27MHz colpitts oscillator qestions

This circuit was intended as 27MHz colpitts oscillator:


and whem simulated using multisim10 sofware I get:


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:


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

Starting with question 3; maybe your timestep was too small. I set min timestep = 100pS for reasonable accuracy, and got 27MHz output (as close as I can see).

To try to answer the rest:
The feedback path is the current from collector to emitter, through the 4.7pF capacitor. The base is effectively grounded at high frequencies.

The tuned frequency is set by the coil and the parallel combination of 695pF, 4.7pF and the collector-base capacitance of the transistor.

At resonance, the collector current is 90 degrees out of phase with the current through those capacitors, but at slightly higher frequencies, they get closer to being in phase. The phase shift between emitter current and collector current makes up the difference.

godfreyl said:

Starting with question 3; maybe your timestep

was too small. I set min timestep = 100pS for reasonable accuracy, and

got 27MHz output (as close as I can see).

Click to expand...

yes when changing the timestep from 3.6e-8 to 1e-10 i got 26.9MHz,

please what is the rule if my oscillator say, to operate at 100MHz, is

it (1/freq)?

The feedback path is the current from collector to emitter,

through the 4.7pF capacitor. The base is effectively grounded at high

frequencies.
At resonance, the collector current is 90 degrees out of phase with the

current through those capacitors, but at slightly higher frequencies,

they get closer to being in phase. The phase shift between emitter

current and collector current makes up the difference.

Click to expand...

I did not understand all your words here, but let me build on your idea

and say: At 27MHz, the transistor works as a common base which has a no

phase diffirence B/W collector o/p and emitter i/p.
the FB caps (in parallel) don't affect current.
caps make voltage lag.
is it true?

The tuned frequency is set by the coil and the parallel

combination of 695pF, 4.7pF and the collector-base capacitance of the

transistor.

Click to expand...

Yes you all right, when I introduce these capacitances in calc i get

the 26.85MHz as in multisim, so I have to take those capacitances into

acount when design another one.

thank you very much, it was a very good help.

samy555 said:

yes when changing the timestep from 3.6e-8 to 1e-10 i got 26.9MHz,

please what is the rule if my oscillator say, to operate at 100MHz....

Click to expand...

No fixed rule. Reducing the timestep makes the results more accurate, but if the timestep is too small, then the simulation takes too long to run. (I get bored if I have to wait more than 1 or 2 seconds).

When I simulate audio circuits, I normally use 1000 steps for each cycle of the waveform, but that is only because I want very accurate distortion measurements. e.g. For a 1KHz signal, I set min timestep = 1uS.

For this kind of circuit, accuracy is less important so 100 steps per cycle is probably OK. If you don't know what timestep to use for a circuit, reduce it in stages. When reducing it further doesn't make a difference, then it is low enough.

btw, If you want to make a radio transmitter, have a look at this website: Harry's Homebrew Homepage. It has some great circuits for FM radio and radio controlled models. There's even a couple of simple TV transmitters.

samy555 said:

and say: At 27MHz, the transistor works as a common base which has no phase diffirence B/W collector o/p and emitter i/p.

Click to expand...

Yes the transistor works as common base, but at high frequencies there is some phase difference B/W collector o/p and emitter i/p.

For example, if the transition frequency of the transistor = 200MHz, then the phase difference will be 45 degrees at 200MHz, 30 degrees at 100MHz, and 6 Degrees at 20 MHz.