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

Bob Camp kb8tq at n1k.org
Mon Jul 27 07:17:37 EDT 2015


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

>  Would that be the same as the crystal current?

You can’t have an oscillator with just a crystal. You also need other “stuff”….


> Thanks.
> Jerry
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