Here's the rub Bob. I have been trying to find a way or have you explain how a high harmonic oscillator stage is even possible and zip. You don't know and I certainly don't know. So there's that.
Jerry On Jul 27, 2015 9:33 AM, "Bob Camp" <[email protected]> wrote: > Hi > > Here’s the basic point: > > What is *required* for low phase noise? > > If you can build *one* oscillator that violates a “law” then that “law” is > not > valid. In tis case the question is “do you *need* low harmonics in the > oscillator > stage to get low phase noise?” > > Here on the list, we get obsessed about all sorts of stuff. That’s fine. > It’s fun. > We learn things taking stuff past “the limit”. The gotcha is that can make > it > hard to keep track of “what is necessary ”. > > > On Jul 27, 2015, at 12:47 AM, jerry shirᴀr <[email protected]> wrote: > > > > Thanks Tim. I love reading these papers. However my copy states "In > fact, > > were it not for this slight non-linearity, it would be virtually > > impossible to build a simple lamp-stabilized RC oscillator with good > > envelope stability over a wide frequency range." rather than "In fact, > were > > it not for [amplifier] nonlinearity, it would be impossible to build a > > simple oscillator with good envelope stability." The meaning changes a > > little bit. > > > > Thanks Bob, > > > > Even looking at Tim's article, they are talking about a low degree of > > distortion with an RC oscillator. I am assuming that the Q of the RC > would > > be quite low with respect to the overtone crystals you speak, and yet the > > RC oscillator described here has low distortion from the oscillator > stage. > > The objective of an RC lab oscillator design *is* low harmonic distortion. > They > have awful phase noise. > > > > > Putting a filter in the feedback path with the high Q crystal seems like > > you would be de-Q-ing the crystal and losing the high Q characteristics > of > > the crystal. > > The oscillator must be a closed loop to operate. There will *always* be > things > “in series” with the crystal. > > > Any changes of filter components over time seems like it > > would necessarily add drift to the oscillator. > > Since you *must* tune the oscillator on frequency and you *must* select > the overtone, you will have caps and inductors in the loop. > > > What do you think? Of > > course I am not saying that you can't put filters in the crystal circuit > > but rather that is something I would never recommend doing that in a > > precision oscillator design. > > Except you have to do it. Since you have to do it, every example out there > of a low phase noise oscillator has at least some caps in series with the > crystal. The vast majority have both coils and caps. > > > > > I realize what the impedance plot looks like of AT-cut and SC-cut > crystals > > but my question was specifically about harmonics. That is the topic of > > this thread. Are you thinking that crystals are rich in harmonics? I am > > not really seeing an idea of where you are saying the harmonic components > > come from in these high precision oscillators in the oscillator circuit. > > The limiting action in the oscillator device creates harmonics. > > > > > What are the "impedance properties" of the crystal? > > There are literally thousands of papers on this. The simple answer is that > they have *many* resonant modes. > > > Why use a crystal > > rather than slapping a cap and a coil in there to get your desired > > frequency? > > 1) Because it’s Q is higher > 2) Because it’s more stable > > > > > When you "pick off" the collector current, wouldn't that include the > > amplified base to emitter junction noise inherent in simple transistor > > oscillator circuits? > > Again, it’s a loop. The current goes around in circles. There is no magic > “clean here” current. If you are looking at an OCXO that doubles the > crystal > before the output is created, it’s a really good bet they pulled the signal > off the collector of the oscillator. The net result is still a low phase > noise > oscillator. > > > Would that be the same as the crystal current? > > You can’t have an oscillator with just a crystal. You also need other > “stuff”…. > > Bob > > > > > Thanks. > > > > Jerry > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
