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walid

27MHz colpitts oscillator qestion

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This circuit was intended as 27MHz colpitts oscillator:
jb13354379381.jpg

and whem simulated using multisim10 sofware I get:
jb13354379382.jpg

I have some questions:
Any osci. Needs a FB to sustain oscillations, I think that this FB from collector to base path through the parallel two caps; the internal transistor capacitance b/w collector and Emitter, and the C4(=4.7p) cap.
From the Fairchild datasheets of the 2n3904 transistor I get:

[img width=680 height=70]
[img width=680 height=91]
Also, there is a curve related the capacitance with REVERSE BIAS VOLTAGE (V) 
My questions:
(1) What is the approxi. value of the internal transistor capacitance b/w collector and Emitter at 27MHz?
(2) How the phase shifted from 180 degree at the collector to 0 or 360 degree at the base? Please give me a step by step illustration.
(3) The freq of the osci. was 25.4MHz while my calculaed value of the resonant freq was 27MHz? what factors affect this?
thanks 

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