# Colpitts oscilator - purpose of voltage divider

I'm confused by the Colpitts oscillator shown below:

I've marked my understanding of the various components, but I'm having trouble understanding the tank circuit.

My understanding is that C1 and C2 form a voltage divider. However, I'm assuming Vin is coming from the emitter, between the two caps which seems unusual given the standard representation:

But intuitively I can see that the voltage is split between C1 and C2. But what is the purpose of this? From my understanding of tank circuits, L1 and C1 alone in parallel should resonate? Why is a voltage divider required?

Yes - it looks like a capacitive voltage divider - however, it isnt one because the signal input is NOT at the top of C1 but between both caps). If you try to look into the feedback circuit (starting at the emitter node which has a dynamic output small-signal resistance r,out) we face a third order circuit. This circuit is a first order grounded RC block (r,out-C2) that is connected with a second order grounded CL block (C1-L1). Both blocks together provide something like a bandpass response, however, with a squared s in the numerator and a third order denominator (EDIT: First order RC lowpass in series with a second order highpass).