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| Colpitts oscillator - role of feedback resistor |
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| aneevuser:
--- Quote from: Wimberleytech on February 05, 2019, 10:48:22 am --- --- Quote from: emece67 on February 05, 2019, 10:31:04 am ---Opening the loop between R1 and C1 and applying signal to C1. --- End quote --- 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. --- End quote --- That makes sense. I'll re-measure the frequency like this when I get the time - got to put this to one side for now though. I'll redo the whole set of experiments at some point - I'm suspicious that I've screwed something up given the discrepancies in some of my measurements. |
| T3sl4co1l:
--- Quote from: aneevuser on February 05, 2019, 09:55:37 am ---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? --- End quote --- Not really, unfortunately; I'm not thoroughly studied/practiced in the arts of low noise, I just know what I've seen others talk about in the space. A lower gain amplifier can have lower noise, though. Technically, an amplifier acts as a refrigerator for the feedback resistor -- the amp consumes power to oppose the noise power of the feedback resistor. (I think I have that right?) Kind of sad that the electrical noise from a resistor is an extremely low power connection (~picowatts?), so you basically can't insulate something well enough to measure this, let alone make it practical. (To put it another way, you have two degrees of freedom by connecting to the resistor through a 1-dimensional transmission line; the resistor to space and surroundings, however, has zillions of degrees of freedom, because of the spacial fields (photons and phonons) that connect to it. Thermo counts degrees of freedom, so ambient heat wins.) --- Quote ---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. --- End quote --- For a well-damped network, there is no gain peak, and the phase shift at resonance (which isn't really resonating) needn't be exactly 180 at that point; by the point it is, gain is very low, so more gain is needed to make it oscillate. Well, if you define resonance as 180 degrees, then that's resonance by definition; I suppose it's as much a semantic as a practical difference. For a network more complicated than an all-series or all-parallel RLC, the different kinds of resonance need not match up (impedance becomes real / voltage or current or power reaches peak / phase shift equals a magic number, usually a multiple of 90 degrees / etc.), and it matters what you're doing. Tim |
| Wimberleytech:
BTW, ran a quick simulation with LTSPice and my results were as expected from theoretical calculations. |
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