[time-nuts] Square to sine wave symmetrical conversion (part 2)

jerry shirᴀr radio.n9xr at gmail.com
Mon Jul 27 15:52:00 UTC 2015


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" <kb8tq at n1k.org> 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 <radio.n9xr at gmail.com> 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
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