# Always starting LC colpitts oscillator

Is there a way to make an LC oscillator that works in most circumstances? Which is able to start up an keeps oscillating for a wide range of inductors?

I'm asking this because I made a LC oscillator which can switch between several capacitor values (with analog switches) for operation in different frequency ranges.

In the simulation (falstad) is working for every possible capacitor combination. But in reality it is only working when both capacitors are enabled.

I don't need a perfect sine wave, but I do need a frequency dependent on the external inductor and adjustable in frequency range.

So I'm looking for an oscillator that starts in most conditions and is able to keep its oscillation going. Might this be possible by over-sizing the amplifier gain or using other components like opamp's. The frequency will be used to measure the inductance of the external inductor. Or any tips on how to dimension all the parts to make it more stable?

Update:

• The transformer is used to provide galvanic isolation from the external inductor which will be placed in the pavement as a vehicle detector

• Is there a way to make an LC oscillator that works in most circumstances? Sure, an oscillator only oscillates if it has sufficient loopgain. To learn about loopgain read: learnabout-electronics.org/Oscillators/osc11.php and electronics-tutorials.ws/oscillator/oscillators.html Commented Jun 11, 2019 at 13:04
• "I'm looking for an oscillator that starts in most conditions and is able to keep its oscillation going." To me that is what the definition of an oscillator is. Can you try and explain what you are searching for or what your question really is. Also, why do you use a transformer to connect your inductor AND, quite importantly, can you show a schematic that doesn't look like it suffers from the black death or smallpox? Commented Jun 11, 2019 at 13:28
• @Andyaka this should be better Commented Jun 11, 2019 at 13:46
• In the document as referenced by Bimpelrekkie thetre is an error: "Therefore the closed loop gain must be 1 (unity)" This is wrong. Theoretically, the closed-loop gain is infinite. Rather, it is the loop gain (open-loop conditions) which must be unity in accordance with the oscillation criterion.
– LvW
Commented Jun 11, 2019 at 14:16
• Bruce....while changing the value of the feedback capacitor in the feedback network you not only changes the frequency (that fulfills the oscillation condition) but also the damping factor of the feedback circuit (that means: The loop gain). Hence, the amplitude condition for oscillation is not met anymore. Hence, the gain of the active block must be modified at the same time. In general, it is not easy to find an oscillator that allows frequency tuning with one element only.
– LvW
Commented Jun 11, 2019 at 14:33