Electronics > Beginners
Colpitts oscillator - role of feedback resistor
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emece67:
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aneevuser:

--- Quote from: T3sl4co1l on February 03, 2019, 08:06:20 pm ---Peaking includes the total loop gain of course, so you can have something like a passive RC network and enough gain to fix it up; but the phase noise will be worse than a sharper tuned network.
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I know nothing about analysis of phase noise but this is an intriguing comment -  can you expand on it? I take it that you're saying that "more gain in feedback network (=> less gain is amplifier) => better phase noise".

Is that a general principle? In fact, I'm not even sure what the source of phase noise is that you're referring to here - is it some effect inherent to the resonance of the tank circuit, rather than thermal noise, or whatever?


--- Quote ---It's not necessary to stick to that format, though, so you can indeed compensate for less gain peaking with more loop gain.  It's not an absolute statement.

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I don't quite follow this either - are you saying that you can, say, choose to design a Colpitt's oscillator to oscillate away from the resonant frequency of the tank circuit by adjusting the amp. gain suitably? If so, I don't see how.
aneevuser:

--- Quote from: emece67 on February 05, 2019, 09:55:17 am ---
--- Quote from: aneevuser on February 05, 2019, 09:34:38 am ---However, I've finally had the time to play around with this op amp based Colpitt's circuit, and whereas I've been able to get it to oscillate reliably, I've found that the frequency of oscillation has varied by up to +10% from the resonant frequency of the tank circuit (the frequency of circuit oscillation has always been greater than the resonant frequency, when it's differed). I don't have time to check these results at the moment, but if I haven't screwed up, it suggests to me that your analysis of oscillation frequency is not the whole story.

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Have you measured (in circuit) the resonance frequency of the C1LC2Ri network?

Regards.

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By "in circuit", do you mean while physically present in the Colpitt's oscillator circuit? If so, no. I measured it as a separate subcircuit, then connected it to the amp.

I'm not even sure how I would measure it when it's part of the whole circuit, apart from measuring the oscillation frequency.
emece67:
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Wimberleytech:

--- Quote from: emece67 on February 05, 2019, 10:31:04 am ---Opening the loop between R1 and C1 and applying signal to C1.

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I would think breaking the loop at the output of the opamp ahead of RF and driving into RF.  Otherwise, C1 will not see the resistance it normally sees when the loop is closed.

The signal source will probably have a 50ohm output resistance, so that will add to the value of RF and give a small error.
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