Also, their capacitance and inductance value can not vary that much to obtain $f_{0}$ - 100x$f_{0}$ oscillation frequency range.

So here is my question, how can I make this oscillator adjustable in a way that it can oscillate to 100x$f_{0}$?

Note: We can add any digital or analog block to the circuit to realize the goal.

• None of the common LC oscillator designs really lend themselves to adjustability, and certainly not over that kind of range. Why do you want this? Have you considered DDS instead? – Jack B Sep 14 '16 at 15:23
• If you can add a digital block, then fix the oscillation at the top end and add a PLL at the output. Or just use a crystal like everyone else. – pjc50 Sep 14 '16 at 15:30
• Notice the square root sign in the equation? That means you have to vary L or C, or their product, by 10000:1 to achieve 100:1 tuning range. Practically you can vary C by about 10:1 for 3;1 tuning range. To extend the range further, you'll need to switch in different inductors. (Or use frequency synthesis instead) – Brian Drummond Sep 14 '16 at 16:00
• Crazy idea: what if you use Miller effect for multiplying the capacitors? You would need an amplifier with a bandwidth much higher than the maximum frequency you intend to tune the oscillator to – Daniel V Sep 14 '16 at 16:16