So what is the operation of this oscillator?
Using an edited version of one of Mikes simulations:
Resistors R1 and R2 set the DC operating conditions for the transistor.
The Collector of the transistor gets its DC supply via the inductor L2.
Editing here:
(for the second time, because the first edit just dissapeared into the bit bucket!)
L2 provides a collector load which is very low resistance at DC and equal to the reactance of L2 at the operating frequency.
This statement is over simplified and is just plain wrong. What I should have said is:
The collector load for Q1 is provided by the overall impedance of L2 in parallel with the series imedance of L1 and C1.
The resulting impedance will be a maximum at the resonant frequency of the tuned circuit, and lower at all other frequencies above and below the resonant frequency.
This gives the amplifier maximum gain at the resonant frequency of the tuned circuit.
End of edit.
Capacitor C2 decouples the DC supply so that no AC can exist on the supply line.
The transistor is in a common emitter configuration, and so a signal at the base will be inverted at the collector (ie for a sine wave it will be phase shifted by 180 degrees).
The tuned circuit comprising L1, L2 and C1, at its resonant frequency the voltage at point "c" will be phase shifted by 180 degrees compared to point "a". This can be clearly seen from Mikes simulation plots.
The DC supply to the base of the transistor is via L1 and R1.
When the circuit is oscillating there will be an AC signal superimposed on the DC supply to the base.
That signal to the base will be 180 degrees phase shifted ffrom the signal at the collector.
So, we have 180 deg phase shift in the transistor, 180 deg phase shift in the tuned circuit, so the feedback signal is in phase with the original signal at the base, and provide that the gain around the circuit is greater than 1 at the resonant frequency of the tuned circuit, we will have a sustained oscillation.
I think that just about does it.
JimB
Edited at 11:30 26/12/13